Disease and Pest Resistance Screening
Distinguish genetic resistance, tolerance, susceptibility, and disease escape through controlled challenge, replication, pathogen identity, and multi-stage evidence.
Distinguish genetic resistance, tolerance, susceptibility, and disease escape through controlled challenge, replication, pathogen identity, and multi-stage evidence.
Core science
Resistance reduces infection, colonization, reproduction, or damage by a defined organism under defined conditions. Tolerance allows acceptable performance despite infection or injury. Escape occurs when a plant avoids exposure or favorable conditions. A plant with no visible disease in one room may be resistant, unexposed, younger, spatially protected, or incorrectly scored.
A resistance screen requires verified pest or pathogen identity, a reproducible challenge, susceptible and resistant checks, adequate inoculum or infestation, randomization, replication, and environmental control. Rate incidence, severity, pathogen reproduction, plant growth, and time. Different isolates can overcome different resistance genes, and development stage can change response. Greenhouse screens and field trials answer complementary questions.
Cannabis powdery mildew research illustrates multiple architectures: a disrupted susceptibility gene near CsMLO1, a dominant PM2 locus on chromosome 9, additional smaller QTL, and diverse germplasm with low disease across years. A marker is useful only after validation in the breeding population and with the relevant pathogen. Strong single-gene resistance can be introgressed, but durability may require combining mechanisms and monitoring pathogen change.
Why this matters in cultivation
- Treat promising resistance as a hypothesis until repeated with verified challenge and progeny. Preserve isolates, check reactions, images, raw scores, and marker data.
Measure and record
Record 1
Record host genotype and stage, pest or pathogen identity to the most reliable taxonomic or isolate level available, challenge method, inoculum or infestation intensity, susceptible and resistant checks, and environmental conditions that influence disease or pest development.
Record 2
Measure incidence, severity, reproduction or population growth, symptom development over time, host damage, and yield or fitness effects. Record escapes, failed inoculations, missing plants, and any treatment or environmental deviation.
Record 3
Repeat promising resistance in additional experiments, isolates, life stages, or environments appropriate to the target risk. Preserve samples or diagnostic evidence when possible so resistance is not inferred from an unverified challenge.
Common misconceptions
Evidence limits and uncertainty
Resistance is specific to the host genotype, pest or pathogen population, isolate, developmental stage, environment, scoring method, and challenge intensity.
Absence of symptoms is only interpretable when exposure was confirmed and susceptible checks demonstrate that the challenge could produce disease or damage.
Check your reasoning
- For "Disease and Pest Resistance Screening", explain the mechanism behind this objective: Distinguish genetic resistance, tolerance, susceptibility, and disease escape through controlled challenge, replication, pathogen identity, and multi-stage evidence. Which observation or measurement would best test whether that mechanism is operating in the real crop?
- A learner claims, "No visible disease proves resistance." Use the lesson’s science and evidence limits to explain why that claim is unreliable, then name one observation or measurement that could separate the competing explanations.
- Applied case — Treat promising resistance as a hypothesis until repeated with verified challenge and progeny. Preserve isolates, check reactions, images, raw scores, and marker data. Build a verification plan using the lesson’s record set (Host IDs and stage; pathogen/pest species and isolate; inoculum or infestation method/dose; checks; environment; randomization/replication; incidence, severity and reproduction over time; yield/tolerance; markers; escapes; repeat environments.). What would you compare before and after the action, and what result would make you revise the original interpretation?
Require lesson-specific evidence, not memorized universal targets. Open the rationales after you have written or discussed your own answer.
Answer rationale 1: Mechanism / workflow rationale
- A strong answer should connect the response to the lesson objective: Distinguish genetic resistance, tolerance, susceptibility, and disease escape through controlled challenge, replication, pathogen identity, and multi-stage evidence.
- Resistance reduces infection, colonization, reproduction, or damage by a defined organism under defined conditions. Tolerance allows acceptable performance despite infection or injury. Escape occurs when a plant avoids exposure or favorable conditions. A plant with no visible disease in one room may be resistant, unexposed, younger, spatially protected, or incorrectly scored.
- A resistance screen requires verified pest or pathogen identity, a reproducible challenge, susceptible and resistant checks, adequate inoculum or infestation, randomization, replication, and environmental control. Rate incidence, severity, pathogen reproduction, plant growth, and time. Different isolates can overcome different resistance genes, and development stage can change response. Greenhouse screens and field trials answer complementary questions.
- The most useful verification evidence includes Record host genotype and stage, pest or pathogen identity to the most reliable taxonomic or isolate level available, challenge method, inoculum or infestation intensity, susceptible and resistant checks, and environmental conditions that influence disease or pest development..
- Keep this limit explicit: Resistance is specific to the host genotype, pest or pathogen population, isolate, developmental stage, environment, scoring method, and challenge intensity.
Answer rationale 2: Misconception rationale
- The shortcut is unreliable because the lesson explicitly teaches a more conditional explanation.
- Representative misconception: No visible disease proves resistance. A plant can escape exposure, receive insufficient inoculum, or encounter an environment unfavorable to disease development.
- Resistance reduces infection, colonization, reproduction, or damage by a defined organism under defined conditions. Tolerance allows acceptable performance despite infection or injury. Escape occurs when a plant avoids exposure or favorable conditions. A plant with no visible disease in one room may be resistant, unexposed, younger, spatially protected, or incorrectly scored.
- A useful discriminator is Measure incidence, severity, reproduction or population growth, symptom development over time, host damage, and yield or fitness effects. Record escapes, failed inoculations, missing plants, and any treatment or environmental deviation..
- Do not overextend the conclusion beyond this limit: Resistance is specific to the host genotype, pest or pathogen population, isolate, developmental stage, environment, scoring method, and challenge intensity.
Answer rationale 3: Applied verification rationale
- In practice: Treat promising resistance as a hypothesis until repeated with verified challenge and progeny. Preserve isolates, check reactions, images, raw scores, and marker data.
- Record before action: Record host genotype and stage, pest or pathogen identity to the most reliable taxonomic or isolate level available, challenge method, inoculum or infestation intensity, susceptible and resistant checks, and environmental conditions that influence disease or pest development..
- Also record: Measure incidence, severity, reproduction or population growth, symptom development over time, host damage, and yield or fitness effects. Record escapes, failed inoculations, missing plants, and any treatment or environmental deviation..
- After the action, repeat the same measurement or observation so the comparison is valid.
- Revise the interpretation if the result conflicts with the lesson limit or the expected response: Resistance is specific to the host genotype, pest or pathogen population, isolate, developmental stage, environment, scoring method, and challenge intensity.
Related lessons
Sources and evidence
- Stack et al. 2024 — CsMLO1 powdery mildew susceptibility locusV20-SRC-026
Large F2 mapping populations, major and minor QTL, and markers; pathogen and population specific.
- Seifi et al. 2025 — PM2 powdery mildew resistance locusV20-SRC-027
Dominant chromosome-9 resistance locus and associated markers; requires background and isolate validation.
- Vignale et al. 2026 — Diverse germplasm screening for powdery mildew resistanceV20-SRC-028
Two-year screening of USDA, breeding, and commercial entries; low severity supports further characterization, not automatic release claims.
- Cannabis disease and resistance source registers from Volumes 15–17V20-SRC-029
Organism identity, screening, epidemiology, and IPM boundaries; internal cross-volume source map.
Internal controlled files
Downloads
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