Plant Pathology: Lab Quiz 7

Abiotic injury/disorder
Resembles damage from biotic factors. May affect all life stages. Damage is systemic, not random. Can occur in the field, storage, or market. Symptoms vary with type and severity of abiotic factor. Lacks disease signs. Doesn’t spread from plant to plant. Can be difficult to impossible to identify. Most are irreversible. Associated stress may promote infection of pathogens.
Aeciospore
Produced by macrocyclic and demicyclic rust fungi. Formed in numerous parallel chains in an aecium. In heteroecious rusts they infect the telial host. Aeciospores of different species often have similar morphology.
Aecium
Form aeciospores in parallel chains. Some are cup shaped. Often on the lower surface of a leaf.
Air pollutants
An abiotic disorder. Can be caused by hydrogen fluoride or ozone.
Antibiosis
A mechanism for biological control. Inhibition or destruction of one organism by a metabolic product of another, including antibiotics, enzymes, and volatile substances. A form of indirect interaction with a pathogen. Common in soil-borne fungi and bacteria. Produced by many microbes, usually in small quantities by plant pathogens. Antibiotics in the soil last longer and spread better than foliar applications. Includes Bacillus sp.
Aphelenchus avenae
A nematode which is a hyperparasite of a plant pathogenic fungi. It may feed on mycelia.
Apple powdery mildew
A polyetic, polycyclic disease.
Arthrobotrys sp.
A fungi that is a hyperparasite that predates on nematodes. It constructs a ring of hyphae, which ensnares and invades the nematode’s body.
Asparagus rust
A common polycyclic disease caused by Puccinia asparagi. Affects all aboveground parts of the host. Affects asparagus and occurs wherever asparagus is grown. Symptoms are dark brown to red pustules on stems and needles, which release rust coloured uredospores throughout the summer. Black protruding lesions form during dry periods, producing teliospores. Management strategies include removing infested crop prior to spear emergence, destroying wild or volunteer asparagus, planting away from established plantings, cleaning fields after harvest by cutting spears below the soil level, planting rows in direction of prevailing winds, using tolerant varieties, and fungicides.
Autoecious rust
“Self house”

Monoecius rust

A rust that completes its life cycle on a single host. Includes Puccinia asparagi.

Avoidance
Occurs before the pathogen is present. Prevents disease by selecting a time of year or site where there is no inoculum, or where the environment is not favourable for infection. Plant when inoculum is low or inactive. Grow seed in areas away from commercial production. Don’t plant in poorly drained soils. Avoid wounding. Avoid exess moisture on plants.
Bacillus sp.
An organism which produces an antibiotic that inhibits the growth of many pathogens, including Botrytis sp.
Basidiomycetes
Club fungi

Produce basidiospores. Includes rust fungi and fleshy fungi including mushrooms, puffballs, and bracket fungi.

Bacillus subtilis
A bacterial inoculant used to suppress disease. Suppresses pathogens including Pythium species using antibiosis, competition for nutrients, and volatile compounds that incude systemic resistance. Activate at 8 – 40?C, pH 5 – 9. Produces spores that survive up to 6 years in the soil. Used in the fermentation of chocolate. Triggers plants to close their stomata.
Basidiospore
Produced by all rust fungi. Four are produced on one basidium. Haploid, tiny, colourless spores extremely susceptible to desiccation. Dispersed during humid periods at night. In heteroecious rusts they infect the aecial host. Produce haploid mycelia which form pycnia of either + or – mating type.
Basidium
Germinates from the teliospore, and produces four basidiospores.
Biological control
Involves the stimulation or enhancement of biological activity in order to reduce the amount of inoculum or disease-producing activity. An alternate to synthetic pesticides. Use of living organisms or their products, such as fungi, bacteria, chromista, viruses, nematodes, and insects. Often depends on activity of microbial communities and involves numerous microbial interactions. There are several success cases. Exists in natural systems, in the soil, organic residues, plant surface, or inside the plant as endophytes. Advantages include minimal or no negative effects on the environment, specific targeting of desired organisms, and long-lasting effects. Disadvantages include need for knowledge of pest biology, high cost, host specificity, and labour intensiveness. It can fail if the control organism dies. Mechanisms include competition, antibiosis, hyperparasitism, induced resistance, and hypovirulence.
Black layer
A stagnant, dark layer of anaerobic organic matter in the soil just below a layer of turf. Visible in the sand profile, and has a rotting egg smell. Hydrogen sulphides are produced from anaerobic decomposition, and react with iron to form a black precipitate. Leads to yellowing of leaves and thinning of turf. Caused by heavy thatch, compaction, poor drainage and excess water, and heavy use of the field.

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Boron deficiency
An abiotic disorder.

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Botrytis sp.
A pathogen which may be controlled with antibiosis produced by Bacillus sp.
Burning
A cultural practice. Burning of plant tissue, destroying pathogens in plants, plant residues, and at the soil surface. The heat can kill resting spores of Pythium pathogens. Sugarcane is harvested by burning the field and collecting stalks. May be used for late blight of potato. There are some environmental concerns; it can start a forest fire.
Cedar apple rust
A disease caused by Gymnosporangium juniperi-virginianae. Affects apples, and cedar is the alternate host.

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Cedar hawthorn rust
A demicyclic, heteroecious disease caused by Gymnosporangium globosum. It rarely kills either of its hosts, but can disfigure plants when twigs are infected.
Cedar hawthorn rust dissemination
Basidiospores from galls on cedar hosts are disseminated by wind or insects to the hawthorn host. Aeciospores from hawthorn host leaves are disseminated by wind to the cedar host.
Cedar hawthorn rust hosts
The aecial host can be hawthorn (Crataegus sp.), apple, crab apple, pear, quince, or serviceberry. The telial host is eastern red cedar (Juniperus virginiana), as well as several other junipers.
Cedar hawthorn rust infection in cedar
Aeciospores land on the leaf. It takes 18 months for galls to mature and release basidiospores.
Cedar hawthorn rust infection in hawthorn
Teliospores land on the leaf, turn brown, and infect, causing a yellow spot.
Cedar hawthorn rust management
Prune diseased branches. Fungicidal spray on either host during periods of spore production. Hawthorns may be sprayed prior to bloom on a 7 – 10 day schedule. Cedars are sprayed three times at two-week intervals. Avoid planting hawthorns within a two mile radius of cedars. Use resistant hawthorn varieties.
Cedar hawthorn rust survival
Overwinters as galls on infected junipers.
Cedar hawthorn rust symptoms on cedar
Reddish-brown, raised and elongated galls form on twigs and small branches. A circular dimple-like depression appears, resembling the surface of a golf ball. In the spring, small chestnut-brown structures protrude from the dimples which expand in wet weather into an orange mass of telial horns. Galls take two years to produce spores, and can remain on the host for several years.

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Cedar hawthorn rust symptoms on hawthorn
Orange-yellow spots develop on the upper surface of leaves, fruit, petioles, and new twigs. Spots have black pimple-like pycnia. Later, orange spots, the aecia, appear directly below these spots with tiny projections.

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Cedar quince rust
A disease caused by Gymnosporangium clavipes.

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Chemotherapy
A type of therapy. Includes chlorine spray for postharvest diseases, soil fumigation, and curative pesticides.
Chlorosis
Yellowing

A symptom of a nutritional deficiency. Occurs on the entire leaf, or in veins.

Common barberry
Berberis vulgaris

The aecial host of wheat stem rust. It was brought to North America by early settlers from Europe. A source of primary inoculum of wheat stem rust in the spring, and the pathogen reproduces asexually, allowing it to hybridize and form new races. In 1918, a federal program to eradicate it began. Eradication delays disease onset by 10 days, reduces initial inoculum level, decreases the number of pathogenic races, and stabilizes pathogenic races. It is listed as a noxious weed, and is destroyed wherever possible.

Common rust of corn
A heteroecious disease caused by Puccinia sorghi. Red-brown pustules appear on any shoot part, mostly on leaves. The telial host is corn, and the aecial host is wood sorrel (Oxalis sp.). It is of minor economic importance, byt spring storms can cause early-season infections; it is favoured by high humidity and cool evening temperatures. Symptoms are yellow flecks or spots on leaves that develop into small, brick-red pustules that break the leaf surface and eventually turn black. The pustules release spores. Leaf yellowing occurs around the lesions and dead, brown areas develop. In severe cases the leaf can die. Management strategies include tillage, crop rotation, resistant varieties, and foliar fungicides. Inbred corn, sweet corn, and specialty hybrids have higher susceptibility.

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Common rust of oat
A disease caused by Puccinia coronata. Orange pustules burst through the surface, producing uredospores. Some areas produce black teliospores.

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Competition
A mechanism for biological control. Competition primarily for food, iron, other elements, and/or space.
Conventional breeding
A type of resistance. Has unpredictable effects.
Cover crop
A cultural practice. A non-host is grown and plowed into the soil.
Cow dung
It was used in the late 1600s to control cankers on tree wounds. It blocks the xylem, and organisms in the dung control pathogens.
Cronartium ribicola
A pathogen with aecia that form in blisters on pine trunks and branches.
Crop rotation
A biological control. In 3000 BC, decline in yield over time was observed when the same crop was grown in the field year after year. This led to crop rotation and intercropping.
Cryphonectria parasitica
A pathogen which causes chestnut blight. Hypovirulent strains can be a curative biological control. Chestnut trees with full resistance were developed using Rhizobacterium radiobacter-mediated transfer of genes from wheat. The trees were too young to reproduce (generation time of a chestnut tree is 10 years), and not yet approved by FDA.
Cultural practices
Includes crop rotation with non-hosts, intercropping, cover crops, fallow, crop rotation, solarisation, plant density, depth of planting, time of planting, site selection, drip irrigation, reduced shading, improved growing conditions, host removal, pruning and thinning, grafting, sanitation, mulching, burning, flooding, and raised beds.
Demicyclic rust
Rust fungi which produce four spore forms: basidiospores, pycniospores, aeciospores, and teliospores (BPAT). Includes Gymnosporangium juniperi-virginianae.
Disease escapee
Plants which by chance were not affected by an epidemic. A source of resistance.
Diseased plant
Something interferes with the normal functions of the plant, by a pathogen or adverse environmental conditions. Cells malfunction and/or die.
Disorder
An interaction between the plant and environment that is usually associated with an imbalance in the physical/chemical requirements for plant growth. Includes nutrient imbalances and synthetic chemicals.
Dodder
A stem holoparasite or hemiparasitic flowering plant. A noxious weed. It has spaghetti-like stems, with leaves reduced to small scales. Light yellow in colour, with minimal chlorophyll content. Stems develop suckers that enter the host. Causes physical damage to the host, reducing nutrients and water, weakening it. Shades the crop, decreases yield, increases harvesting costs, and can vector diseases. Elevated levels in fodder can be toxic to horses and cattle, causing liver damage and diarrhea.

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Drip irrigation
A cultural practice. Irrigation without getting the leaves wet.
Dutch elm disease
Cultural practices include removal of dead and damaged branches. Pesticides may be used for the tree and beetles. Partial resistance may be found from genetic crosses between American and Asian elm species. Biological control using Verticillium dahlia, and ELMguard. Exclusion using quarantine and trenches between trees.

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Dwarf mistletoe
Arceuthobium sp.

One of the most economically damaging mistletoes. Very specific, affecting only conifers. Smaller than true mistletoes.

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ELMguard
A natural elicitor protein which may be used for biological control of Dutch elm disease.
Endocronartium harknessii
Peridermium harknessii

An endocyclic, autoecious rust pathogen that causes western gall rust of pine. Spores infect the branches of pine in May and June, and produce galls the next summer. Produces growth regulators in the vasculature of the host, stimulating formation of galls and witches’ broom. Galls produce spores after one year, and can continue to do so for several years.

Endoparasite
A parasitic flowering plant that lives primarily within host tissue. Composed mainly of haustoria. May be photosynthetic or not. Often in the root, stem, or climbing vine.
Epiparasite
A parasitic flowering plant that lives primarily on the surface of the host.
Eradication
Eliminate, destroy, or inactivate the inoculum at the source after introduction. Rarely effective in large areas. Eliminate alternate hosts, and destroy weeds that are reservoirs. Fumigation and pesticides. Burning. Crop rotation.
Erysiphe cichoracearum
A pathogen that causes powdery mildew of cucurbit. Produces chasmothecia.
Exclusion
Prevents the introduction of inoculum to a continent, country, region, or field. Quarantine policies and programs to prevent introduction. Disease-free planting materials as transplants and seeds. Certification programs. Tissue culture propagation. Exclusion of vectors. Cleaning equipment. Cutting root grafts.
Fallow
A cultural practice. No crops are grown, and no residue is produced for the pathogen.
Flooding
A cultural practice. Reduces the number or eliminates propagules from the soil, including fungal, insect, bacterial, weed, and nematode propagules. It can lead to an increase in some pathogens such as oomycetes. Rice is typically grown flooded to reduce weeds.
Formae specialis
Morphologically indistinguishable lines of a rust fungi physiologically specialized to infect different host species.
Fusarium sp.
A fungal pathogen that can be used as biocontrol for witchweed.
Grafting
A cultural practice. Can be done for watermelons and apples.
Gymnosporangium clavipes
A rust pathogen that causes cedar quince rust. The telial horn is tube shaped.
Gymnosporangium globosum
A demicyclic, heteroecious rust pathogen that causes cedar hawthorn rust.
Gymnosporangium juniperi-virginianae
A heteroecious rust pathogen that causes cedar apple rust.
Gymnosporangium sabinae
A demicyclic, heteroecious rust pathogen that causes pear trellis rust.
Gymnosporangium sp.
Can infect most species of red cedar (Juniperus virginiana), as well as many other junipers and alternate hosts. Aecia are deep and walls shread downards, like a banana peel.
Harvest
Harvest under appropriate environmental conditions. Clean harvesting equipment.
Haustoria
Feeding structures of obligate parasites, which take nutrients from living plants. In parasitic flowering plants, they are a modified lateral root that transports nutrients and water from the host.
Healthy plant
Cells divide and differentiate as needed, absorbing water and nutrients from the soil, performing photosynthesis and translocating, metabolising, or storing photosynthates, and producing seeds and reproductive organs.
Hemiparasite
A parasitic flowering plant that can survive without the host. Some nutrients, mostly water and minerals, are obtained from the host. It may live in or on the host independently. They are green, and produce their own sugars. Typically less economically damaging than holoparasites.
Heteroecious rust
“Different house”

A rust that requires two alternate hosts to complete its life cycle. Includes Puccinia graminis and Gymnosporangium juniperi-virginianae.

Hollyhock rust
An autoecious, microcyclic rust disease caused by Puccinia malvacearum. Tends to become more severe as the summer progresses, killing most of the leaves on infected plants by early fall.
Hollyhock rust conditions
Favours wet or humid conditions. The disease may still be aggressive in dry years.
Hollyhock rust dissemination
Basidiospores are spread by wind and rain. The fungus has not been found in the seed, but the seed can carry spores on accompanying bracts or floral parts.
Hollyhock rust hosts
Hollyhock (Althaea rosea), common mallow (Malva sylvestris), mallow (Malva rotundifolia), Malva, Abutilon, Hibiscus, Lavatera, Malvastrum, and Sidalcea.
Hollyhock rust infection
A basidiospore lands on the leaf, causing a new infection and producing a spot at the site of infection.
Hollyhock rust management
Check plants regularly, and remove infected plants or plant parts. Use seeds from unaffected plants. Monitor brought-in plants closely. Grow hollyhocks as a biennial, discarding after flowering. Cut plants to the soil level in the autumn, and dispose of all debris. Avoid dense plantings of hollyhock. Encourage good roots by keeping soil neither too wet nor too dry. Control common mallow and other weeds in the Malvaceae family. Preventative fungicides with regular applications prior symptoms, twice a week when plants are active in growth, beginning in the spring when leaves expand. Sulphur may damage leaves if temperature exceeds 30?C within 24 hours. Irrigate in the morning so that leaves can dry quickly. Improve air circulation.
Hollyhock rust survival
Overwinters on a few green leaves remaining at the base of hollyhocks and perennial mallows. May survive within the crown of the plant.
Hollyhock rust symptoms
There is a latent period of a few days to several weeks. Bright yellow or orange spots form on the upper leaf surface, or on the stems or calyx. Reddish-brown pustules form on the lower leaf surface beneath lesions. Lesions turn grey in high humidity, and produce basidiospores. Eventually spots turn brown and drop out. Lower leaves are affected first. Severely infected plants may shrivel and fall.

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Holoparasite
A parasitic flowering plant that is an obligate parasite that cannot survive without the host. All nutrients are derived from the host. They are not green, and do not produce their own sugars. Typically more economically damaging than hemiparasites.
Hybridization cross
A source of resistance. Combining agronomic qualities and disease qualities.
Hyperparasitism
A mechanism for biological control. The hyperparasite parasitizes the plant pathogen. Generally the parasite uses the pathogen as a food source; extracellular enzymes may digest the pathogen’s cell walls. Both organisms usually co-exist for an extended period of time. A form of direct interaction with the pathogen. Includes Trochoderma atroviridae, Aphelenchus avenae, Arthrobotrys, and vampyrellid amoebae.
Hypovirulence
A mechanism for biological control. The reduced ability of some isolates in a population of a given pathogen to cause disease. Can be caused by a virus or virus-like agent within the fungus. Isolation of viral genetic material, transferred to other isolates. Hypovirulent strains tend to have reduced growth and reproduction compared to other strains.
Indian pipe
Ghost plant

An endoparasitic flowering plant.

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Induced resistance
A mechanism for biological control. Heightened resistance in a plant towards pathogens as a result of a previous treatment with a pathogen, attentuated pathogen with reduced virulence, or a non-pathogen or chemical that is not itself a pesticide or antimicrobial. Includes systemic induced resistance and systemic acquired resistance.
Injury
A destructive, physical occurrence to the plant. Includes drought, heat, frost, and mechanical injury.
Integrated disease management (IDM)
Minimizing the impact of disease through cultural practices, pesticides, host resistance, biological control, and exclusion. There is an emphasis on cultural management and host resistance in organic agriculture.
Intercropping
A cultural practice. A non-host is grown with a host. Intercropping with a legume increases nitrogen in the soil. Height difference between the crops improves air circulation in the field.
Japanese barberry
Berberis thumbergii

A species of barberry which is not susceptible to wheat stem rust, and can be grown in Canada as an ornamental plant.

Kudzu
Pueraria sp.

Japanese arrowroot

A legume in the family Fabaceae. A structural parasite that is a climbing, coiling, and trailing perennial vine. It is the alternate host for soybean rust. Native to eastern Asia, southeastern Asia, and some Pacific islands. It edible, but often sprayed with herbicides. Introduced to the USA in 1876 as an ornamental bush. It was used to stop eriosion in the 1930s, and people were paid to sow it into topsoil. It spread at a rate of 150,000 acres annually, and now the estimate is 2,500 acres annually.

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Late blight of potato
May be eradicated by burning the shoot prior to harvest, to prevent tuber infection. Raised beds may be used to reduce tuber infection.

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Leaf blight of rubber
Not a serious disease unless the crop is grown in monoculture.

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Leaf scorch
An abiotic injury.

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Lenape potato
A variety of potato which is resistant to potato virus A and late blight infection. Its tubers are high in solanine, which is toxic. The variety was removed from production.
Light
The amount of light can lead to low-light bean, sunscald, and sunspots. Water drops on the plant surface do not cause sunburn or leaf scorch.
Macrocyclic rust
Rust fungi which produce all five spore forms: basidiospores, pycniospores, aeciospores, uredospores, and teliospores (BPAUT). Includes Puccinia graminis.
Management
Reduces the progress and maintains disease development below an acceptable threshold. Requires correct identification of pathogen and disease, and knowledge of the disease cycle, targeting vulnerable stages. Practices are often based on models. Successful management requires scouting, accuracy of diagnosis, identification of the stages of the disease, and early implementation of management practices.
Microbial inoculants
May be used for disease control. Increases the populations of disease-suppressive microbes in the soil or on/in the plant. Numerous products are registered as biopesticides, and are commercially available. Repeated applications are usaully necessary to maintain population levels for disease control. Can be fungi, bacteria, and oomycetes.
Microcyclic rust
Rust fungi which produce two spore forms: basidiospores and teliospores (BT). Includes Puccinia malvacearum.
Mistletoe
An epiparasitic plant. An evergreen shrub. There are over 1,000 species. May be a stem hemiparasite or a holoparasite. It is photosynthetic, but may kill trees. Includes dwarf and true mistletoe. Has separate male and female plants. Symptoms include dieback and reduced shoot growth, chlorosis, distortions on branches at site of infection, witches’ broom, swollen branches, cankers, weak wood, and death after several years. Increased dead tissues near the ground increases risk of forest fire.
Mulching
A cultural practice. Application of a covering layer to the soil surface. May be crop residue, dried plant material, other natural materials, or synthetic materials. It can inhibit inoculum, but too much is not good because it holds moisture. Improves soil drainage to prevent soil-borne diseases, prevents direct contact of foliage and fruits with teh soil, reduces drought stress, and returns nutrients to the soil. It has the potential for phytotoxicity and slugs.
Necrosis
Cell death of plant tissue. A symptom of a nutritional deficiency.
Nutritional deficiency
An abiotic disorder. Symptoms include chlorosis, necrosis, reduction in growth and development, and reddish-purple colour from anthocyanins that accumulate when metabolic processes are disrupted. Symptoms typically do not have distinct borders, and colour changes are faded, gradient-like. Includes nitrogen, magnesium, calcium, iron, potassium, and phosphorus deficiencies.
Nutritional excess
An abiotic disorder
Obligate parasite
Organisms which grow and reproduce only on or in living plants, deriving nutrients from living hosts with haustoria. Includes rust fungi, powdery mildews, and downy mildews.
Oil
May be sprayed onto the plant surface to form a barrier against disease.
Organic amendments
May be used for biocontrol. Increases the numbers and activity of beneficial microbes associated with plants which have competition, antibiosis, hyperparasitism, and induced resistance (SAR and ISR). Introduces a diverse range of microbes into the soil, and enhances activity and population of native organisms. Control is dependent on the disease, nature of the compost, and how the material is composted. Numerous products are registered as biopesticides and are commercially available. Repeated applications are usually necessary to maintain population levels for disease control. May be compost, mulch, manure, or sludge.
Overhead watering
The water dropplets on the plant surface from overhead watering on a sunny day does not cause sunburn or leaf scorch. It is not water efficient because the water will evaporate.
Papaya ring spot virus (PRSV)
A virus that affects papayas. A resistant variety was developed using conventional breeding.
Parasitic flowering plant
An angiosperm that attaches to and parasitizes another plant with haustoria. There are over 4,000 species. Usually doesn’t kill the host. Many are economically devastating weeds. Includes holoparasites and hemiparasites, and epiparasites and endoparasites. Typical establishment involves seeds, germination, haustoria, growth, attachment, host penetration, haustoria penetration, and nutrient flow.
Pear trellis rust
A demicyclic, heteroecious disease caused by Gymnosporangium sabinae. Can cause total defoliation and loss of yield. Has been confirmed in several locations in southern Ontario; it is a regulated disease.
Pear trellis rust dissemination
Basidiospores formed in juniper telia are wind-borne, and can be dispersed up to 6 km. Pycniospores form on pycnia on pear upper leaf surface, and are disseminated by insects which are attracted by a sticky substance. Aeciospores are wind-borne.
Pear trellis rust hosts
The aecial host is pear (Pyrus sp.). The telial host is juniper (Juniperus sp.).
Pear trellis rust survival
Overwinters in galls in juniper hosts. The disease cannot develop on the pear host once leaves have fallen in the autumn.
Pear trellis rust management
Fungicides used for other diseases. Keep pears and junipers at least 11 km apart. Inspect juniper plants periodically and prune out swellings or galls.
Pear trellis rust symptoms in juniper
Woody galls develop in the spring. After two years, the galls form horn-like projections which are telia. During wet weather the telia swell and become orange and gelatinous, and produce basidiospores.

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Pear trellis rust symptoms in pear
Bright yellow-orange spots form on leaves which have dark, dot-like pycnia on the upper surface. Spots turn red around the margins in early summer. The underside of the lesions turn brown and swell and blister. Lantern-shaped aecia form in the late summer on blisters on the lower surface, and produce aeciospores.

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Pesticide
The more you use a pesticide, the more likely the pathogen will develop resistance. Can cause abiotic disorders when herbicides or fungicides are applied at too high of a rate, especially in stressed plants.
Pesticide injection
Often with tree diseases, the pesticide needs to be underneath the bark, so spraying will not work, and an injection is necessary.
Phakopsora meibomiae
A pathogen which causes a disease similar to soybean rust, that also infects soybeans and numerous legumes. Less aggressive than P. pachyrhizi, and the two can be distinguished by a molecular test. Occurs primarily in the western hemisphere.
Phakopsora pachyrhizi
A macrocyclic, heteroecious rust pathogen which causes soybean rust. Overwinters in the USA. It has an anamorph with obovoid, sessile spores. Teliospores are one-celled, irregularly arranged in 2 – 7 layers in the telia.
Phosphorus deficiency
A nutritional deficiency.

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Phragmanthera crassicaulis
A parasitic flowering plant.

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Physical injury
An abiotic injury. May be caused by lawn mowers, lightning, hail, or wind.
Phytotoxicity
“Plant toxic”

Mulches have some risk of phytotoxicity.

Polyetic
A multi-year disease cycle where the amount of inoculum increases from year to year. It may be monocyclic or polycyclic within each growing season. Includes apple powdery mildew.
Post-harvest
Clean the storage area. The storage area should be climate controlled.
Powdery mildew of cucurbit
A disease caused by Erysiphe cichoracearum.

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Prevention
Always considered most important, because disease does most of its increasing before the visual threshold.
Protection
Assumes the pathogen is present. Prevents infection using a chemical or other barrier between host and pathogen. Treat healthy plants prior to infection to reduce the rate of increase of infection. Biological control. Spacing of plants. Temporal spacing of plantings. Chemicals such as fungicides and nematicides. Row covers. Trenching.
Pseudomonas sp.
A bacterial inoculant used to suppress disease.
Puccinia asparagi
A macrocyclic, autoecious rust pathogen which causes asparagus rust. Overwinters as teliospores in lesions of host residue. Basidiospores are initial inoculum for new infections. Aeciospores are produced which infect the host. Uredospores are rust-coloured and spread the disease rapidly during warm days with cool nights and dew formation.
Puccinia coronata
A rust pathogen that causes common rust of oat.
Puccinia graminis
A macrocyclic, heteroecious rust fungi which causes stem rust.
Puccinia graminis f.sp. avenae
A rust fungi that affects oats.
Puccinia graminis f.sp. graminis
A rust fungi that affects wheat.
Puccinia graminis f.sp. secalis
A rust fungi that affects rye.
Puccinia graminis f.sp. tritici
A macrocyclic, heteroecious rust pathogen which causes stem rust of wheat. Produces pycnia and aecia on common barberry, and uredia and telia on wheat.
Puccinia malvacearum
A microcyclic, autoecious rust pathogen which causes hollyhock rust.
Puccinia Pathway
The pathway on which the uredospores of Puccinia graminis f.sp. tritici and other pathogens blow north on the wind from the USA and northern Mexico, where the pathogen is maintained on susceptible crops year-round.
Puccinia sorghi
A macrocyclic, heteroecious rust pathogen that causes common rust of corn. Infects after tasselling, and is present in almost every corn field in Ontario. Overwinters in the southern USA and Mexico, and spores are blown into Ontario.
Pynia
Produce pycniospores. Can resemble a pycnidia under a dissecting microscope. Often found on the upper surface of a leaf.
Pycniospore
Spermatia

Produced by macrocyclic and demicyclic rust fungi. Formed in the pycnia. Not technically a spore; it is a gamete and cannot infect a host. It fuses with receptive hyphae of a compatible mating type to become dikaryotic hyphae that form aecia.

Pythium root rot
A disease which has high incidence in overwatered plants.

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Pythium sp.
A chromista inoculant used to suppress disease.
Race
Lines of a pathogen which can attack only certain varieties or cultivars within a host species, and can be detected and identified by a set of differential cultivars.
Rafflesia sp.
Corpse flower

A holoparasitic, endoparasitic flowering plant. Causes little damage to its host. Found in rainforests in southeast Asia. Embedded inside the host for most of its life, except for flowering. Lacks chlorophyll and has no leaves, stems, or true roots. Only the flower is visible on the surface, up to 1 m in diameter. Can only grow on the Tetrastigma vine. Germination and haustoria penetration require damaged vines. The flowers smell like rotting flesh, and flowering lasts 5 – 7 days. Buds are produced near host tendrils, or “roots”, 9 – 12 months after flowering. The seeds are sticky and attach to animals to disperse. The smallest member of this genus, 9.73 cm in diameter, was discovered in the Philippines, and may be critically endangered.

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Raised beds
A cultural practice. Helps with drainage, frost control, and organic material. Can be used for late blight of potato.
Resistance
Use of cultivars that are resistant to or tolerant of infection, particularly initial infection. It is the best and easiest management practice, if available. Includes natural resistance, genetic engineering, and conventional breeding. Rhizobacterium radiobacter may be used to transform plants with T-DNA. Sources of resistance include disease escapees, surviving plants, and hybridization crosses. Best with annuals and biennials. Resistance is better with more specialized pathogens such as rusts and viruses, and less with general pathogens. A problem is development of races to specific lines of hosts; there may be many genes involved in resistance. Trees take a long time to reach reproductive maturity, and breeding programs may be slow.
Rice
It may be grown in a dry field, but flooding helps the crop compete with weeds.
Row cover
A type of prevention. Prevents leaves and fruits from touching the soil.
Rust fungi
A group of basidiomycetes. Over 8,000 species in 130 genera. Date back to the Cretaceous period, possibly into the Pennsylvanian period (325 million years ago). Highly specialized parasites, one of the most widespread and destructive plant diseases. Affects wheat, barley, oats, beans, soybeans, sunflowers, coffee, pine, asparagus, carnation, apple, ornamental crab apple, rose, quince, raspberry, geranium, and others. Produce pustules from which enormous numbers of spores may be produced. Have a complex life cycle that can include five spore forms and two alternate hosts. Includes macrocyclic, demicyclic, and microcyclic rusts.
Salt injury
An abiotic disorder caused by salt. Salt damage usually follows a pattern, such as where a snowbank was.

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Sanitation
A cultural practice. Culls are taken far away from the field.
Septoria rudbeckia
A pathogen which affects black-eyed Susan.
Sludge
Liquefied manure. May be used as an organic amendment.
Solarisation
A cultural practice. Clear plastic mulch is laid, with a drip irrigation hose in the centre underneath the plastic. The well-sealed mulch edges and fogs from water vapour. Temperature in the soil can reach up to 35 – 60?C, depending on soil type, season, location, soil depth, and other factors. The heat kills or suppresses pathogens and weeds.
Soybean rust
Asian soybean rust

A serious, heteroecious disease caused by Phakopsora pachyrhizi. Initial inoculum is carried on the wind from the USA. First occurred in Japan in 1902. It spread to South America in 2001, then to North America in 2004. Affects soybeans (Glycine max), Glycine soja, Pachyrhizus erosus, Pueraria lobate (kudzu), Vigna unguiculata, legume species including kidney bean, and clover. Resistant soybean varieties are not yet available.

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Stem rust
A disease caused by Puccinia graminis.

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Stem rust of wheat
Wheat stem rust

A heteroecious, macrocyclic disease caused by Puccinia graminis f.sp. tritici. There was an outbreak in 1916, causing loss of 200 million bushels of wheat, and increases in wheat and flour prices. In Canada, the main source of primary inoculum is uredospores blown northward on the Puccinia Pathway.

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Stem rust of wheat hosts
Wheat is the telial host. Common barberry is the aecial host.
Stem rust of wheat infection
Requires spore production in the overwintering area, favourable conditions for spore liberation and movement into upper air masses, favourable conditions for transport of viable spores, rain to deposit the spore on a susceptible host cultivar, and favourable environmental conditions.
Telia
A pustule which produces teliospores.
Telial horns
Orange masses of spore-bearing jelly-like tendrils which emerge from galls of cedar hawthorn rust on a cedar host during wet weather.
Teliospore
Produced by all rust fungi in telia. Differ in morphology, and are the main basis for distinguishing genera of rust fungi. They germinate to produce a basidium. Initially it is dikaryotic, and nuclei fuse to form a diploid nucleus which migrates into the basidium and undergoes meiosis.
Temperature
Can cause frost damage, bolting, and bean heat stress.
Tetrastigma vine
A member of the grape family. It is the only host of Rafflesia sp. parasitic flowering plants.
Therapy
Curing plants that are already infected. Includes chemotherapy and thermotherapy.
Thermotherapy
A type of therapy. Hot water treatment of seeds.
Thurber’s stemsucker
Pilostyles thurberi

An epiparasitic flowering plant.

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Thyme broomrape
Orobranche sp.

A holoparasitic flowering plant.

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Trichoderma atroviride
A hyperparasite which may be used to control sclerotia of plant pathogens.
Trichoderma sp.
A fungus present in nearly all agricultural soils. It can be used as a fungal inoculant for biocontrol from induced resistance, nutrient competition, and antibiosis. It can be used as a plant growth promoter; it increases nutrient availability to plants, increasing plant vigour.
True mistletoe
Mistletoe which is larger and less damaging than dwarf mistletoe. Has many hosts, including oak and pear. It is green and photosynthesizes more. It includes the traditional Christmas mistletoe.

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Typhula ishikariensis
A pathogen that causes grey snow mould. A biocontrol method is competition with T. phacorrhiza.
Typhula phacorrhiza
A low-temperature tolerant saprotroph that can grow under snow, colonize tissue of creepign bentgrass, and suppress development of grey snow mould.
Uredium
A pustule which produces uredospores. A single uredium can produce 350,000 uredospores.
Uredospore
Produced by macrocyclic rust fungi. Dikaryotic, yellow spores produced in uredia. Blown on the wind up to 100s of kilometers. In heteroecious rusts they infect the same host on which they were formed. Infection cycles can be as little as 5 – 6 days in favourable conditions; they are the reason smuts have explosive growth.
Vampyrellid amoeba
A spiky amoeba that is a hyperparasite of fungi. It forms holes in the mycelium.
Verticillium dahlia
An organism which may be used for biological control of Dutch elm disease.
Visual threshold
Occurs at approximately 1% of infected seeds. From one infected seed, the disease will increase more before the visual theshold than afterwards.
Water
The amount of soil water can cause underwatered or overwatered plants. Overwatered plants will not have proper root growth due to lack of oxygen, and have increased risk of Pythium root rot.
Western gall rust of pine
Pine-pine gall rust

A disease caused by Endocronartium harknessii. Affects two- and three-needled Pinus species, including hard pine, spruce, jack, ponderosa, lodgepole, Scots, and mugo pines. Found in northern and eastern North America. Symptoms are globose, woody galls on stems and branches that can grow for many years. Galls erupt in the spring with bright orange spores. Galls can weaken the stems, resulting in wind breakage. Symptoms can also include witches’ broom and girdling of the stem with cankers associated with galls. Affects trees of all ages, and can lead to death in young trees. Can make up to 10% of nursery pines or Christmas trees unsalable. Management strategies include removing galls and/or felling infected trees before sporulation in the spring, a single application fo maneb at the beginning of sporulation, use of disease-free stock, and use of resistant varieties.

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Winter injury
An abiotic injury.

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Witchweed
Striga sp.

A hemiparasitic flowering plant. Affects rice and corn. Causes economic losses from 40 – 100% in Africa and Asia. Most damage is below-ground until it is too late. Symptoms include yellowing of veins and foliage, leaf withering and death, and decreased growth, fruiting, and yield. Seeds germinate close to host roots. There is a three-week window to detect the weed and take action. The tiny seeds (0.4 mm) require an after-ripening period, waiting for correct temperature (20 – 35?C) and moisture conditions. One plant can produce over 500,000 seeds, which remain viable in the soil for several years, at temperatures as low as -15?C. The seeds may be dispersed by water, wind, animals, or humans. After germination, haustoria form underground with host roots, and alter the host by increasing root formation. It grows a shoot and emerges from the soil to produce chlorophyll and photosynthesize, flower, and produce seeds. Management is difficult, because host and parasite share a vascular system: sanitation hand removal, crop rotation, biocontrol, herbicides (expensive), burrying flowers, and biological control such as Fusarium sp. Resistant crop varieties usually have decreased production of germination stimulants in the roots.

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Yellow rattle
Rhinanthus minor

A hemiparasitic flowering plant.

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