Plant Pathology Exam Review
Sclerotinia Stem Rot
Definition & Importance
Sclerotinia stem rot (Sclerotinia sclerotiorum) is a significant disease affecting over 400 host plants, including canola, pulses, and vegetables.
This disease can cause major yield losses when infection occurs during early flowering.
Disease Cycle
The pathogen overwinters in the soil in the form of sclerotia.
These sclerotia germinate into apothecia when the soil remains wet for approximately 10 days at temperatures ranging from 10 to 20°C.
Each apothecium releases ascospores that are capable of infecting the petals of plants.
As petals fall into leaf axils and remain moist, they initiate a stem infection.
The monocyclic nature of this disease means that early-season conditions greatly influence its severity.
Environmental Requirements
Sclerotinia stem rot requires prolonged moisture, cool-to-moderate temperatures, and dense canopies that can trap humidity for optimal development.
Management Strategies
Fungicides: Application of fungicides from Groups 3, 7, 11, 9+12 at 10% bloom is critical.
Petal Tests: Conducting tests to predict disease risk.
Risk Maps: Utilization of maps to assess disease risk based on environmental conditions.
Historical Records: Reference of past field records for effective planning and management.
Rotation: While rotation can mitigate damage, it is only partially effective.
Row Spacing & Seeding Rates: Wider row spacing or lower seeding rates can help reduce canopy humidity.
Varietal Tolerance: Some varieties may exhibit tolerance to the disease.
Fusarium Head Blight (FHB)
Definition & Importance
Fusarium head blight, commonly referred to as “scab,” affects a variety of crops including wheat, barley, oats, and corn.
The disease is primarily caused by Fusarium graminearum and is detrimental due to yield reduction, lower grain grades, and the production of the mycotoxin deoxynivalenol (DON).
Disease Cycle
The disease is initiated from residue-borne ascomycete fungi, which produce perithecia.
Ascospores are released during the flowering of cereals, leading to infection.
Infection is facilitated when humidity remains above or equal to 12 hours and temperatures are maintained between 16 to 30°C.
Symptoms
Bleached spikelets, shriveled chalky kernels, and salmon-colored spore masses are key indicators of this disease.
Management Strategies
Application of triazole fungicides at early flowering is crucial; applications should be done at the time when yellow anthers are present.
Avoidance of Group 11 fungicides is recommended due to their negative effect on toxin levels.
Implement crop rotation, control grassy weeds, ensure seed cleanliness, and adopt a higher seeding rate to achieve uniform flowering.
Residue management is critical, as seed treatments mainly assist in preventing seedling blight rather than head infection.
Stripe Rust
Definition
Stripe rust is caused by Puccinia striiformis f.sp. tritici, which is known for rapid epidemic growth due to its polycyclic nature and production of urediniospores.
Biology & Spread
Stripe rust thrives in cool (optimal temperatures of 7-12°C) and humid conditions.
Urediniospores are dispersed into Canada from the southern regions of Mexico and the USA annually.
New fungal races, such as PstS1, have exhibited the ability to tolerate warmer temperatures.
Symptoms
The disease manifests in yellow-orange pustules that appear in long stripes along the leaf veins.
Management Strategies
Genetic Resistance: Incorporating both Adult Plant Resistance (APR) and all-stage resistance (ASR) varieties into planting strategies.
Fungicides: Use of groups 3, 7, and 11 during appropriate development stages.
Early Seeding: Timing seeding earlier can avoid peak infections.
Green-Bridge Management: Effective management of crop residues that can harbor pathogens is necessary.
Compare & Contrast: Qualitative vs Quantitative Resistance
Qualitative Resistance
Characteristics: Determined by a single R gene; displays an all-or-nothing response; tends to be non-durable.
Vulnerability: Easily defeated due to gene-for-gene interactions with pathogens.
Quantitative Resistance
Characteristics: Governed by multiple genes (polygenic); provides partial resistance; contributes to reducing epidemic rates.
Durability: More durable and stable over time, forming Adult Plant Resistance (APR).
Exam Tip: Use examples of rust resistance breakdown to illustrate points on resistance durability.
Pathogen Classes & Host-Specific Diseases
Ascomycetes
Characteristics: Produce asci containing 8 ascospores; many are necrotrophic pathogens that include FHB, Sclerotinia, Ascochyta, and tan spot.
Dispersal Method: Typically residue-borne with spread via splash or wind, commonly found in cereals and pulses.
Basidiomycetes
Characteristics: Includes rusts and smuts; considered obligate biotrophs.
Spore Production: Produce basidiospores on basidia and are known for causing large epidemics, such as stripe rust and stem rust.
Includes smuts/bunts, wherein pathogen replaces kernels with fungal structures.
Pulse Diseases (Chickpea, Lentil, Pea)
Ascochyta in Chickpea
Causal Agent: Didymella rabiei.
Characteristics include multiple infection cycles within a season and lesions that develop concentric pycnidia.
High susceptibility often results in the need for 4 or more fungicide sprays; a maximum of <0.3% infected seed is tolerated.
Ascochyta in Lentil
Symptoms: Features small tan lesions with dark margins; pycnidia formation serves as a diagnostic feature.
Fungicide Application: Essential to apply fungicides before canopy closure to control spread.
Anthracnose in Lentil
Causal Agent: Colletotrichum lentis.
Characterized by the formation of microsclerotia that can last many years and by symptoms including cankers and canopy blight.
Complete resistance is not observed in lentil varieties.
Aphanomyces Root Rot
Symptoms: Honey-brown roots with poor nodulation observed; oospores can survive for over 20 years.
Management: Limited resistance is found, and 8+ years of crop rotation is necessary for effective control.
Canola Diseases
Blackleg
Causal Agent: Leptosphaeria maculans.
Causes stem canker at the plant crown; essential to implement R genes for resistance and practice a 3-4 year rotation.
Seed treatment "Saltro" is effective in slowing the onset of early infections.
Sclerotinia
Disease Cycle: Involves a life cycle from apothecia to ascospores and subsequent petal infections.
It's crucial to spray fungicides at the 10% bloom stage.
Verticillium Stripe
Characterized by microsclerotia present in vascular tissue leading to bronzing symptoms; however, wilting does not occur.
No fungicides are currently available for control.
Clubroot
Causal Agent: Plasmodiophora brassicae, a protist that leads to galls, stunting, and yield losses.
Persistence: The soil can remain infectious for over 15 years post-infection.
Management is based on using resistant varieties, sanitation practices, and early detection of symptoms.
Integrated Disease Management & Key Fungicide Topics
General Long Answer
Rotation:
Avoid planting host crops in consecutive years.
This is essential for managing residue-borne diseases such as tan spot, Ascochyta, and FHB.
Genetic Resistance:
Implement R genes for controlling blackleg and rusts, and use APR strategies for stripe rust.
Cultural Controls:
Adopt early seeding techniques to prevent rust infections, manage row spacing appropriately, and avoid deep seeding that may lead to root rots.
Fungicide Strategy:
Understanding the timing of applications (e.g., at flag leaf or flowering stages) is vital.
Use the correct FRAC groups, avoiding Group 11 applications for FHB.
Rotating active ingredients helps prevent the development of resistance in pathogens.
Scouting & Diagnostics:
Monitor latent periods of pathogens such as Ascochyta (4 days); early detection crucially reduces disease spread.
Environmental Monitoring:
Implement petal tests for Sclerotinia and utilize weather-dependent risk indices for FHB control.
Fungicide Timing — Exam Long Answer Key Principle
- The RIGHT TIME for fungicide application often matters significantly more than merely using the RIGHT PRODUCT.
Barley Fungicide Timing: Apply fungicides at the flag leaf fully emerged stage to maximize protection of the photosynthetic leaf area.
Wheat FHB: Apply fungicides during early flowering when yellow anthers are visible.
Sclerotinia: Implement sprays specifically at the 10% bloom stage and refrain from late applications.
Importance of Timing:
Fungicides are primarily protective rather than curative, indicating that they must be applied before the pathogen establishes.
Targets are only exposed for limited times (example: wheat flowers); thus, after infection has occurred, fungicides cannot reverse damage left by the pathogen.
FLASHCARDS (Key Concepts)
Disease Triangle:
Composed of Host + Pathogen + Favorable Environment.
Teleomorph:
Sexual stage of the fungus.
Anamorph:
Asexual stage of the fungus.
Qualitative Resistance:
Defined as monogenic, all-or-nothing, and typically not durable.
Quantitative Resistance:
Consists of polygenic, partial, and durable characteristics.
FRAC Group 3:
Encompasses triazoles primarily used in FHB suppression.
FRAC Group 7:
Comprises succinate dehydrogenase inhibitors (SDHIs).
FRAC Group 11:
Represents strobilurins, specifically to avoid during FHB due to detrimental effects.
Bacterial Leaf Streak Pathogen:
Xanthomonas translucens.
Aster Yellows Cause:
Caused by phytoplasma, a type of mollicute.
Pythium Spore Type:
Identified as zoospores.
Aphanomyces Hallmark:
Characterized by honey-brown roots and the presence of oospores lasting over 20 years.
Sclerotinia Primary Inoculum:
Primarily consists of ascospores obtained from apothecia.
FHB Primary Inoculum:
Arises from ascospores derived from perithecia.
Stripe Rust Temperature:
Optimal temperature is recorded at 7-12°C.
Memorization Statements
A disease arises from the combination of host, pathogen, and favorable environmental conditions.
A teleomorph represents the sexual stage, while an anamorph represents the asexual stage of fungal life.
Qualitative resistance stems from a monogenic basis and is easily defeated by pathogens.
Quantitative resistance decelerates epidemics while providing durability.
Fusarium head blight predominantly results from the activity of Fusarium graminearum.
FHB produces mycotoxin DON (deoxynivalenol).
Utilizing Group 11 fungicides may exacerbate DON levels in infected hosts.
FHB fungicides must be administered during the early flowering phase.
Sclerotinia requires 10 days of moist soil for viable apothecium formation.
Sclerotinia infections occur primarily via petals that have been contaminated.
Stripe rust favors temperatures between 7 and 12°C for growth.
Stripe rust spores migrate northwards into Canada from southern regions.
Rust pathogens are obligate biotrophs requiring a living host to complete their lifecycle.
Smuts are categorized as monocyclic diseases; in contrast, rusts are polycyclic diseases.
Loose smut infects plants during anthesis, while covered smut infects during seed germination.
Bacterial diseases exhibit no positive response to fungicide applications.
The pathogen Xanthomonas translucens is responsible for bacterial leaf streak (BLS).
Fire blight presents with symptoms known as shepherd’s crook.
The disease aster yellows arises from having phytoplasma.
The pathogen Pythium produces swimming zoospores.
Aphanomyces oospores have a longevity of over 20 years.
Symptoms of blackleg include stem cankers that occur at the plant crown.
Verticillium stripe results in microsclerotia forming within stems.
Clubroot leads to the development of root galls, with infectious soil persisting for over 15 years.
Seed treatments can diminish risks associated with seedling blights but are ineffective against FHB.
Fusarium head blight is categorized as a monocyclic disease.
Sclerotinia is also classified as a monocyclic disease.
Rust pathogens are recognized as polycyclic organisms.
Ascochyta infections in chickpeas are often exacerbated through moisture generated by rain splash.
The latent period for Ascochyta infection is typically around 4 days.
Anthracnose in lentils results in the formation of microsclerotia.
Tan spot lesions can be identified by their dark centers.
Septoria lesions are recognized by their parallel edges.
Spot blotch is a consequence of infection by Cochliobolus sativus.
Net blotch displays a characteristic net-like pattern on barley leaves.
Scald diseases favor cool (12-14°C) and moist conditions for propagation.
Seed lots that exhibit over 14% Fusarium are not permissible for use.
The take-all disease is indicated by roots having a glassy black appearance.
Nematodes are typically distributed across soil through machinery and equipment.
Infection by FHB necessitates humidity lasting for at least 12 hours.
The survival of rust pathogens relies heavily on the migration of urediniospores.
Fungicides aimed at managing Sclerotinia must be applied when blooms reach 10%.
Resistance against blackleg diminishes significantly under tightly practiced rotations.
Crop rotation is recognized as ineffective for rust pathogens.
DON presence is possible even in kernels that appear normal visually.
Two-row barley varieties show greater resistance to FHB compared to six-row varieties.
Fire blight can survive in cankers through overwintering processes.
The organism Rhizoctonia is identified by its characteristic branching at a 90° angle.
Verticillium stripe does not induce wilting in affected plants.
SECTION A — Multiple Choice (20 questions with each worth 2 points)
A disease characterized by a high inoculum potential but featuring only a single infection cycle per year is best managed by:
C. Reducing initial inoculum
Which scenario could exacerbate the severity of FHB even if fungicide timing is correctly observed?
B. Use of Group 11 fungicide
Which pathogen cycle allows for escape through uniform flowering?
C. Fusarium head blight
Regarding smuts, which statement is FALSE?
C. Seed treatment is ineffective.
Damage from aster yellows in canola typically increases in years when:
A. Leafhopper populations build on southern crops
Which statement about Sclerotinia is most accurate?
C. It infects via infected petals during bloom.
In a seed lot with 8% total Fusarium and high germination rates (98%), the best management decision would be:
C. Plant with seed treatment.
The epidemics of stripe rust occur due to:
B. The urediniospore stage migrates north annually.
Which characteristic of Verticillium longisporum most reliably differentiates it from blackleg?
C. Microsclerotia inside vascular tissue.
Symptoms of Aphanomyces can be confused with those of:
B. Fusarium wilt.
Fungicides that exhibit poor redistribution would be least effective in managing:
A. FHB.
The pathogen that cannot be controlled through crop rotation is:
B. Stripe rust.
Which pathogen would most likely thrive under reduced tillage practices?
B. Sclerotinia.
In a field of peas with honey-brown roots but healthy hypocotyls, the likely disease is:
B. Aphanomyces.
The TRUE statement regarding qualitative resistance is:
C. It is easily overcome.
Among pulse diseases, which has the highest chances of reinfecting even after lengthy rotation breaks?
A. Ascochyta.
Which environmental condition favors Phytophthora over Pythium?
B. Warm, saturated soils.
Timing of fungicide application for FHB relates to flowering because:
B. Anthers provide infection sites.
The most diagnostic feature distinguishing smudge from black point is?
B. Amount of surface covered.
Seed treatments are LEAST effective for managing:
A. Loose smut.
SECTION B — Short Answer
Group 11 fungicides can increase DON levels in grains infected with FHB because they inhibit fungal respiration, which stresses the Fusarium fungus and prompts it to produce more DON as a survival response. Additionally, these fungicides do not prevent the infection of florets, allowing the infection to continue unchecked while toxin production escalates.
Early seeding is effective for managing both rusts and aster yellows by helping to avoid peak rust spore arrival and allowing crops to flower earlier, thereby escaping the peak flight times of leafhoppers that transmit phytoplasma.
Two major reasons Aphanomyces root rot management is challenging in Western Canada include:
Oospores of the pathogen can survive for over 20 years in the soil, complicating crop rotation effectiveness.
The absence of resistant lentil varieties hampers management efforts, especially in wet soil conditions that favor zoospore infection.
Bacterial leaf streak (BLS) can be severe even in “clean” fields due to its spread through rain splash, and the bacteria survive on leaf surfaces without needing residual evidence, making hygiene monitoring crucial.
Smuts benefit from systemic seed treatments because they infect plants internally, reaching the pathogen through the seed treatment. In contrast, leaf spots infect externally, making seed treatments ineffective against them.
Environmental conditions can affect fungicide performance such as:
Sclerotinia: Requires humidity for fungicides to work effectively; if the canopy is too dry or if application timing is late, failure occurs.
Barley Leaf Diseases: Depend on leaf coverage for effectiveness; adverse weather can impair coverage and therefore reduce control effectiveness.
Pulse crops can experience perfect inoculation conditions for Ascochyta yet show minimal disease expression due to warm, dry conditions following infection, which inhibit lesion development and allow only latent infections with few visible symptoms.
The increase in Verticillium stripe in recent years, despite plant resistance to blackleg, can be attributed to the presence of tight rotations and high canola frequency, which create heightened selection pressure allowing the pathogen to persist.
SECTION C — Compare/Contrast
Fusarium graminearum (FHB) vs. Fusarium avenaceum (root rots)
Pathogen Biology: F. graminearum produces perithecia and airborne ascospores and infects heads, requiring humidity and warm temperatures, while F. avenaceum is associated with soil infection and does not produce DON.
Environmental Needs: F. graminearum thrives under warm and humid conditions; F. avenaceum prefers cool, wet soils.
Control Strategies: F. graminearum is managed through fungicidal treatments during flowering and crop rotation, while F. avenaceum is more effectively controlled using seed treatments and improved drainage.
Stripe Rust vs. Leaf Rust
Epidemiology: Stripe rust features airborne urediniospores and undergoes annual migration, with quick resistance breakdown; Leaf rust also prefers warm temperatures but has a less explosive nature and a potentially more stable resistance.
Management: Fungicides are effective against stripe rust, whereas leaf rust may require a combination of local overwintering management.
Aster Yellows (phytoplasma) vs. Barley Yellow Dwarf Virus (BYDV)
Vector Biology: Aster yellows is transmitted by leafhoppers while BYDV is spread by aphids.
Disease Symptoms: Aster yellows result in deformities and phyllody while BYDV results in yellowing and stunting.
Management: Aster yellows require early seeding strategies, whereas BYDV can be controlled using resistant varieties and monitoring aphid flights.
SECTION D — Difficult Diagnostics
Take-all Clues:
Black roots.
Whiteheads in the wheat field.
High soil pH (7.8).
Net Blotch Clues:
Lesions with dark margins and surrounding chlorotic halos on barley.
Presence of pseudothecia visible under a hand lens, which manifest predominantly in no-till fields.
Verticillium Stripe Clues:
Observed bronzing on stem surfaces that appear late in the season.
Presence of pepper-like dots within the stem when split, but no breaks in the stem.
Pythium Damping-off Clues:
Seedlings showing damping-off symptoms in cold damp soil conditions.
White, water-soaked lesions present at the soil line in poorly drained fields.
SECTION E — Long Answer
Integrated Disease Management Plan for Wheat with Tan Spot, FHB, and Stripe Rust:
Must include diverse rotation strategies avoiding host crops in consecutive years.
Use of FHB-resistant cultivars alongside APR strategies for stripe rust resistance.
Fungicide applications timed effectively using Group 3 and 7 treatments during flowering, with an emphasis on residue management and seeding practices to optimize growth.
Incorporate environmental monitoring with weather-based spraying decisions.
Residue-borne vs. Windborne Diseases:
Residue-borne pathogens (e.g., tan spot) thrive on remaining crop residues, hence rotating crops directly reduces their inoculum source.
Windborne diseases, like rusts, migrate from external sources and are less affected by crop rotations; disease management must address inoculum sources in other regions.
Management Plan for Canola Field Infected by Multiple Pathogens:
Utilize treated, vigorous seeds, improving drainage systems, and aim for early planting into warm soils to promote good establishment.
Strategies for Sclerotinia, specifically applying fungicides at 10% bloom and ensuring sanitation practices are maintained to reduce incidences of dense canopies.
ANSWER KEY – MOCK EXAM #2 (HARD VERSION)
SECTION A — Multiple Choice
C
B
C
C
A
C
C
B
C
A & B possible; BEST is A (honey-brown roots + no lateral roots = Aphanomyces)
A
B
C
B
C
D
B
B
B
C
SECTION B — Short Answer
Group 11 fungicides inhibit fungal respiration but exacerbate DON production as a survival tactic of Fusarium.
Early seeding reduces infection risks by avoiding peak spore arrival and allows crops to emerge before the peak leafhopper flight for aster yellows.
Aphanomyces root rot management is complicated due to the longevity of its oospores and the lack of resistant variety options.
BLS symptoms can emerge from unnoticed pathogen survival without visible residues.
Smuts are more responsive to systemic treatments due to their internal infection strategy compared to external infection of pathogens like leaf spots.
Fungicide performance is affected by humidity and canopy coverage; Sclerotinia requires moist conditions for fungicide efficacy.
Ascochyta infections may not express visible symptoms due to environmental conditions limiting symptom development following initial infection.
Increased frequency of blackleg in canola leads to greater selection pressure favoring Verticillium persistency.
SECTION C — Compare/Contrast
F. graminearum vs. F. avenaceum:
Pathogen type, environmental needs, and control strategies differ significantly between these pathogenic fungi affecting different developmental parts of the crop.
Contrast between Stripe rust and Leaf Rust focuses on management strategies, resistance durability, and their respective epidemiological behaviors demonstrates nuanced differences between pathogen types.
The vectors, transmission routes, and management strategies for Aster yellows vs. BYDV exhibit how different biological agents require tailored approaches to successful management.
SECTION D — Diagnostics
Take-all symptoms include black roots and patches of whiteheads in areas with high soil pH.
Net Blotch identified by dark lesion margins indicates the need for residue management.
Verticillium Stripe indicated through bronzing on stems and internal microsclerotia.
Pythium Damping-off characterized by water-soaked lesions at the soil line in poorly drained conditions.
SECTION E — Long Answer Points
Must consider diverse rotation, resistant cultivars, timely sprays with groups 3+7, and proactive environmental monitoring.
Residue-borne diseases utilize remaining crop matter for overwintering, making rotations effective, unlike windborne pathogens which travel from afar with minimal impact from the rotational strategies.
Season-long management should include treated seeds, early sowing, Sclerotinia action plans, and optimizing environmental conditions for successful establishment.
Glossary of Key Terms
Anamorph
Asexual stage of a fungus.
Antibiosis
Direct biocontrol mechanism involving antimicrobial compound production.
Apothecia
Fruiting bodies of some ascomycetes that release ascospores in pathogenic cycles.
Blackleg
Disease affecting canola, caused by Leptosphaeria maculans.
Fusarium Head Blight (FHB)
Disease affecting a variety of cereals caused by Fusarium spp. known to produce DON.
Qualitative Resistance
Results from the presence of significant R genes, often not durable across pathogen evolution.
Quantitative Resistance
Stems from numerous minor genes, resistant to a wider array of pathogens and more durable.
Sclerotia
Survival structures formed by fungi, often long-lived, allowing for disease continuation in the absence of hosts.