Plant Pathology: Lab Exam
A phenoxy herbicide. A synthetic auxin. Causes distortions in grape, cotton, tomato, and other plants. May be confused with a viral disease. Can be used to control dodder, however it kills the host as well as the weed.
35S promoter
DNA found in cauliflower mosaic virus. Encodes 7 vital proteins: movement protein, insect transmission factor, structural protein with DNA binding capabilities, capsid protein, capsid protein, protease/reverse transciprtase, RNaseH, translational activator, and inclusion body formation/trafficking. A sequence overlap exists between it and the coding sequence of P6. Used to transform dicots. There are 54 transgenic events certified for release in the USA, with up to 528 bp of ORF.
Abiotic disorder

Abiotic disease

Unfavourable soil properties, fertility imbalances, moisture extremes, temperature extremes, chemical toxicity, physical injuries, and other problems. Reduce plant health and can kill plants, and can predispose them to diseases caused by pathogens. Very common: about half of plant samples in diagnostic clinics suffer from abiotic disorders. When all plants in an area are affected, it is likely to be caused by an abiotic disorder.

Plants need aeration of the root zone. This is the most important factor. Too much mulch, or soil compaction can reduce air in the root zone.
Air pollution
Can cause phytotoxicity. May be caused by ozone, sulphur dioxide, or ethylene. Air pollution levels may be reported in weather reports.
Animal damage

A form of mechanical injury. May predispose plants to diseases by weakening the plant and providing entry points for pathogens. Includes feeding by insects. Feeding by voles and rabbits results in girdling and death. Porcupines and squirrels remove bark and may leave small twigs on the ground, and can cause branch dieback. Deer may feed on buds and young stems of trees, as well as crops. Male deer may cause limb damage when they rub antlers on trees. Sapsuckers, inluding yellow-bellied sapsucker, drill holes in a straight line, oriented vertically or horizontally, on trees, causing reduced vigour and susceptibility to environmental stress, disease, and insects. May be corrected with protective barriers, live-trapping, repellants, or scare tactics.


Angie Liao
Worked from 2003 – 2006 on research sensitivity of Sclerotinia homoeocarpa to DMI fungicides.
Anne-Miet Van Den Nieuwelaar
Worked from 2013 – 2015 on research on sensitivity of Sclerotinia homoeocarpa to DMI fungicides.
Aphelenchoides besseyi

On the top-ten list of plant pathogenic nematodes. An important widespread pathogen. Causes white tip disease in rice. Seed-borne, and infected plants have reduced size. Formation of chlorotic tips on new leaves, which may then have necrosis. The flag leaf may be distorted, leading to a reduction in number and size of seeds produced. Can feed on other plants.


A tiny, green, pear-shaped insect with a soft body, two cornicles on the abdomen, and two antennae. It has a stylet piercing and sucking mouth part. Lives in colonies on succulent shoots, leaves, and stems. It is born pregnant. Gives live birth in the summer, and lays eggs in the fall for overwintering. Usually has a winged morph near the end of the summer, when it can fly to transmit plum pox virus.
Apple scab

Infections occur in mid-spring. Symptoms include blackish-purple blotches which become scabs in late summer. There is chlorosis, defoliation, and dieback. Mulch the leaves. Choose resistant cultivars such as Sutyzam. Use fungicides at budbreak.


A weak-wooded tree, fast-growing. Affected by emerald ash borer.
Area under the disease progess curve. A mathematical tool used for mathematical models of disease epidemics. A useful quantitative summary of disease intensity over time, for comparisons across years, locations, or management tactics. Can be deceiving; can produce the same number for very different situations. Uses calculus.
Bacterial spot of tomato

Xanthomonas vesicatora

X. euvesicatoria

X. perforans


Baermann funnel method
A method of detecting nematodes. Soil or plant tissue is placed in a funnel with filter paper, and nematodes move through water and sink to the bottom of rubber tubing. After 24 – 48 hours the nematodes are recovered in a shallow dish of water.
Bait test
Grow test plant sin soil from sites under investigation for viruses. Diagnostic symptoms may develop on the plant. Indicator plants may be mechanically inoculated with sap from test plant roots, and observed for symptom development.
Banner MAXX
A fungicide. Toxic dose is 180 L, or 1 L of undiluted product. It has a bad smell.
Bark beetle
Invades dead pine trees, and transmits blue-stain fungi.
Bean common mosaic virus

Symptoms may be similar to cucumber mosaic virus. Symptoms include leaf necrosis.


Beech bark disease

Neonectria ditissima

N. faginata

A complex of fungi and beech scale insect


Beet necrotic yellow vein virus

Symptoms include root proliferation.


Best Management Practices
A book published in 2012. Has recommendations to help reduce impact of plum pox virus in Ontario orchards and nurseries.


Increases the pH of soil. Causes root dieback and disease, and excess P.

A 96-well plate used for identifying bacteria. Uses colour reaction of substrates.
Biological assay
A pathogenicity way to diagnose plant viruses. Inoculate plants and observe for symptoms for days or weeks. Simple, requires minimal knowledge of the pathogen. Polyvalent. It is the only choice for uncharacterized graft-transmissible agents. Some isolates can induce no symptoms. Takes a long time to obtain results.
Black knot

Apiosporina morbosa

Manage by pruning branches.


Black pod disease of cacao

Phytophthora capsici

P. citrophthora

P. megakarya

P. palmivora

Losses are as high as 100%. An oomycetes. Quarantines are in different countries. Infected beans have a bad odour.


Black root rot

Teilaviopsis brasicola

Can become collar rot. Reproduces only asexually. Common in greenhouses. Reduce soil pH, and improve drainage.


Black root rot of hop

Phytophthora citricola

Affects hops as well as other hosts.



A symptom of high temperature damage. Leaves become white in colour. Occurs in geraniums when exposed to temperatures above 35?C.


Blossom end rot

A symptom of calcium deficiency. Can often lead to secondary colonization by fungi.


Blue stain fungus

Grosmannia clavigera

An ascomycetes fungus that evolved in a relationship with mountain pine beetles. lives in dead and dying wood of pines. Produces cobalt-blue discolouration that is prized for its interesting designs. Pine wilt nematode feeds on it. Transmitted by bark beetles. It blocks a tree’s vascualr system and can lead to plant death. The tree may be asymptomatic for over a year, especially in colder climates.


Boron (B)
A micronutrient. Non-mobile, and may be remedied with borax.

A symptom of phytotoxicity from copper-containing fungicides or insecticides, or ozone.



An odd growth that can be small, or grow to a large size. Bark covers it, and remains intact, not cracked or sunken.



A symptom of de-icing salts. A chlorotic scorch symptom of leaf blades, with or without a well-defined lesion margin.


Burrowing nematode

Radopholus simils

On the top-ten list of plant pathogenic nematodes. A migratory, endoparasitic nematode. Affects banana, citrus, and pepper, causing toppling disease. Causes economic problems in warmer regions, and has been introduced to temperate regions where it causes problems in greenhouses. Infection causes damage to roots, which have dark lesions caused as the nematode migrates through tissue. Tissue rots occur from secondary bacterial and/or fungal infections, weakening root systems, leaving plants susceptible to topping, especially in wind.


A drug with an LD50 of 192 mg/kg. In order to die from overdose, you would have to drink 100 cups without going pee, or eat 1.5 kg of beans, or 70 kg of chocolate.
Calcium (Ca)
A secondary macronutrient. Non-mobile, and excess can limit other nutrients. An important signal regulator, and strengthens cell membranes and cell walls. A component of resopnses to pathogen toxins including oxalic acid produced by Sclerotium roflsii. Fertilizers include calcitic lime and gypsum.
Calcium deficiency

Occurs in fruiting vegetables. A problem in acidic soils. A result of inadequate Ca levels in soil, poor transpiration, or fluctuations in soil moisture. Symptoms include blossom end rot, stunting, localized tissue necrosis, leaf marginal chlorosis, death of terminal buds and root tips, and inhibited root growth. Symptoms start on new leaves. May be corrected by increasing soil pH and applying Ca fertilizer. Increases susceptibility to certain plant diseases due to weakened cell walls. Increases susceptibility to Sclerotirum rolfsii, because calsium is a component of host response to oxalic acid, a toxin of this pathogen. May be caused by over-applicaiton of Mg, causing competition for cation exchange sites.




A variety of apple with bright red, 1 cm fruits. Fruiting is persistent. Very resistant to apple scab. White flowers, with densely branched habit.

Canada leaf tulip
A new tulip that has white petals with a red “maple leaf”-shaped streak.
Cauliflower mosaic virus (CaMV)


A pararetrovirus. Affects Brassicaceae and Solanaceae species. Vectored by 27 species of aphid. Symptoms include mosaic and mottle. Its genome has been mapped. Has 35S promoter which can be used to transform dicots.


Cedar leaf miner

Leaves look hollowed out.


Cellular changes
A way that nematodes cause damage to plants. Host cells have necrosis and abnormal cell growth. During feeding they inject saliva containing enzymes that decompose cells, producing necrosis and distortion of plant tissues.
Centrifuge method

Sugar floatation method

A method of detecting nematodes. After sieving, soil or plant tissues are placed into centrifuge tubes and spun at 3000 RPM for 4 minutes. Nematodes and debris sink to the bottom. Supernatant is discarded and the tube is filled with 1/2 sugar solution and shaken. The tube is centrifuged again at 3000 RPM for 1/2 to 2 minutes, and the clean nematodes are flushed into a dish or tube.

Ceral cyst nematode (CCN)

Heterodera avenae

H. filipjevi

Loss can be up to 90% in some environments.


Chilling injury

Low temperature damage that occurs above 0?C. Plants native to tropical regiosn are susceptible. Damages newly expanded plant parts, causing purplish colouration of foliage, and possible necrosis.


Chlormequat chloride

A PGR, a gibberellin inhibitor. Can cause temporary chlorosis in ornamentals.



Loss of chlorophyll. Yellowing. Occurs in older growth, or the whole plant.


Active ingredient
The fungicidal portion of a fungicide. Includes chlorothalonil, azoxystrobin, and metalaxyl.
Basal neck rot

A disease that affects onions. Caused by Fusarium oxysporum and Botrytis allii. May be detected with profiling of 25 VOCs identified by Prithiviraj et al, 2004.


Found in Ascomycetes. Differentiated by hyphal appendages and number of asci.
Chrysanthemum foliar nematode


A plant parasitic nematode. Causes angular leaf spots on coral bells.


Citrus canker

A disease which can be detected using hyperspectral imaging. Economic impact of missing a cankerous fruit is higher than that of misclassifying a fruit without a canker. Minimize false negative errors by properly selecting the threshold value with a cost of reducing the overall classification accuracy.


Citrus ring spot virus

Symptoms include bark scaling in sweet orange.


A Pythium-specific selective medium. Used for diagnosis of plant diseases.
Coffee leaf rust

Hemileia vastatrisx

Originated in Sri Lanka. A macrocyclic rust. Resistant varieties produce lower quality coffee.


A common pathogen, causing leaf blight and rings.
Colour breaking

A symptom of cucumber mosaic virus in yellow squash lacking the precocious gene. Also a symptom of watermelon mosaic potyvirus. Green blotchy patterns.


When soil structure has reduced pore space. May occur due to field traffic, raindrop impact, tilling, minimal crop rotation, or use of heavy equipment. There is less space for root growth. There is reduced water availability, and increased runoff. There is low oxygen availability for root respiration. Some species are more tolerant of compaction than others. Cracking at the soil surface in dry periods, and water pooling after rain may be an indication of compaction.
Copper (Cu)
A micronutrient. Non-mobile, and deficiencies are rare.
Cucumber mosaic virus (CMV)
In the genus Cucumovirus in the family Bromoviridae. Related to tomato aspermy virus and peanut stunt virus. First described in 1916 by Doolittle and Jagger. Occurs worldwide in temperate and tropical climates. Causes significant economic loss. Infection rates can reach 100%, but 10 – 20% is common. May reduce quality of product. Consists of three spherical particles, each 28 nm in diameter, with three negative-sense ssRNA molecules, RNA 1 – 3, each in a distinct particle. There may be a satellite RNA. Replicates RNA and produces proteins in the cytoplasm. May have intercellular movement through plasmodesmata and phloem. Includes the groups IA, IB, and II, distinguished by serological relationships and pathogenicity in cowpea. Some strains are host-specific.
Cucumber mosaic virus conditions
When it is cool and wet in the spring or fall, aphid numbers are decrased and the virus is sporadic, mostly near edges of fields. If weather is drier, aphid numbers are higher, and the virus may spread rapidly in young crops.
Cucumber mosaic virus dissemination
Seed-borne in chickweed, spinach, and legumes. Transmitted by over 80 species of aphid, including Myzus persicae, Aphis gossypii, soybean aphid (Aphis glycines), yellow clover aphid (Terioaphis trifolii), and pea aphid (Acyrthosiphon pisum). There is non-persistent, stylet-borne transmission. The virus is on the aphid for only a few minutes. Transmission efficiency depends on aphid species, virus strain, host, and environmental conditions. It may be transmitted mechanically in experiments, but is unstable.
Cucumber mosaic virus hosts
Affects 1,200 species in over 100 families, including cucumber and other cucurbits, artichoke, bean, beet, carrot, celery, lettuce, pea, pepper, potato, sweet potato, spinach, tomato, alfalfa, banana, chickpea, lupin, oilseed rape, subterranean clover, yam, anemone, candytuft, columbine, dahlia, delphinium, geranium, gladiolus, lily, marigold, petunia, phlox, viola, and zinnia. Weeds can be reservoirs, including milkweed (Asclepias syriaca), yellow rocket (Barbarea vulgaris), marsh yellowcress (Rorippa islandica), yellow toadflax (Linaria vulgaris), and common chickweed (Stellaria media). Resistant varieties have been around since 1927, including Chinese Long and Tokyo Long Green cucumbers, forming the basis of resistance today. Some zucchini squash have intermediate resistance, and some have transgenic resistance. Resistance in melons is derived from oriental melons. Most varieties of watermelon are resistant. Spinash variety Virginia Savoy has incomplete resistance.
Cucumber mosaic virus management
Genetic resistance is the most effective method. Isolation of the crop by growing non-host crop around it, such as corn, may delay initial infection. Insecticides and mineral oils may control aphid vectors; best used with high plant density. Insecticides provide transient protection due to non-persistent transmission. Reemay floating row cover can exclude aphids in early weeks. Inoculation with mild, protective strains. Application of resistance-inducing bacteria; often multiple species come in one commercial product.
Cucumber mosaic virus SAR
Infected cucumber cotyletonds with TMV, inducing SAR in other leaves of the same plants two weeks later with tobacco necrosis virus. Diameters of lesions that developed after the second inoculation were 35% less, and lesion numbers were 30% fewer than in control plants.
Cucumber mosaic virus symptoms
Systemic infection. May remain symptomless in some plants including alfalfa and many weeds. Symptoms vary depending on host species and age.
Cucumber mosaic virus symptoms in bean

Symptoms vary depending on variety. May be confused with bean common mosaic virus. Leaf curl, green mottle, and blistering, with zupperlike roughness along main veins. When infection occurs before bloom, there may be no yield due to flower abortion and abnormal development. When infection occurs after bloom, plants may recover and have normal growth with limited yield loss.


Cucumber mosaic virus symptoms in celery

Southern mosaic virus

Symptoms can be transient, and may be muted in cool environments. Leaves have vein clearing mosaic, with yellowing and veinal chlorosis. Petioles have long, sunken, beige lesions, and are unmarketable.


Cucumber mosaic virus symptoms in cucurbits

Severe epinasty common in summer squash. Early infections cause severe stunting, malformed leaves, and rough texture on fruit surface, making fruits unmarketable. Vines have stunted growing tips. Colour breaking may occur on some yellow squash. Pumpkins have mosaic pattern on the fruit and are unmarketable.


Cucumber mosaic virus symptoms in lettuce

Leaf molting, severe roughness of leaf, occasional necrosis. With early infections, the plant is stunted.


Cucumber mosaic virus symptoms in pepper

Symptoms vary depending on plant age, and time of infection. Initial symptoms include chlorosis of young leaves. Oak leaf and ringspot patterns develop as the plant ages. New leaves have a chlorotic mosaic pattern on the entire leaf, and later new leaves may have deformations including sunken interveinal lamina with protruding primary veins, with a dull light green appearance. The plant may be stunted. Fruits may have ringspot and roughness, and are unmarketable.


Cucumber mosaic virus symptoms in spinach

Spinach blight

Symptoms vary depending on variety, age of plant, temperature, and virus strain. Leaf chlorosis can progress and cause severe blighting of the growing point and plant death. Leaves are narrow, wrinkled, with vein distortion and inward roll.


Cucumber mosaic virus symptoms in tomato

Symptoms may be transient, and may appear on new and old leaves only, with normal middle leaves. Leaves are yellow, with mottle similar to tomato mosaic virus, but with shoestring-like blades. The plant is bushy and stunted. Fruits are small, often mottled or necrotic, with delayed maturity. In a greenhouse, symptoms are transient and new leaves are small with varying deformation, with reduced internode and bushy growth habit. In the field, there may be upward cupping of leaves, and mosaic is more obvious in late infections.


Curative fungicide
Stops infection progress. Infection occurs a few hours or days prior to application.
Cypress canker

Seiridium cardinale

A disease found in many areas around the world. Kills up to 95% of native cypress trees including junipers and cedars.


Cyst nematode



On the top-ten list of plant pathogenic nematodes. An obligate biotroph. A sedenray, endoparasitic nematode. Includes a complex feeding structure in the roots of hosts that supplies the nematode with a long-lasting food source. The female’s body becomes swollen into a cyst to protect eggs. Includes SCN, PCN, and CCN.


A strain of plum pox virus. Causes 15% yield loss. Spreads very quickly. It was confirmed in Pennsylvania in 1999, and was eradicated by burning trees. It was confirmed in Canada in 2000.
Dagger nematode


A migratory, ectoparasitic nematode. Affects raspberry and rose. Transmits grapevine fanleaf virus.




An herbicide that can be used to control doder. Applied to the soil in the spring prior to seed germination.

De-icing salts
Can cause phytotoxicity. Used to lower the melting point of ice on roadways and sidewalks. Sodium chloride is toxic to plants in high amounts when dissolved in water. Sodium impacts nutrient availability of Mg and K by competing for cation exchange sites. Chloride is taken up by plants and can accumulate in leaves where it may reach toxic levels. Salt affects the osmotic potential of soil, so that water flows out of the root instead of in, which can cause burn. Common in the spring after snow melt. Occurs in plants near roads or sidewalks.
Degree days

Measures the accumulation of heat and average daily temperature.

?(((max temp + min temp) / 2) – 10?C)

Demethylation-inhibiting fungicide (DMI)
The most important group of fungicides used for plant diseases. Interfere with sterol biosynthesis in fungi, and not in animals. Potent against ascomycetes and basidiomycetes in low doses. Systemic activity. Risk of fungicide resistance is high. Includes propiconazole, myclobutanil, metconazole, and triticonazole.
Monitoring of health and detection of diseases in plants is crucial for sustainable agriculture. Scouting is the most widely used mechanism. Molecular techniques such as PCR and ELISA require detailed sampling and processing procedure. Early information on crop health and disease detection can facilitate control of diseases. Needs to be rapid, cost-effective, and reliable.
An aspect of IPM. Needs to be accurate.
Diplodia ear rot of corn

Stenocarpella maydis

Looks like white mould, and can cover an entire ear. Can’t spray for it.


Disease-gradient curve

Dispersal curve

The amount of inoculum is greater near the source, and decreases with increasing distance from the source.

Disease progress curve
A mathematical tool used for mathematical models of disease epidemics. Follows the amount of disease in an epidemic over time. Time is on the x-axis, and a quantifiable parameter is on the y-axis. Curves transformed to log to obtain linear regressions provide slopes that represent disease increase. Enables you to know when to treat, if treatment is working, and compare with other years.
Diseases of Herbaceous Perennials
A book for IPM.
Diseases of Trees and Shrubs, 2nd edition
A book for IPM.
An herbicide that causes discolouration of veins in grape, which may be confused with viral disease or nutritional problems.



Devil’s guts

Devil’s hair

Devil’s ringlet





Lone vine



In the family Cuscutaceae. A twining, yellow or orange parasitic plant, sometimes tinged with purple or red, sometimes almost white. Stems are thin and thread-like, or relatively stout. Have haustoria which are modified roots that press up against host stem and penetrate tissue. Flowers are white, pink, or yellowish, small (2 – 4 mm). Fruit is 3 mm in diameter, containing 1 – 4 brown-black seeds. Rarely kills its host, but stunts its growth. Its vascular system becomes continuous with its host’s, and it can transmit viruses between plants that are not closely related.


Dodder dissemination
It can spread quickly and transmit phytoplasma pathogens, and can have plant pathogenic bacteria Rickettsia inhabiting its phloem. Seeds may be spread in irrigation water, livestock manure, or mixed in with crop seed.
Dodder hosts
Parasitizes various plants, especially alfalfa, lespedeza, flax, clover, potatoes, chrysanthemum, dahlia, helenium, Virginia-creeper, trumpet-vine, English ivy, and petunias.
Dodder lifecycle
Seedlings attach to a host after germinating, or they die. The seedling quickly twines around the host’s stem, always counter-clockwise. The plant then loses all connections to the soil, and all water, minerals, and carbohydrates are absorbed from the host through haustoria. Seeds may be dormant for five years.
Dodder management
Hard to control or eradicate. The best practice is to pull and destroy plants before it produces seeds. DCPA and 2,4-D can be used to control it.
Dollar spot

Sclerotinia homoeocarpa

The most common turf disease in the Great Lakes region. Affects Agrostis stolonifera, Poa pratensis, and Lolium perenne.


Double stranded DNA (dsDNA)
There are no plant viruses in this group. Includes herpes virus.
Double stranded RNA (dsRNA)
Some plant viruses and many mycoviruses are in this group.
Dwarf flowering almond
A common ornamental shrub found on University of Guelph campus. May be a host for plum pox virus.
The effective concentration of fungicide where 50% of fungi die. It is increased with repeated fungicide application.
A nematode where the body remains outside of plant tissue while feeding. There is direct penetration of the plant.

A symptom of water excess. Corky, blister-like swellings on the underside of leaves. May be worse during cloudy, overcast periods.


Electronic olfactory system (EOS)
An electronic nose used for environmental odour monitoring. Can detect fireblight. It measures wind speed and direction to determine where odours come from.

Enzyme-linked immunosorbent assay

A way of diagnosing plant viruses, based on specificity of animal-produced antibodies to protein in virus protein coats. The screening test used for HIV. High sensitivity. An antibody is attached to a solid surface, with affinity for the substance of interest, such as hormone, bacteria, or virus. Tubes are filled with antigen solution to be assayed. Any antigen molecules present bind to immobilized antibody molecules. Substrate solution is added after washing away unbound conjugate. The reaction is stopped after a set interval, and concentration of coloured product is measured by a spectrophotometer. Intensity of colour is proportional to concentration of bound antigen. Similar to bacterial ID assay. More reliable than biological assay, and widely used.

A nematode where the whole nematode enters plant tissues while feeding. May have direct penetration or penetration through stoma.
An axis of the plant disease pyramid. Factors include moisture and temperature. Moisture is a dominant factor for diseases caused by oomycetes, bacteria, and nematodes. Temperature affects disease cycles of the pathogen.
The development and spread of a pathogen on a particular plant cultivated over a large area. Often involves monocultures. Slows down or ends when most host plants or tissues are killed by the pathogen, become resistant, are harvested, or unfavourable conditions develop. May be mild or severe, and may develop slowly or rapidly.
The study of epidemics and diseases in populations. The ecology of diseases. The study of spread of diseases in time and space, with the objective of tracing factors that are responsible for, or contribute to epidemic occurrence. Used to forecast disease, help prevent epidemics, predict yield loss, and manage plant disease epidemics. Provides a conceptual outline of the biology of diseases. New tools include PCR, ELISA, DNA Diagnostic Clinic, GIS, GPS, remote sensing, and disease modeling and forecasting. Provides a tool for the refined analysis of traditional and contemporary biological/ecological information on plant diseases. Aids practicioners in design of disease management IPM programs on a regional scale on a continuing basis.

A symptom of cucumber mosaic virus in cucurbits. Downward bending of the petiole and leaf surface, with leaf reduction.


Equipment injury
A form of mechanical injury. Human-inflicted, caused by lawn mowers, weed trimmers, or improper pruning that gouges, rips, or splits. Wounded areas can be infection points of pathogens and insects, causing lasting damage. Avoid by creating barriers around plants. Heavy equipment can cause soil compaction. Excavation can sever root portions, leading to dieback on the same side of the tree as the damage. Young plants may die within a season or two, and older plants may slowly decline over many years.

May cause air pollution phytotoxicity. Can build up in closed production areas such as greenhouses. A gaseous plant hormone that promotes ripening and senescence. May be a byproduct of incomplete combustion in heating systems. Colourless and odourless. Symptoms vary depending on plant species, age, and ethylene concentration and duration of exposure. Symptoms include curled leaves, flower abortion, distorted and twisted stems, leaf and petal abscission, and stunting. May be mistaken for viral infection or herbicide damage. Tomatoes, peppers, and vinca are susceptible. Can occur at concentrations as low as 0.01 – 0.1 ppm, and 1 – 10 ppm causes major damage or plant death.


A fungicide with an LD50 of 1077 mg/kg.




A mathematical model for disease epidemics.

False root-knot nematode

Nacobbus aberrans

On the top-ten list of plant pathogenic nematodes. Found in South America and USA. Migratory with veniform juveniles which move through the root and feed on cells, causing cavities and lesions in root tissues. Juveniles can exit and re-enter the root, causing additional damage. Mature females are sedentary and induce partial dissolution of cell walls and fusion of protoplasts, resulting in specialized syncytia that can be up to 8 mm in size, disrupting the stele. Root galls form around feeding sites. Most eggs are laid in an egg sac.


Fire blight

Sanitation is essential. Equipment for shearing and grafting should be cleaned with alcohol between each use. Wounds are entry points for a disease. Can be detected using an EOS; olfactory fingerprints are determined, and different patterns are found for healthy and sick plants.


Fluorescence imaging
An imaging technique. Using fluorescence microscopy, autofluorescence is observed around lesions induced by infection or wounding. Fluorescence is excited by UV or blue light. Multicolour fluorescence imaging can be used for detection of early hypersensitive reaction to tobacco mosaic virus. Fluorescence images are obtained with a camera. Xenon or halogen lamp is used as a UV light source, and fluorescence at specific wavelengths are recorded using a charge-coupled device (CCD)-based camera system.
Fluorescence spectroscopy
A spectroscopy technique. Fluorescence from the object is measured after excitation with a beam of light, usually UV. For the last twenty years this has been used to monitor stress levels and physiological states of plants. Blue-green fluorescence in the 400 – 600 nm. Chlorophyll fluorescence in the 650 – 800 nm range is produced by green leaves. Can be used to monitor for nutrient deficiencies, environmental conditions based on stress levels, and plant disease.
Inert emulsifiers, carriers, or diluents of a fungicide. May be adjuvants that increase effectiveness, such as wetting agents or penetrants.
A fungicide with an LD50 of 2860 mg/kg.

Low temperature damage that occurs below 0?C. Sustained sub-zero temperatures can kill plants native to tropical regions. Bark can crack, exposing underlying wood, making a tree susceptible to other pathogens including Rhizobacterium radiobacter and insects. Temperatures below -30?C can kill plants. When water freezes it expands tissues and can kill distal portions of the plant. Can cause wilt in July at the first dry period of the year, because there is not enough phloem to support leaves.


Fungal effector
A protein virulence factor used by a pathogen to manipulate host cells. Evolved to counter plant defenses. Secreted by the pathogen, using signal peptide secretion. Can interact with the host in the apoplast or cytoplasm. Requires a method of being taken up to get into the cytoplasm. Include cell wall degrading enzymes, oxoreductase activity, protease activity (degrades plant defenses), reverse transcriptases, and transcriptional regulators (activat host cell death). Triggers host immunity; recognized with receptors, and transcriptional activation of defense leads to controlled cell death. Evolves to overcome plant defenses.

Can cause phytotoxicity. Designed to protect plants from diseases. Inappropriate rates and tank-mix causes problems. Some may have negative-growth regulating effects, stronger in particular species or cultivars, or more severe in hot weather. Fungicides are toxic to animals only in very high doses, but there is concern about long-term effects, and effects when in mixtures. Suppresses, limits, kills, or inhibits growth and/or developemt of fungi. Increases crop yields, lowers food cost, inexpensive for farmers (we pay the real cost), rapid action, predictable level of control, and reduce mycotoxins. Control when there are no effective alternatives. May also kill non-targets, there may be residues and drift, food contamination, environmental contamination, habitat alteration, and biomagnification. May be toxic to humans, livestock, and wildlife. Repeated use kills off most sensitive fungi, and the remaining resistant types reproduce to form the next generation. Creates pest problems including resistance, resurgence, and secondary pests. Includes the active ingredient, and formulants. Classified based on structure, use (preventative, curative), activity (protectant, systemic), mode of action, and site of action. In Canada it is difficult to register new fungicides, and there are only about 15 available, including DMIs.


Fungicide resistance
A random mutation is present prior to application, which is stable and heritable. More frequent with repeated spraying without fungicide rotation. More frequent with pathogens with large reproductive capacity, short life cycle, and long periods of disease-favourable conditions. Cross resistance can occur between fungicides with the same mode of action. To avoid, use tank mixes, label rates, and alternate fungicides.
Gas chromatography mass spectrometry (GC-MS)
A commonly used technique for a qualitative and quantitative analysis of volatile metabolites released by plants in different environmental and physiological conditions.
Gator bag
A bag filled with water at the base of a tree, which waters the tree. It costs $20.


A group of virus species sharing certain common characters.

A statistical model appropriate for estimates across continuous areas. Creates surface maps based on point samples or observations. A map with an area shaded in a colour keyed to a variable. Slight elevation changes can affect plant disease.

A form of mechanical injury. Guy wires, string, trunk wraps, vines, or roots. Pressure or constriction of the bark may collapse phloem tissue, and sometimes xylem tissue. Fewer photosynthates can travel to the root system, leading to development of a smaller root system with less water and nutrient uptake potential. Trees are smaller, have poor leaf emergence, and are less vigorous. Recovery is difficult.



Geographic information system

A new tool for epidemiology. A computer system capable of assembling, storing, manipulating, and displaying data referenced by geographic coordinates. Relates data collected by GPS to other sources of geo-referenced information. Can integrate spatial information and uncover possible relationships. May make vector and raster maps.

A mathematical model for disease epidemics. Appropriate for polycyclic diseases as an alternative to logistic models. Has an absolute rate curve that reaches a maximum more quickly and declines more gradually than logistic models.
Sampled populations of Microdochium nivale from BC and Ontario: 10 locations with 50 – 100 isolates per location. Assessed sensitivity to dicarboximide, DMI, and SDHI fungicides in the lab (strip agar tests) and field. Tested for resistance-associated fitness costs, and sequenced genomes of sensitive and resistant isolates to look for polymorphisms. Biomass fitness tests were inconclusive due to large variation between technical replicates. There is some difference between how temperature affects resistant and sensitive isolates. Insensitivity in the lab may not translate to control failure in the field. Future testing will involve pathogenicity, virulence, reproduction with spore tests, and comparative genomics with RNA and DNA sequencing, INDELs, frameshifts, polymorphisms, and expression differences.

Global positioning system

A new tool for epidemiology. Determines location data using satellites.

Graduate studies
Doesn’t pay very much because it is school as well as a job. You get credentials, participate in research, and may be published.
Can transmit viruses between plants that are closely related, either from the rootstock or scion.
Grapevine fanleaf virus (GFLV)

A nepovirus that is transmitted by dagger nematode. One of the most important viruses of grape. Particles attach to the cuticular lining and lumen of the odontophore and pharynx of the nematode. It can be retained on a nematode for four yeras, but is not retained when it moults, and is not passed to offspring.


Green peach aphid
Transmits 27% of plum pox virus. Peach is its preferred host.
Hand lens



Pocket lens

Can be 10x to 16x magnification. Made by Boreal Labs, Efston Science, and BioQuip. Used to look at tiny insects and symptoms.


Harvest Gold

A variety of apple with bright yellow, 1 cm fruits. Fruits into early winter. Some resistance to apple scab. White flowers.


Can cause phytotoxicity. Used to control weeds. May be non-selective or selective, and have varying modes of action including growth hormone regulation, amino acid biosynthesis inhibitor, lipid biosynthesis inhibitor, membrane disruptor, and respiration inhibitor. May affect photosynthesis and growth, and cause root stunting or swelling, which could be confused with nematodes. May cause necrotic or chlorotic blotches or spots, mottled colour, and vein distortion or banding that could be confused with fungal or viral disease. May have long residual effect in the soil, causing damage to subsequent crops, depending on weather and soil factors. Herbicides may drift on the wind from neighboring properties during application. Confirmed with soil or plant tissue analysis. Includes 2,4-D and diuron.


High temperature damage
Plants adapted to cooler climates are sensitive. Results in physiological changes: roots and shoots may stop growing, roots may die, abnormal colour and growth habits, such as bleaching. Coupled with low soil moisture, there may be scorching, premature leaf drop, or plant death. May cause poor pollination.
It is diagnosed using ELISA. It is difficult to cure because it integrates its DNA into host DNA.
Killing a person.
An axis of the plant disease pyramid. Factors include genetic resistance (horizontal or vertical), degree of genetic uniformity, type of crop (annual/perennial, foliar/root, woody/herbaceous), and age. Some crops are susceptible only during growth periods, and become resistant when mature.
Horizontal resistance
Low level of resistance to many diseases.
Huanglongbing disease

A disease that affects citrus. May be detected using VOC analysis.


Plants are susceptible to oomycetes.
Hypersensitive response
A response that plants may have to a virus or bacteria. Cell death around the infection point. Prevents systemic spread of the pathogen and eliminates local infections. Causes necrotic lesions. The plant produces reactive oxygen species, inducing apoptosis.
Hyperspectral imagin

Multispectral imaging

An imaging technique. Gaining interest for its application in precision agriculture. Each pixel is acquired for a range of wavelengths in the electromagnetic spectra. Similar to multispectral imaging, but has a broader range of wavelengths being scanned for each pixel. May be used to detect citrus canker. A hyperspectral camera may be embedded on a drone. Cost of acquiring hyperspectral images is high for specific crops in specific climates. Remote sensing use is increasing. May be used to map varieties in a vineyard and develop early warning systems for disease outbreak.

Ice storm of 2013
Its effects are still being felt. Affects may be confused for verticillium wilt. It killed many trees infected with emerald ash borer. Impacted even hardy species.
Immigration marker
New polymorphisms in fungi. Detected with DNA technology.
Infrared spectroscopy
A spectroscopy technique. Used for disease detectioin in plants in combination with visible spectroscopy.
Infrared thermography
An imaging technique. Not cost effective.
Initial inoculum (X0)
A factor of monocyclic and polycyclic disease models. If reduced, it decreases disease incidence. Depends on inoculum from previous crops in the field, and inoculum from crops in adjacent fields. Affected by destroying infected plant debris, removing diseased plants, chemical seed treatments, protective fungicides, race-specific disease resistance, and biological control agents targeted at initial inoculum.

Can cause phytotoxicity. Designed to protect plants from arthropod pests. Inappropriate rates and tank-mix causes problems. Combining captan or sulphur with certian oils can cause damage, especially in hot weather.


Integrated pest management (IPM)
A decision making process that uses all available tools to reduce a pest population to an acceptable level, in a cost-effective, environmentally rational manner. Includes pest prevention, regular monitoring, diagnosis, action thresholds, management methods, and evaluation. Common problems are light, water, air, and heat units.
International Code of Virus Classification and Nomenclature
The favoured form of nomenclature for viruses. Names of orders and families are italicized. Families and genera are classified by shape, type of nucleic acid, and protein coat. Species name is not italicized.
Interveinal chlorosis

A symptom of iron deficiency and manganese deficiency. Occurs in high pH soils. Treated with chelated Fe or Mn, or acidifying water.


A dicarboximide fungicide. Disrupts osmotic signal transduction, causing problems in water uptake. Five genes are associated with resistance. Works for several species. There are two verified cases of resistance in Microdochium nivale. Found that there were resistant variants in BC, which were also resistant to propiconazole: these had reduced fitness cost.
Iron (Fe)
A micronutrient. Non-mobile, and has high availability in low soil pH and low availability in high pH. A key component in the production of chlorophyll. Most soils have adequate amounts.
Iron deficiency

Iron chlorosis

A problem in calcareous soils. Prevalent in some areas of the USA. Leaves are a normal size and shape, but have interveinal chlorosis, yellow-green, often with a striped appearance. Symptoms develop in new leaves first. May be corrected by lowering soil pH below 7. May occur when soil oxygen levels are low, such as compacted soil. It can be caused by microbial activity, and may be reduced in low temperature, low light, or moisture extremes. Common in potted plants.


Label rate
A method of managing fungicide resistance. Always use the label rate. Lower or higher rates both cause resistance problems.


A variety of apple with gold, 1 cm fruits. Fruits into fall. Good resistance to apple scab. White flowers. Dwarf with upright spreading habit.

Lesion nematode


A migratory, endoparasitic nematode. Affects peanut.



An odd growth that often has a cluster of buds that sprout in the spring. Gnarled growth, but doesn’t impact the health of the tree.


Little cherry virus

Symptoms include small cherry fruits.


A mathematical model for disease epidemics. Appropriate for polycyclic diseases. Most widely used for describing epidemics.
Low temperature damage

Symptoms are caused by ice crystals that form in plant cells, causing damage to cell membranes and organelles. May cause water deficiency. Newly expanded shoots are more susceptible. Combined with freezing rain, can cause ice accumulation on limbs, which may cause mechanical damage. May be mistaken for an infectious root disease, chemical injury, or gas injury. It may be necessary to remove damaged plant parts. Includes chilling injury and freezing.


A strain of plum pox virus. Causes losses greater than 15%. Doesn’t spread as quickly as strain D.
Magnesium (Mg)
A secondary macronutrient. Mobile, and can leach if Ca is not present. An important component of chlorophyll, and is a cofactor in the production of ATP. Fertilizers include foliar and granular applications of Epsom salt.
Magnesium deficiency

A problem in alkaline soils, and in acidic sandy soils where magnesium is leached away. Symptoms include reduced photosynthesis, yellowing of leaf margins, early leaf senescence, and interveinal chlorosis and necrosis with an orange, red, or brownish colour. May be mistaken for a potassium deficiency or viral disease. Symptoms appear in older leaves first. May be caused by over-application of K or Ca, causing competition for cation exchange sites. Common in field plants.


Maize fine stripe virus

Symptoms include yellow stripes.


Malus baccata


A variety of apple with small, red, orange, or yellow fruit. Fruits into late fall. Very resistant to apple scab. White flowers, with various habit.

Malus sargentii


A variety of apple with small, bright red fruits. Fruits into late fall. Good resistance to apple scab. Red buds open to white flowers. Rounded form.

Manganese (Mn)
A micronutrient. Non-mobile, and is absorbed via leaves. Has low availability in high soil pH.
A mnemonic to help remember that Iron and Manganese have low availability in high pH soils.
Mathematical model
Used to describe and summarize epidemics. Provides an understanding of spatial-temporal dynamics. Describe disease progress in quantitative terms. Requires a measurable variable, something that can be enumerated or measured, and changes with time. Variables may be propagules, number or portion of plants infected, portion of tissue area with symptoms, enzyme activity, growth parameters, discolouration, or yield loss. Allows for disease forecasting. When all measurable components are considered, it can assess virulence of initial inoculum, effect of the environment, crop disease resistance, crop growth rate, length of time plant and pathogen interact, and effectiveness of management strategies. May use disease progress curves, AUDPC, and computer simulations such as MYCOS, PLASMO, and EPIDEMIC. Often models are limited to specific climates and regions. Some are better than others. May be refined over time when additional data is included. Simplifies reality to provide an estimate of a relationship. Includes monomolecular, exponential, logistic, and Gompertz models.
Mechanical injury (abiotic disorder)

May be caused by storms, misuse of equipment, or animal activity. Linked to a specific event. Symptoms  include broken limbs, flattened tree tops, and torn bark. Symptoms may be subtle when roots are affected. Open wounds may be entry points for pathogens, which can lead to slow decline and death.


Mechanical injury (nematode)
A way that nematodes can cause damage to plants. Penetration and movement through tissues, damaging tissues and creating entry sites for other microorganisms. Pierce plant cells, often leaving a yellow spot.
Mechanical transmission
Can transmit viruses. Often host cells need to be damaged, or injected into.
Meloidogyne graminis
A root knot nematode. Can reproduce only on grasses.
Meloidogyne javanica

A root knot nematode. Sweet pepper is less susceptible to it.


It was used as a control method for nematodes, until it was discovered to be toxic to humans.
Mercury chloride
A fungicide with an LD50 of 100 mg/kg.


A DMI registered for use on dollar spot. Introduced in 2012.

Methyl bromide
A fumigant that may be used to control root knot nematode. It is being phased out.
Microdochium nivale

A fungal pathogen which attacks cereals including wheat (Triticum aestevum), and barley (Hordeum vulgare), and cool season grasses including bentgrass (Agrostis sp.) and bluegrass (Poa sp.). Asexual reproduction is rare.


A nematode that can move around throughout its life. It cannot move very far. Can vector viruses.
Migratory ectoparasite
Nematodes which never enter the host, and migrate through soil using roots as an ephemeral food source as they encounter them.
Migratory endoparasite
Nematodes which enter the host, and migrate through tissues causing extensive damage.
Mode of action
A way that fungicides may be classified. Relevant to resistance development. How it affects metabolic processes. Includes respiration inhibitors, synthesis inhibitors, regulation disruptors, and single- and multi-site.
Moisture extremes
An abiotic disorder. Water needs vary greatly based on species and environment. Water deficiency or excess can cause injury to plants. Injury may be short-lived, and plants can recover, but as duration increases, likelihood of recovery decreases. Diagnosis can be difficult. Symptoms may be confused with salt or herbicide damage, root rot, or vascular wilt.
Molybdenum (Mo)
A micronutrient. Non-mobile, and is important for legumes.
Monocyclic disease

X = X0RT

X = Disease intensity

X0 = Initial inoculum

R = Rate of epidemic once it starts

T = Time or duration of epidemic.

The disease progress curve is linear in early phase, with a slope of R. To reduce disease incidence, reduce initial inoculum, rate of infection, or duration of epidemic. Typical R values  are 0.02/day. There is initial inoculum only, and management focuses on reducing amount or efficacy of initial inoculum. Low reproduction and death rate. Often soil-borne. Includes root diseases and wilt diseases.


Negative exponential model

A mathematical model for disease epidemics.


A symptom of many viruses. Lesions are bordered by veins. Occurs when the virus can travel through plasmodesmata, but cannot get into vasculature.


A way of differentiating nematodes. Different structure depending on if the nematode is a fungal feeder, plant feeder, predator, or omnivore.
Multi-site mode of action
A mode of action of fungicides. Resistance is rare.
Mummy berry

Monilinia vaccinii-corymbosi




A DMI registered for use on dollar spot. Introduced in 2000. LD50 is 2100 mg/kg.

A virus that affects fungi.
Needle necrosis

A symptom of water deficiency in conifers.


Needle nematode

A migratory, ectoparasitic nematode.


Negative sense single stranded RNA (ssRNA-)
A virus where some or all genes are translated into protein from an RNA strand. There are some plent viruses in this group. Includes rabies.
Nema wool

Eelworm wool

The overwintering structure of nematodes. Forms at the end of a growth period. An anhydrobiotic structure, formed of millions of nematodes in a dormant state. Reactivated with moisture, and is a potential source of infestation for subsequent crops. Can survive for hundreds of years, so crop rotations may be ineffective.



The largest plant pathogen, just on the edge of eyesight. In the kingdom Animalia, in the phylum Nematoda. Over 6,500 species. Worm-shaped in at least part of its life cycle, 0.3 – 3.0 mm in length, with a round cross section, smooth and unsegmented, without appendages. Some females may become swollen. There is a thick cuticle and egg wall. No skeleton, respiratory system, or circulatory system. May be differentiated by tail shape, stylet, and mouthparts. May have sexual reproduction or parthenogenesis. Females are larger than males, and are usually parasitic. Almost all plants are attacked by nematodes. There can be up to 50,000 nematodes in one plant. Can cause 10% crop loss. Can migrate up wet stems and leaves. May be an ectoparasite, endoparasite, semi-endoparasite, sedendary, migratory, or various combinations. Inoculume is infested soil, nema wool, plant debris, cloves and bulbs, seeds, or weed hosts. Disseminated by movement of inoculum, irrigation or rainwater, equipment and tools, or human movement. Optimum conditions are 15 – 20?C, moist environments with oxygen. Live mostly in soil, but move through soil when conditions in plant tissue are unfavourable. Detected with Baermann funnel, sieving, or centrifuge methods. Some are saprophytic, not pathogens.

Nematode life cycle
A nematode hatches from an egg and has four larval stages. Moulting between stages increases size and sheds the cuticle. The first moult occurs within the egg, which hatches about two days after oviposition. The most common stage for infection is juvenile stage 2 (J2). When conditions are wet in the spring, eggs hatch, or the fourth juvenile stage is activated, and they swim towards plants, attracted by plant chemicals. Females oviposit 4 days after their final moult, laying 8 – 10 eggs every day. One female can lay 500 eggs in her lifetime. Live for 40 – 75 days. The entire lifecycle takes 19 – 23 days in optimum conditions and can produce 4 – 10 generations per season, except in cold weather. Overwinters as nema wool.
Nematode management
Two or more control methods are usually necessary. Nematodes may be managed by exclusion, using nematode free plants and material, sanitation, rotation with poor hosts or non-host species, suppressive cover crops such as Brassica (produce natural nematicides), green manure, or fallow. Plant resistance is mainly to root knot nematode. Improve plant health by adding water, fertilizer, and amendments. Most chemicals to control them such as nematicides, methyl bromide, mercury, and other chemicals, are banned. A few successful biological control measures have been found, including bacteria such as Pasteuria penetrans, and nematode-trapping fungi.
Nematode symptoms

Symptoms are caused by mechanical injury and cellular and physiological changes in the host. May interact with other pathogens, transmit viruses, and increase susceptibility of the host to environmental stresses. Aboveground symptoms include stunting, decline, chlorosis, reduced yield and quality, wilting, nutrient deficiencies, deformed tissue, and death. May cause angular spots on leaves; damage is in distinct zones. Roots may have galls, enlarged tips, lesions, proliferation of lateral roots, and reduced growth. Symptoms are usually caused by large numbers of nematodes, and the more nematodes the more numerous and larger the galls. A field may have a spotty, irregular patch usually oval shaped, or along a row, due to the way nematodes migrate. Damage is influenced by how the field was initially infected, and is more serious in humid climates. Often damage occurs in a dip in the field. Wilting is seen later in the season when there is water stress. Once you see symptoms, nematodes may have already left the plant.


Niagara Quarantine Zone
An area in southern Ontario where plum pox virus is present, and quarantined. Air from Lake Ontario makes the climate very mild and warm. Produces 80% of Canada’s stone fruits. Nursery propagation must occur outside the zone, subjecting the trees to winter injury.
A drug with an LD50 of 53 mg/kg.
Nitrogen (N)
A primary macronutrient. Mobile, and easily leached. Use of N fertilizer is higher than any other nutrient. Used in the production of chlorophyll. May be absorbed in two ionic forms, NO3 and NH4. Fertilizers include synthetic fertilizers, cover crops, and compost.
Nitrogen deficiency

A major limitation in non-leguminous plants. Symptoms are a pale yellow colour, plant stunting, and poor vigour. Symptoms are observed in older leaves first. May result from an infection of root pathogens such as root knot nematode. Cause increased susceptibility to Alternaria solani.


Nitrogen toxicity

A nutrient toxicity. Typical in hot, dry conditions. Plants are an overly deep shade of green. Lesions occur on stems of annual seedlings, and can be confused with a canker disease. Tomato plants twist and distort, and may appear similar to a viral disease. Can be a problem in greenhouses due to lack of microorganisms that convert ammonium to nitrate. Cause increased susceptibility to Botrytis cinerea and Rhizoctonia solani.


Non-persistent transmission
Virus transmission on insects where the virus becomes attached to the distal tip of the stylet. The virus does not circulate within the insect. It is acquired in seconds during a test-probe, and is carried on the mouthparts to the next plant. The insect can transmit the virus only to the first few plants it feeds on after acquiring the virus. Retained for 2 – 3 hours. Spread is favoured by winged species that move through the field or orchard in search of host plants. Insecticides do not reduce spread, because new insects come in after application.
Northern corn leaf blight

Exserohilum tucicum

Symptoms occur before tasselling.


Northern root knot nematode

Meloidogyne hapla

Produces galls less than half the size of M. incognita. Cannot reproduce on grasses.


Nuclear magnetic resonance spectroscopy (NMR)
A spectroscopy technique. Not cost effective. Uses the magnetic properties of certain atomic nuclei to determine the physical and chemical properties of atoms or molecules in which they are contained.
Nutrient deficiency
Results from lack of plant nutrients in the soil, or pool soil conditions such as pH. There is a lack of pathogen signs, and relatively uniform distribution in the field. Best diagnosed with plant tissue analysis; allows determination of plant nutrient uptake rather than availability in the soil. May be mistaken for viral diseases. Mobile nutrients show symptoms in older leaves, and immobile nutrients show symptoms in younger leaves.
Nutrient toxicity

Nutrient excess

Damage from excessive nutrient levels. May occur from over-application of fertilizers or manures. Common in many production systems, especially greenhouse floriculture. Symptoms often include chlorosis or necrosis on leaf margins or tips. Leaf spotting, flecking, and other symptoms can occur. May be triggered by extreme soil pH. May occur when irrigation water or soil has concentrations of micronutrients.

Oak wilt

Ceratocystis fagacearum

Tyloses block pathogen spread through root grafts. Affects red oak.


Odd growths
May be confused with a disease. Bumpy bark, burls, lignotubers, and other growths that can look like a disease, but they are usually caused by injury.
Oral LD50
The oral dose where 50% of test subjects die. Usually tested on small animals such as mice or rats. A value less than 500 mg/kg is very toxic. A value of 500 – 5,000 mg/kg is moderately toxic. A value of 5,000 – 15,000 mg/kg is slightly toxic.
Oxidative burst
May be triggered by PAMPs. Leads to plant cell death and pathogen death.

May cause air pollution phytotoxicity. Produced when compounds of combustion emissions react with oxygen and sunlight. Forms at the ground level. Symptoms include flecking, bronzing, chlorosis, and necrosis. Can cause needle-banding and tip-burn in conifers. May be mistaken as a disease or mite infection.


A multifunctional protein with sequence that overlaps with 35S of cauliflower mosaic virus. Not all functions are known. There is some concern that expression may have unforseen consequences in transgenic organisms, such as allergies.
An epidemic that is widespread, and/or has a high level of disease.
A virus that replicates through transverse replication like a retrovirus, but the viral particle contains DNA instead of RNA.
A Phytophthora-specific selective medium. Used for diagnosis of plant diseases.
Asexual, monosexual reproduction of nematodes. The female produces eggs without fertilization from a male. Many root knot nematodes are parthenogenic. Males may be rare or absent.
Pasteuria penetrans
A bacteria which may be introduced to the soil as a biocontrol agent for nematodes. The spores adhere to the cuticle of the nematode. Produces penetration tubes that penetrate the nematode. The cuticle grows over the bacteria once inside. There may be up to 2 million spores on one nematode. The nematode usually survives, but has lower reproduction. A lot of bacteria may be needed for this to be effective.
An axis of the plant disease pyramid. Factors include level of virulence, quality of primary inoculum, reproduction (monocyclic, polycyclic, polyetic), ecology (foliar, root, surface), and mode of dissemination (insect, wind, water).
Pathogen associated molecular pattern (PAMP)
A molecular tag found on pathogens. Triggers host immunity. Recognized by plant receptors, activating defense genes, resulting in oxidative burst.
Pathogen isolation
Symptomatic leaves are surface sterilized with ethanol and hypochlorite, then grown on antibiotic media such as streptomycin and tetracycline.
A way that plant viruses are diagnosed. Biological indexing and biological assays.
Peanut pod nematode

Ditylenchus africanus

Affects peanuts in South Africa.

A succinate dehydrogenase inhibitor fungicide. Inhibits ATP synthesis. Its mode of action is disruption of respiration. Four genes are associated with resistance. There are no cases of resistance in Microdochium nivale.
Pepper mild mottle virus (PMMoV)

A plant virus which may be transmitted to humans. Causes fever, abdominal pains, and pruritus.


The region surrounding the vulva and anus of a root knot nematode. May be used to distinguish Meloidogyne species. There are patterns and ridges characteristic to the species.
Persistent transmission
Virus transmission on insects where the virus circulates in the insect’s body and enters the foregut of the insect. Can replicate in both host and insect. The insect can transmit the virus to every plant it feeds on after acquring the virus, for the rest of its life.
Pest prevention
An aspect of IPM. Includes sanitation, exclusion, and the environment.
Kills a pest.
Timing of flowering, bud break, and fruiting of common ornamental plants.
Phenylmercuric acetate
A fungicide with an LD50 of 30 mg/kg.
Phomopsis seed mould

Phomopsis longicolla

Affects soybeans in warm, wet conditions. Infects through wounds. Harvest as soon as possible. Reduces seed grade, depending on amount damaged.


Phosphorus (P)
A primary macronutrient. Mobile, and has low availability in low or high soil pH. Typically not mobile in soils. Cool environmental conditions and poor oxygen availability can make it difficult for plants to absorb and translate. Soil type and biology affects availability. Used for photosynthesis, ATP, and DNA. Important for flowering and seed production. Fertilizers include commercial fertilizers, natural rock minerals, and animal residues.
Phosphorus deficiency

Can be a serious problem in some plants. Plants have weaker stems, which can result in lodging in grain crops. There is poor growth, stunting, blue/green hinge to leaves and/or purple colouration in stems and underside of leaves.


Physiological changes
A way that nematodes cause damage to plants. The host has reduced uptake and flow of water and nutrients from the roots, and reduced flow of photosynthates from leaves to roots. Can cause wilting later in the season when there is water stress. Nematodes steal nutrients from the plant.
Chemical injury. Includes herbicides, insecticides, fungicides, plant growth regulators, de-icing salts, and air pollution. Caused by use of inappropriate products, over-application, or certain product mixtures. Always test chemicals on a small scale first. Chemicals with copper, such as fungicides and insecticides, can cause tissue bronzing. The label on chemicals will advise about proper usage, rates, and mixture incompatibilities. Chemicals should be used to prevent disease.
Pine wilt nematode

Bursaphelenchus xylophilus

In the family Parasitaphelenchidae, in Clade 10. On the top-ten list of plant pathogenic nematodes. Native to North America, distributed in the Midwest of the USA. Introduced to Portugal, Spain, Japan, China, Korea, and Taiwan where it causes enormous damage. Causes loss of 2 x 106 cubic metres of timbre in some conditions. A migratory, endoparasitic, insect-vectored nematode, 1 mm in length. Feeds on blue-stain fungi that live in dead wood. It destroys pine resin canals, causing clogged water and resin flow. Can kill a tree in a few weeks or months. All infected trees can die when conditions are right.

Pine wilt nematode hosts
Causes little damage to trees native to North America, and profound damage on non-native species. Affects mostly Scots pine, but also Austrian (Pinus nigra), jack, (P. banksiana), mugo (P. mugo), red (P. resinosa), and white pine (P. strobus), as well as spruces, firs, hemlocks and some junipers. Japanese black (P. thunbergiana) and Japanese red (P. densiflora) are particularly susceptible. Only affects trees over 10 years old, so Christmas trees tend to be safe. In resistant hosts, pine wilt nematode dies before it can feed and multiply.
Pine wilt nematode lifecycle
Has the shortest life cycle of any plant parasitic nematode, lasting 4 – 5 days. It is attracted to the pupae of sawyer beetle, and the adult transmits the nematode. Enters feeding wounds made by sawyer beetle in healthy trees, and feeds and multiplies in resin channels. Can spread through a tree rapidly. May be transmitted to dying trees, where it feeds on blue-stain fungi.
Pine wilt nematode management
Dead pines should be cut promptly and burned, burried, or chipped, and should not be saved as firewood because beetles can emerge from logs. Don’t pile chips near trees. Plant resistant trees.
Pine wilt nematode symptoms

Needles turn greyish green, then tan to brown. Resin flow from wood causes decline. Needles remain dead on the tree for a year or more. Infection may start with scattered branches or affect the tree all at once.



Plant disease pyramid
Axes are: host, pathogen, environment, and time. Disease in a population of host plants follows a pattern of increase and decrease over time and space. Interactions of pathogen populations with host populations, under influence of the environment. Humans can have major influences on disease patterns.
Plant growth regulator (PGR)
Can cause phytotoxicity. A chemical that alters the physiology of plants, affecting flowering, elongation, root growth, or other functions. Used to produce fruit crops and ornamentals. Can reduce vegetative growth, thin fruit, improve fruit quality, reduce shoot elongation, make plants stronger and more compact, or reduce seed production. May be used to reduce mowing frequency in turfgrass. Causes phytotoxicity if used at inappropriate rate, time, or environmental conditions. Can cause long-lasting discolouration, stunted growth, or other symptoms. Includes chlormequat chloride.
Plant parasitic nematode
There are over 4,100 species. Represent an important constraint on delivery of global food security. Cause loss of $80 billion US each year. Many growers are unaware of them. Small, soil-borne pathogens that cause nonspecific symptoms. Have hollow, protusible styles that penetrate plant cells and allow feeding, and sometimes entry into the host. Difficult to study because they are difficult to culture.
Plant to human virus transmission
Very rare and unlikely. The University of the Mediterranean in Marseille, France found slight evidence that pepper mild mottle virus may infect humans.
Pleiotropic effects
Includes resistance-associated fitness costs. Genes rarely have one function so a mutation which causes fungicide resistance may also affect reproduction (spores may not germinate), virulence (less damaging), or growth. Important for resistance management. Sensitive variants will out-compete resistant variants in the absence of the fungicide.
Plum pox virus (PPV)

Plum pox potyvirus


A regulated and quarantined plant disease. The most serious disease in stone fruits. There are different strains, which affect different hosts and cause different levels of damage. Strains include: D, M, C (cherry), CR (cherry Russian), EA, W, Rec, An (ancestral M), and T (Turkey). Laboratory tests are required to distinguish strains. In 1999, the USDA found PPV in Pennsylvania and in Michigan in 2006, but declared it eradicated in 2009. Canada suspended imports from the USA, but detected it in 2000 in Ontario and Nova Scotia, in trees imported from Pennsylvania. An eradication program was implemented, but was given up in 2011. In New York it was found at sites from 2006 to 2015, probably spreading in from Canada.

Plum pox virus eradication
Eradication requires removal of all infected trees, destroying a $24 – 30 million/year industry in Ontario. There were political pressures to end the eradication program, and manage disease. A quarantine is established in the Niagara Quarantine Zone. Elsewhere there was eradication, and some growers lost entire productions for several years. Some orchards planted in 2007 with certified material still had the disease. The program was ended in 2011 due to costs. However, CFIA will continue to monitor outside of the quarantine zone. Resistant varieties will be bred.
Plum pox virus hosts
Affects stone fruits of the genus Prunus including peaches, nectarines, plums, apricots, almonds, cherry, purple leaf sand cherry, dwarf flowering almond, American plum, and Canada plum.
Plum pox virus management
Plant tested virus-free certified nursery stock. Scout and inspect orchards. Educate and train workers to identify it, and remove and dispose of diseased trees immediately. Manage aphids. Manage alternative virus reservoirs. Select new sites away from disease sources. Plant tolerant and resistant varieties.
Plum pox virus symptoms

Symptoms appear several years after infectioin, and are not evenly distributed in the tree. May disappear during extended periods of temperatures over 30?C. Leaves have chlorotic rings, mottling, vein clearing, chlorotic blotching, and appear small and deformed. Often symptoms appear on basal leaves of current year’s growth. Does not kill the tree, but reduces vigour and fruit yield. Fruits fall prematurely and are unmarketable.


Plum pox virus symptoms in apricot

Deformed, unmarketable fruit with low sugar content, poor taste, and gummy texture. Pock marks on the pit. Premature fruit fall. There can be 80 – 100% yield loss.


Plum pox virus symptoms in nectarine

Rings and blotches on fruits.


Plum pox virus symptoms in peach

Rings, lines, blotches, and spots on fruits.


Plum pox virus symptoms in plum

Deformed, unmarketable fruit with low sugar content, poor taste, and gummy texture. Premature fruit fall. Can cause 80 – 100% yield loss.


Plum pox virus transmission
Transmitted in infected propagation material such as budwood, nursery stock, and rootstock. Transmitted by aphids with non-persistent transmission, usually by migrant aphids in the spring. About 30,000 aphids visit a tree in a year.
Polycyclic disease

X = X0ert

X = final amount of disease

X0 = initial inoculum

e = base of the natural logarithm

r = rate of an epidemic once it starts

t = time or duration of epidemic

The disease progress curve is a sigmoid curve. An exponential model during early phase. Rate of increase is proportional to the amount of disease at any one time. Absolute rate of disease increase is proportional to disease intensity. To reduce incidence, reduce initial inoculum, rate of infection, or duration of epidemic. High reproduction and death rate. Wind-borne vectors. Includes leaf rust, leaf blight, powdery mildew, tobacco mosaic virus, and late blight. Control by stopping pathogen reproduction and using resistant varieties. Typical r values are 0.3 – 0.5/day. Several to many generations occur in a growing season. Secondary inoculum is produced and causes additional infections. Management focuses on reduction of primary inoculum (hard due to explosive growth), and reducing the rate of increase.

Polyetic cycle

Multi-year cycle

Amount of inoculum increases from year to year. Pathogen can cause severe outbreaks over several years. Monocyclic or polycyclic within each growing season. Inoculum produced in one season can be carried over to the next, leading to a buildup of inoculum over the years. In the tropics there are no breaks between growing seasons, and can lead to accumulation of inoculum. Includes Dutch elm disease (monocyclic), and apple powdery mildew (polycylic).

Positive sense single stranded RNA (ssRNA+)
Most plant viruses are in this group. Includes SARS and coronavirus.
Potassium (K)
A primary macronutrient. Mobile, and affects fruit quality. Plays a key role in cellular signalling, growth regulation, and photosynthesis. Soil levels vary significantly based on parental material and weathering. Availability is reduced by cations such as Ca2+ and NH4+. May be leached from sandy soils. Uptake is reduced by temperature, soil moisture, and oxygen availability. Fertilizers include conventional fertilizers and rock phosphate (potash).
Potassium deficiency

Symptoms show first in older leaves. Potassium is important for fruit quality, and is important in fruit and vegetable production. Necrosis on leaf margins, leaf curling and browning, and interveinal chlorosis. Increases susceptibility to frost damage and certain diseases.


Potato cyst nematode (PCN)

Globodera pallida

G. rostochiensis

Causes 9% of losses in potato production worldwide.


Potato leaf roll virus

Symptoms include reduced growth and leaf rolling.


Potato rot nematode

Ditylenchus destructor

Affects potato tubers mainly in Europe.


Potato scab

Streptomyces scabies

Has spiral hyphae. Favoured in acidic soil. No food risk. Affects most root crops.


A variety of apple with dark red, 1.3 cm fruits. Fruiting is very persistent. Resistant to apple scab. Dark red flowers. Rounded form.
Preventative fungicide
Prevent infection and establishment of the pathogen. All fungicides are preventative to some degree.
A variable which may be used in a mathematical model of a disease epidemic. For bacteria it includes cells or colonies. For fungi it may be zoospores or sclerotia. For viruses it may be populations in vectors or populations of vectors. For nematodes it may be plant cysts or eggs.


A DMI fungicide registered for use on dollar spot. Introduced in 1994. LD50 is 1300 mg/kg. An older fungicide. Its mode of action is disruption of ergosterol synthesis. Its site of action is C14-demethylase. Dozens of genes are associated with resistance. There are no cases of resistance in Microdochium nivale. Found that there were resistant variants in BC, which were also resistant to iprodione: these had reduced fitness cost.

Protectant fungicides
Coat the plant and protect against pathogen entry. Present prior to infection. Doesn’t enter plant cells. Protects new growth. Duration of activity is limited by the environment. Includes chlorothalonil.
Publication #841

Guide to Nursery and Landscape Plant Production and IPM

Available as a PDF online. Has information on plant phenology.

Purple sand cherry
A common ornamental shrub with dark purple leaves and white flowers. May be a host for plum pox virus.
A fungicide with an LD50 of 1700 mg/kg.
Doesn’t exist unless resistant varieties exist.
Race T
A race of southern corn leaf blight. Infectious to corn with Texas male sterile cytoplasm. Caused an epidemic in 1970.
A map made with GIS with data organized as a matrix of numerical values, referenced spatially by row and column position.
Rate (R)
A factor of monocyclic and polycyclic disease models. If reduced, it decreases disease incidence. Depends on reproductive potential of the pathogen, virulence of the pathogen, susceptibility of the host, and conduciveness of the environment. Affected by non-specific disease resistance, systemic fungicides, cultural practices that alter the environment, and removal of diseased plants.


A strain of plum pox virus discovered in 2010. A combination of strains D and M. Evolved when both strains infected the same plant. Found in a tree growing in a residential site near Grimsby. Found again in 2013.

Killing a king.
Regular monitoring
An aspect of IPM. Knowledgeable scouts, good records, and information for decision making is important. Monitoring data can lead to modifying pesticide use, reducing applications, and switching to reduced risk, pest-specific products.
Regulation disruption
A mode of action of fungicides. Mimics hormones. Can regulate animal hormones, and may affect people.
Reniform nematode

Rotylenchulus reniformis

On the top-ten list of plant pathogenic nematodes. Sedentary, semi-endoparasitic. Induce a feeding structure within the host in its sedentary stage. Affects a large number of annual and perennial plants. There are two races in India. Race A can reproduce on cowpea, cotton, and castor.


Resistance management
Tries to reduce the number of resistant variants in a pathogen population. Alternate or mix fungicides with different modes of action, minimizing applications of fungicides with resistance development, using label rates, different crop cultivars, and using other control methods such as cultural, genetic, or biological methods. Monitoring is important: easier if you catch it early.
Resistance-related fitness cost
A pleiotropic effect. A fungus with resistance to a fungicide often has slower growth, less reproductive success, or is more sensitive to something else. Speed of development of resistance depends on this cost. The fungus must find an alternative metabolism pathway to the one that the fungicide blocks, and this pathway could be less efficient. Most strains of a fungus are highly sensitive to fungicides.
Reverse-transcribing virus
Has dsDNA or ssDNA genomes, and replication includes synthesis of DNA from RNA by the enzyme reverse transcriptase. Many integrate into their host genomes. There is one family of plant viruses in this group. Includes HIV.
Rice-stem nematode

Ditylenchus angustus

Affects rice by feeding ectoparasitically on meristematic tissues of stems and leaves in deep-water and lowland rice. Occurs in India and southeastern Asia.


An RNA strand of cucumber mosaic virus. Encodes the protein 1a, used for replication of viral genome. Has methyltransferase activity for adding 5′ caps, and helicase activity that unwinds dsRNAs during viral replication.
An RNA strand of cucumber mosaic virus. Encodes the proteins 2a and 2b, which are N-proximal. 2a is used for replication, and has RNA-dependent RNA polymerase activity. 2b interacts with 1a, and inhibits host gene silencing response to the virus.
An RNA strand of cucumber mosaic virus. Encodes the protein 3a. There is also an RNA 4 in the particle, expressing coat protein. 3a allows for intercellular movement of the virus, and affects host specificity. Coat protein determines transmission by aphid vectors, and is involved in intercellular movement.
Root knot nematode


“Apple-shaped female”

On the top-ten list of plant pathogenic nematodes. The most common and destructive plant pathogenic nematode. First reported in 1855 by Berkeley, and further described by Chitwood and Goeldi. Worldwide distribution, primarily tropical to sub-tropical. There are about 100 species, including M. arenaria, M. incognita, M. javanica, and M. hapla. Species may be distinguished by perineum and molecular analysis. A sedentary, endoparasitic nematode. Obligate biotroph.

Root knot nematode hosts
Parasitizes almost every species of vascular plant. Most species have a wide host range. Broad host range enables them to persist in alternate hosts. Affects tomato, potato, sugar beet, and barley. Some tomato varieties have resistance, but there is potential for the nematode to circumvent this resistnace. Resistance may be ineffective at high temperatures.
Root knot nematode lifecycle
The first moult occurs in the egg, forming J2s, which are the infective stage, veniform, no more than 500 ?m in length. It attacks growing root tips and enters the root behind the root cap, then moves into the zone of elongation, where they establish a feeding site. The nematode injects secretory proteins that stimualte plant cells to become a syncytium. While parasitizing, it undergoes two moults, into J4. At maturity males are veniform, 1100 – 2000 ?m in length, with spicules for mating. At maturity females are globose and sedentary with a short neck, visibly by eye, pearly white in colour. There may be parthenogenic reproduction, and she secretes over 1000 eggs in a gelatinous mass. Life cycle may be as short as two weeks, typically longer in cooler regions. Eggs can survive at least a year in the soil, and hatch at random.
Root knot nematode management
Rotate with a non-host crop, although this can be difficult when there is more than one species. Cover crops such as sudangrass and marigold produce natural nematicides. Flooding or solarisation may work in warm climates, but takes a long time. Fumigants including 1,3-dichloropropene, methyl bromide, and dazomet, applied pre-planting: the soil may need to be covered with a tarp to be effective. Chemicals including oxamyl and fenamiphos work, and are the only feasible option in established plants, but are a potential danger to humans and other non-target animals. Plant resistance. Biocontrol with nematode-feeding fungi including Arthrobotrys and Monacrosporium, which employ traps for nematodes, or Pochonia chlamydosporia and Paecilomyces lilacinus which parasitizes eggs and females. Biocontrol with bacteria such as Pasteuria penetrans and Bacillus, which attach to the juvenile and slowly consume it with penetration structures. It is difficult to get enough fungi or bacterai to control the nematodes.
Root knot nematode symptoms

Large galls on the root system. Reduced yields. Size and number of galls depends on nematode population density, species, and race, and plant species and variety. The plant often wilts even in sufficient soil moisture, especially in the afternoon. There may be nutrient deficiency, including nitrogen deficiency, not corrected by fertilization. Stunting is observed, and yield is reduced. Can kill the host if populations are high and occur early. Symptoms appear in clusters of plants because nematodes move slowly in soil. Predispose the host to infection by other pathogens.


Root knot nematode symptoms in carrot

Typically severe forking and galling on lateral roots. Carrot is unmarketable.


Root knot nematode symptoms in grass
Galls are usually small and barely noticeable, no more than slight swellings.
Root knot nematode symptoms in lettuce

Galls are bead-like. There are resistant and susceptible varieties.


Root knot nematode symptoms in onion

Galls are usually small and barely noticeable, no more than slight swellings.


Root lesion nematode


On the top-ten list of plant pathogenic nematodes. Over 60 species, including P. penetrans, P. thornei, P. neglectus, P. zeae, P. vulnus, and P. coffeae. Worldwide distribution. Pholyphagous, migratory intercellular root endoparasites. Life cycle lasts 3 – 8 weeks, depending on species and conditions. Adults and juveniles can enter and leave roots. Females lay eggs inside or adjacent to roots. Males are common in some species and absent in others due to parthenogenesis. Eggs can survive for long periods in adverse conditions. Can survive for more than a year in the soil.


Varieties often have poor fruits, but impart good cold-hardiness, disease resistance, and dwarfing to the tree.
Satellite RNA (satRNA)


An ssRNA molecule that is dependent on cucumber mosaic virus for replication. Provides no essential function, but may intensify or attenuate symptoms, depending on the strain of the virus.

Sawyer beetle

Monochamus sartor

Longhorned beetle

A beetle 2 – 4 cm in length, with long antennae. Attracted to healthy trees for feeding, and dying trees for egg-laying. Transmits pine wilt nematode to both. The nematode is attracted to its pupae, and up to tens of thousands can leave on one beetle when it develops into an adult. It can fly for thousands of miles. Produces feeding wounds on healthy trees that are the entry point of the nematode. Lays eggs under the bark, transmitting nematodes with the eggs. It is transported as a grub in wood used for crates. It lives on spruce Picea species.

Scion Image
Software which may be used to capture, display, analyse, enhance, measure, annotate, and output images. Fungal infection of plant leaves may be quantified by digital analysis. Can be used to assess a wide variety of fungal or foliar diseases including powdery mildew, rust, anthracnose, and scab. Can be used with a scanner or a digital camera. Faster than diagrammatic scales, and more accurate than visual estimation. Free to download at

A symptom of water deficiency in deciduous trees, especially with high temperature. Marginal leaf necrosis.


Scots pine

Pinus sylvestris

A tree which was popular in the Midwest, but is no longer recommended due to pine wilt nematode. More than 90% of trees killed by this nematode are Scots pine.

A mechanism of plant disease detection. Expensive, labour-intensive, time-consuming, and not always reliable.
A nematode that feeds in only one location.
Seed-gall nematode


A plant parasitic nematode. Affects wheat, causing seed galls.


A nematode where a portion of its body is inside the plant tissue while feeding. May have migratory stages, but also penetrates the host to feed at some stage.
Describing disease severity is subjective. Minimalize variance in data by having one person measure all severities in one batch, or by careful training.
Sieving method
A method of detecting nematodes. Soil or plant tissue is filtered with a mesh sieve that catches nematodes, which are then washed from the sieve into a beaker, before the Baermann funnel method.
Single-site mode of action
A mode of action of fungicides. One mutation can infer resistance. Resistance is common.
Single stranded DNA (ssDNA)
There are two families of plant viruses in this group.
Site of action
A way that fungicides may be classified. More specified than mode of action. The specific protein, enzyme, or molecule that is targeted by fungicide.
Sloughing bark

May be confused with a disease. A tree that naturally loses bark every year, such as sycamore trees. Sometimes bark falls off gradually, or it can appear to blow off in a few days.


A fungus-specific selective medium. Used for diagnosis of plant diseases.
When soil is stockpiled, it kills microflora. Soil in developed areas is not the best.
Soil pH
A measure of the H+ ion activity in the soil. Different plant species have different preferences, but generally a range of 6 – 7 is most favourable. Outside this range, there is impact on nutrient availability, which can cause deficiencies or toxicities. Below 5.5, there is low availability of Ca, Mg, and P, and high availability of Al, Fe, and B. Above 7.8, there is low availability of P, B, Fe, Mn, Zn, and Cu, and high availability of Ca and Mg. Calcium carbonate can be used to increase soil pH, and sulphur can be used to decrease soil pH. Nitrogen fertilizers can increase soil pH. Some plant pathogens may be controlled by adjusting soil pH.
Southern corn leaf blight

Bipolaris maydis

Also known by its teleomorph, Cochliobolus heterostrophus. An epidemic in 1970 caused 15% loss of corn in the USA, costing $1 billion. This could have been used to make 30 billion hamburgers.


Southern root knot nematode

Meloidogyne incognita

Produces galls over twice the size of M. hapla. Affects cotton and sweet pepper. Peanut is less susceptible.


Soybean aphid
Transmits 9% of plum pox virus.
Soybean cyst nematode (SCN)

Heterodera glycines

A plant parasitic nematode. Worldwide distribution. The most damaging soybean pest, causing 30% yield loss. Identified in Canada in 1987. Annual losses are estimated at $1.5 million US. Affects soybeans, beans, clover, peas, vetch, and kudzu. The first moult occurs in the egg, forming stage J2, which emerges from the egg and cyst, and lives free in the soil, attacking young roots, causing formation of syncytia. They then moult two more times to form adult nematodes. Males leave the root. Females become an egg-filled sac that may resemble a root nodule, then lays eggs in a brown gelatinous cyst on the root surface. Eggs can survive up to 10 years. Its life cycle is as short as 21 – 24 days. The plant does not produce above-ground symptoms unless there is drought, but may still have yield loss. Integrated disease management includes scouting for soil samples, growing non-host crops, and resistant soybean varieties. Older resistant varieties produce 5 – 10% less yield in absence of the nematode. Populations decline after continued planting of corn.


Spectroscopic and imaging techniques
Visible and infrared regions of the electromagnetic spectra provide maximum information on physiological stress. Some wavebands specific to a disease can be used to detect plant diseases before visible symptoms develop. Can be used to identify stress levels and nutrient deficiencies in plants. There is a demand for automated non-destructive methods for plant disease detection. Includes fluorescent imaging, multispectral/hyperspectral imaging, infrared spectroscopy, fluorescence spectroscopy, visible/multiband spectroscopy, NMR spectroscopy, infrared thermography, Terahertz spectroscopy, and X-ray imaging.
Spirea aphid
Transmits 16% of plum pox virus. Doesn’t like Prunus species, so it is likely to move along after acquiring the virus.
Spot nematode


A plant parasitic nematode. Affects soybean and sugar beet.


Spruce spider mite
It likes phlox plants.
Stem and bulb nematode

Ditylenchus dipsaci

On the top-ten list of plant pathogenic nematodes. A migratory, endoparasitic nematode. Worldwide distribution. A rising problem. Can increase more than 1000 times in an infected plant during one season. Can survive severe desiccation. Survives in plant debris, soil, and weed hosts. Causes significant yield loss, feeding on leaves, stems, and bulbs. Can swim through water on the plant surface and enter stomata. Leave a plant once it is no longer suitable for feeding.

Stem and bulb nematode hosts
Has a wide range, including rye and potato. Affects over 450 genera, but most races primarily infect a limited number of species. The race which attacks garlic also affects onion, chive, leek, celery, parsley, salsify, Shasta pea, hairy nightshade, and miner’s lettuce.
Stem and bulb nematode lifecycle
The first moult occurs in the egg, forming J2s, which emerge from the egg and have two more moults, forming J4s which penetrate the plant. This takes about 20 days at 16?C. Adults develop in the leaves and females lay 8 – 10 eggs in the leaf every day for 25 – 50 days. Adults can live for 43 – 73 days. Can survive multiple years in a desiccated state in plant debris, seed, and soil.
Stem and bulb nematode management
Use clean seed. Hot water treatment of seed only reduces the number of nematodes, and can damage seeds. Strict sanitation procedures prevent spread. Clean all tools and equipment. Crop rotation with non-host crops for four years, such as carrot, potato, spinach, corn, and wheat. Control weed hosts and volunteers. Green manure such as mustard, rapeseed, oilseed radish, and sorghum-sudangrass, which produce natural nematicides.
Stem and bulb nematode symptoms

Infections cause slight hypertrophy and hyperplasia. Spicules may develop on leaves, which remain short and thick at the base. Nematodes migrate downwards intercellularly on the plant surface, to the scales of the bulb, which appear as mealy or discoloured rings. Juveniles may cling to the bulb surface as nema wool. Foliage may fall over and juveniles leave heavily infected bulbs.


Stem and bulb nematode symptoms in garlic

Yellow leaves, wilt, stunted plants, and collapse. Infected plants are in patches. Bulbs are brown, lightweight, and shriveled, with easily cracked and detachable wrapper layers. Secondary infections may cause rot, and damage can progress in storage. Healthy looking bulbs may be infested with small numbers, but are still marketable.


Stem gall nematode

A nematode that causes deformed tissue in knapweed.


Sting nematode


A migratory, ectoparasitic nematode. Affects bean.


Storm damage

A form of mechanical injury. Caused by high winds, heavy snow, or ice. Ice storms may deposit an ice coating on trees, leading to breakage of limbs. High winds can lock down trees predisposed by root and butt rots or wood decay. Hail damage is common, producing tattered leaves. Lightning strikes can cause long, narrow bands of dead tissue running tree length, shattered wood, bark blown off the trunk, or tree death. Tornadoes can flatten trees, blow them over, rip off major limbs, or blow off th ebark. Removal of torn limbs and good care after the storm helps plants recover. Significantly damage trees should be removed.


Strip agar test
A plug of mycelia from a pathogen isolation is placed onto new growing media, and its growth is measured after 48 hours. Measures fungal growth, EC50, growth rate, biomass, and discriminatory concentration.
Stripe rust

Puccinia striiformis f.sp. tritici

Yellow rust

Affects barberry and other grasses. A macrocyclic rust. Spores travel on the Puccinia Pathway.


Stubby root nematode


A plant parasitic nematode. Affects corn.




A part of a nematode, 10 – 12 ?m long. Functions in mechanical penetration of host walls to deposit eggs, secretion, and ingestion of food. Size and shape may be used to classify nematodes. All plant pathogenic nematodes have a stylet.

Sulphur (S)
A secondary macronutrient. Non-mobile, and may decrease soil pH.

Sugar Tyme

A variety of apple with bright red, 1.3 cm fruits. Fruits into spring. Moderate to high resistance to apple scab. White flowers. Oval form.



A plant cell that a root knot nematode or cyst nematode has stimulated to become large and multinucleate. Produces proteins that the nematode ingests. Acts as a nutrient sink for the plant, feeding the nematode. The nematode’s stylet is inserted into the cytoplasm, and filters cytosol that it ingests. Has increased production of plant growth regulators, causing gall formation. If the nematode dies, so does the syncytium.

Synthesis inhibition
A mode of action of fungicides. Includes inhibition of sterols, proteins, or chitin.
Systemic acquired resistance (SAR)
A management practice for viruses. Inoculation with TCV can induce resistance. Reduced symptoms, and reduction of pathogen growth. Increased levels of salicylic acid. Leaves of treated plants consistently show less extensive formation of lesions/
Systemic fungicide
Taken up by the plant, and move within the tissues. May protect new growth. May have protectant activity. Not subjected to the environment. Includes strobilurin.
Systemic infection
Infection throughout the entire plant body. For viruses, there must be movement in the vascular system, usually phloem and plasmodesmata.
Tail shape
A way of differentiating nematodes. The tail may be pointed, sharp, tapering, filiform, rounded, bluntly conical, or with setae.
Tank mixing
Using more than one fungicide at a time. May be a way to manage fungicide resistance.
Tar spot

Rhytisma acerinum

Affects sycamore and maple. Visually unappealing but does not harm the plant. Can indicate low levels of pollution in the environment.


Temperature extreme
Detrimental to plants. Injury depends on plant species and age, duration of temperature event, time of year, or interaction with other stresses. Roots and shoots may have varying levels of tolerance to thermal stress. Shoots experience a wider range of temperatures than roots, but roots may be exposed to extreme temperatures for longer periods. To avoid, use plants adapted to the climate.
Tender fruit processing plant
Located in St. David’s, Ontario. The largest tender fruit processing plant east of the Rocky Mountains. Processed stone fruits and pears, producing fruit cups. Imports exceeded domestic supply for the first time in 2001, then again in 2002 and 2005, due to eradication procedures for plum pox virus. In March 2008 it closed due to foreign competition.
Terahertz spectroscopy
An imaging technique. Not cost effective.
Test probe
An aphid inserts its stylet into the leaf to taste the plant, to see if it is its preferred host. It will fly away if the plant is not. It will stay and feed on the plant if it is. Transmits non-persistent viruses on the mouthparts.
A fungicide with an LD50 of 780 mg/kg.
Thistle aphid
Transmits 4% of plum pox virus.
An axis of the plant disease pyramid. Factors include season, duration and frequency of favourable conditions, and appearance of vectors.
Time or duration of epidemic (T)
A factor of monocyclic and polycyclic disease models. Decrease the value by planting later or harvesting earlier.
A variety of apple with red, 0.6 cm fruits that drop in-season. Very good resistance to apple scab. White flowers. Dwarf form.
Tip dieback

May be caused by insects, maggots, winter dieback, or disease.


Tobacco ringspot virus

A virus that is vectored by the nematode Xiphinema americanum.


Tom Hsiang et al (1997 – 2013)
Professor Hsiang’s study on sensitivity of Sclerotinia homoeocarpa to DMI fungicides. In 1994 examined 435 isolates from 8 populations. In 2003 examined 465 isolates from 9 populations. In 2013 examined over 1000 isolates from 13 populations. About 100 samples were collected from each site, and the pathogen isolated, and plated on fungicide amended agar strips with different concentrations of propiconazole. The fungal growth was measured a day later. Concluded that to shift a population’s EC50 from 0.01 ?g/mL to 0.1 ?g/mL requires 42.3 applications of DMI fungicides. Conducted field experiments in 2003, with 0.5 x 0.5 m plots inoculated with strains of varying sensitivity. It was hard to find plots where fungicides had never been used before. Plots were treated with Banner MAXX. Found that isolates with decreased sensitivity were knocked down by fungicide application, but were able to overcome inhibitory effects more quickly, reducing duration of control from 2 weeks to 1 week. New data suggests that it takes 64 applications of DMI to go from sensitive to resistant pathogen; this takes into account re-invasion with sensitive wild-type isolates, and less sensitive isolates being outcompeted due to resistance-related fitness cost. Funded by NSERC, Ontario Turfgrass Research Foundation, and Syngenta. Cost $25,000 per year. Money went 60 – 75% to wages, 10 – 20% to materials and supplies, 5 – 10% to field work and meetings, and 10 – 30% to university overhead.
Tomato mosaic virus (ToMV)

The first virus to be discovered. In 1883 it was discovered to be smaller than bacteria. In 1898 Beijerinck discovered it caused disease. In 1935 it was demonstrated that it was not a cell. Symptoms may be similar to cucumber mosaic virus. Symptoms include fern-leaf, where the blade leaflet is not as completely suppressed as in shoestring leaflet. May be used to induce SAR for cucumber mosaic virus.


Tomato wilt virus

A virus vectored by tobacco and western flower thrips. Affects various hosts.


Tree planting
The trunk flare should be visible, with mulch 1 – 2 inches from the trunk. The root ball and backfill should be gently packed and settled with water. Tree roots grow horizontally. Often trees are planted too deep.


A DMI registered for use on dollar spot. Introduced in 2010.

Tulip breaking virus

Symptoms include breaking of flower colour in tulips. During tulip mania, in 1637, infected bulbs sold for more than 10 times the annual income of a skilled craftsman. The multicolour effects of intricate lines and flame-like streaks on petals are very beautiful.


Tulip mania
Occurred in the 1630s. Tulips were seen as a social status symbol. Bulbs infected with tulip breaking virus sold for huge amounts of money.
A high value crop. Low tolerance for aesthetic damage. Fungal diseases are a large source of damage. Revenue in Ontario was $2.61 billion in 2007.
Turnip crinkle virus (TCV)
Inoculated to induce SAR against Pseudomonas syringae bacteria and TCV.
Two spotted spider mite
A pest which may be viewed with a hand lens.
Variegated plant

May be confused with a disease. Plants with areas of leaves white, lacking chlorophyll. Grown for horticulture. Caused by a genetic mutation, often not stable through sexual reproduction. Plants tend to be smaller because they produce less chlorophyll.


A map made with GIS that features points, lines, and polygons manipulated in a database.
Vertical resistance
Strong resistance to one disease.
Verticillium wilt

Verticillium albo-atrum

V. dahliae

A deuteromycetes. Symptoms are similar to other plant diseases.




Lack protein-synthesizing and energy-producing apparatus. Composed of nucleic acid, a protein coat, and some have an outer envelope. Classified by shape, type of nucleic acid, and protein coat. There are over 100 that affect plants. Obligate intracellular biotrophs. Can grow and reproduce only in living hosts. Not considered to be living organisms. Small, visible with an electron microscope. Diagnosed using pathogenicity, morphology, ELISA, symptoms, structures inside plant cells, protein or nucleic acid properties, and transmission. Plants may respond to viruses with immunity, lack of intercellular movement, lack of systemic movement, hypersensitive response, or recovery. Viruses may be engineered to interfere with their ability to infect plants, and introduced to protect from viral and insect attacks. May be used to vector genetic material that instructs plants to make products such as antibiotics or other drugs. There are additional studies on gene function. May be used for drug delivery to diseased tissue such as chemotherapeutic drugs. Affects plants less than humans because plants don’t have as much contact with each other and cannot spread them as efficiently.

Virus dissemination
Most are transmitted by a vector including insects (non-persistent or persistent) such as aphids, leafhoppers, thrips, beetles, and whiteflies. Some are transmitted by nematodes, protozoa, seeds, pollen, mechanically, plasmodiophorales (flagellate fungi). May be transmitted in sap when wounds of plants have contact. Each virus is usually transmitted by one type of vector.
Virus host
Determined by the host of the vector.
Virus lifecycle
Can be seconds to days. Can’t penetrate cell walls or cuticle unaided. Some enter through wounds, or are transmitted mechanically. Some can replicate within host as well as vector. Attachment to the host cell with lock-and-key fit. Entry initiates infection. The protein coat is removed, and the viral genome is replicated and viral proteins are manufactured and assembled. Host cell’s resources are diverted to virus production. Most viruses are inactive in dead tissues, but can survive for up to 50 years.
Virus management
There is no cure for a plant once infected by a virus. Prevention with virus-free planting material and exclusion. Eradicate diseased plants. Biological or chemical control of vector. Use resistant plants. Systemic acquired resistance.
Virus symptoms
Depends on host, season, environmental conditions, other organisms, and strain of virus. May be symptomless. May be local or systemic. Overall reduced size, and yellowing of leaves. The plant has reduced photosynthesis and sugar production.
Visible spectroscopy

Multiband spectroscopy

A spectroscopy technique. Has been used as a rapid, non-destructive, and cost-effective method for detection of plant diseases. Used for disease detection in plants in combination with infrared spectroscopy.

Volatile organic compound (VOC)
Any airborne molecule containing carbon. Many are produced by plants, depending on physiochemical factors such as humidity, temperature, light, soil conditions, fertilization, and biological factors such as plant growth and development stage, insects, and other plants. Can change when the plant is infected with a disease, due to changes in its physiology. May be extracted from leaves, and may be used as a biomarker to identify huanglongbing disease in citrus. Includes hesperidin, naringenin, and quercetin.
Volumetric spore sampler
A device which sucks up air and counts spores.


A strain of plum pox virus discovered in 2004. Found in three plum trees on a residential site in Stoney Creek.

Plants should be grouped according to water needs. Leaf wetness period should be in the mid-morning. Overhead irrigation is more efficient for growers, but shouldn’t be used in the afternoon, to minimize leaf wetness duration. Sometimes there is no infrastructure to water at appropriate times.
Water deficiency

A moisture extreme. If water requirements of a plant are not met for any species, its physiology and biochemistry are affected. Acute deficiency causes minor wilted shoot, which may be temporary, and occurs in the warmest part of the day. Chronic deficiency produces more severe injury: plants grow slowly or stop growing, young leaves do not expand, foliage loses colour, and there may be scorching or needle necrosis. Includes winter desiccation.


Water excess

A moisture extreme. Soil appears wet, is discoloured, or smells of rotten eggs due to sulphur gasses. May be caused by irrigation. Increases susceptibility to root rots. Reduces oxygen availability to roots, which may lead to wilt. Acute water excess (flooding), causes root cell weakening and/or death, producing discoloured and/or water-soaked, mushy roots which are susceptible to pathogens such as Phytophthora and pathogens with flagellate spores. Flooding can reduce nematode populations, and may be used to disinfect soils prior to planting. Chronic water excess causes stunted plants with underdeveloped shoots, bleeding cankers on stems, adventitious roots at the crown, split bark, water-soaked and discoloured wood, and edema. May be corrected by improving drainage.


Watermelon mosaic potyvirus (WMV)

Symptoms include colour breaking.


Wayne Barton
Worked from 1994 – 1997 on research on the sensitivity of Sclerotinia homoeocarpa to DMI fungicides. His wife Hilda helped.
Weather stations or sensors
Placed over the crop canopy for modeling epidemics. Includes a volumetric spore sampler.
Wetting agent
An adjuvant formulant of fungicides which allows it to fill in nooks and crannies easier.
Wheat leaf rust

Puccinia triticina

Visible-NIR hyperspectral imaging may be used to detect it in winter wheat; the classification model yielded 92 – 98% classification accuracy while classifying diseased plants.


White pine blister rust

Cronartium ribicola

Affects pine trees. Occurs in cool areas, such as depressions.


Winter desiccation

Water deficiency that occurs in winter months when soil is frozen, especially during windy and/or sunny weather when transpiration is high. Foliage appears off-colour, scorched, or necrotic. Occurs on the southwest side of the tree, which gets more light and loses the most water. Evergreens are susceptible.


Woody plants
Harder to cure of disease because more money is invested in them.
X-ray imaging
An imaging technique. Not cost effective.
Xiphinema americanum
A nematode which transmits tobacco ringspot virus.
Xiphinema index

A nematode in the family Longidoridae in Clade 2. On the top-ten list of plant pathogenic nematodes. A vector of grapevine fanleaf virus. A significant parasite of grape and fig, with worldwide distribution. Significantly larger than other plant parasitic nematodes, up to 3 mm in length. Biotrophic in all stages. Some stages induce complex multinucleate feeding structures in host roots. Has reproductive parthenogenesis, with rare sexual reproduction. Feeding slows root extension, causing swelling and gall formation, reducing growth of the plant. Gall tissue cells are enlarged and multinucleate.



Yellow-bellied sapsucker

A woodpecker that feeds on insects and tree sap. May cause animal damage. Protected by federal law under the Migratory Bird Treaty Act. It is illegal to kill them.


Zinc (Zn)
A micronutrient. Non-mobile, and has low availability in high soil pH.
Zucchini yellow mosaic virus

Symptoms include fruit deformities. May be confused with insect damage. Can be desirable for “spooky”-looking pumpkins.


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