Plant pathology Study Guide

📖 Core Concepts Plant pathology – scientific study of plant diseases caused by pathogens and abiotic stresses. Disease triangle – disease occurs only when susceptible host + pathogen + conducive environment intersect. Pathogen types – fungi, oomycetes, bacteria, viruses (plus viroids, phytoplasmas, nematodes, parasitic plants). Cell‑wall‑degrading enzymes – esterases, glycosyl‑hydrolases, lyases, oxidoreductases that break pectin, cellulose, hemicellulose → provide nutrients & breach barriers. Effectors & toxins – proteins or small molecules secreted to suppress host immunity (effectors) or damage tissue (toxins). Resistance mechanisms – structural barriers, receptor‑mediated signaling, resistance (R) genes, and breeding‑based introgression. Abiotic disorders – drought, frost, nutrient deficiencies, salts, etc.; mimic disease symptoms but have non‑infectious origins. Integrated disease management (IDM) – coordinated use of resistant varieties, accurate diagnostics, cultural practices, and targeted chemicals. --- 📌 Must Remember Disease triangle: all three components must be present; removing any one stops the disease. Gene‑for‑gene (Flor): each plant R gene recognizes a specific pathogen avirulence (Avr) gene. Host‑specific vs. non‑host‑specific toxins: the former affect only certain cultivars, the latter have broad activity. Key enzymes for cell‑wall degradation: Pectinesterase – demethylates pectin. Pectate lyase – cleaves de‑methylated pectin. Pectinases – hydrolyze pectin backbone. Major detection tools: PCR, RT‑PCR, LAMP (DNA/RNA), ELISA (protein). Primary cultural control: crop rotation reduces soil‑borne inoculum. Major epidemiological factors: weather, host density, inoculum level, cultural practices. --- 🔄 Key Processes Infection Cycle (generic) Inoculum lands on host surface. Germination → penetration (via appressoria, wounds, or natural openings). Secretion of cell‑wall‑degrading enzymes → breach barrier. Delivery of effectors/toxins → suppress immunity & cause symptoms. Colonization & sporulation → new inoculum produced. Effector Deployment (Type‑III secretion) Bacterial pathogen assembles needle‑like apparatus → injects effectors directly into host cytoplasm → interferes with signaling or antimicrobial synthesis. Diagnostic Workflow Sample collection → nucleic‑acid extraction → PCR/LAMP (DNA) or RT‑PCR (RNA) → gel/fluorescence readout → confirm pathogen identity. Integrated Management Decision Tree Identify pathogen → check resistant cultivar availability → apply cultural control (crop rotation, sanitation) → if needed, add targeted chemical → monitor with diagnostics → adjust. --- 🔍 Key Comparisons Fungi vs. Oomycetes Cell wall: chitin (fungi) vs. cellulose + β‑glucans (oomycetes). Phylogeny: true fungi (Kingdom Fungi) vs. Stramenopiles (closer to algae). Host‑specific toxin vs. Non‑host‑specific toxin Target range: single susceptible cultivar vs. many plant species. Genetic basis: toxin gene often linked to pathogen race; broad toxins are conserved. Abiotic disorder vs. Biotic disease Cause: environmental/chemical stress vs. living pathogen. Diagnostic clue: uniform pattern, absence of pathogen structures, correlation with weather or soil data. PCR vs. LAMP Equipment: thermocycler required for PCR; simple heat block sufficient for LAMP. Speed: LAMP delivers results in ≤30 min; PCR typically 1–2 h. --- ⚠️ Common Misunderstandings “All wilt is fungal.” – Vascular wilts can be caused by bacteria, nematodes, or viruses. “If symptoms look like drought, it’s abiotic.” – Pathogen‑induced chlorosis can mimic nutrient deficiency; always confirm with a diagnostic test. “Resistant varieties are forever immune.” – Pathogens can evolve new Avr genes, breaking resistance (gene‑for‑gene arms race). “Chemical control alone solves the problem.” – Overreliance leads to resistance; integration with cultural methods is essential. --- 🧠 Mental Models / Intuition Triangle = Switch: Think of the disease triangle as a three‑switch light panel – if any switch is OFF, the light (disease) stays out. Key‑Lock (Gene‑for‑Gene): Plant R gene = lock; pathogen Avr gene = key. Only matching key opens (triggers defense). Wall Breach Analogy: Enzymes = demolition crew, effectors = saboteurs inside the building; both needed for successful invasion. --- 🚩 Exceptions & Edge Cases Latent infections – pathogen present without symptoms until stress triggers disease expression. Mixed infections – two or more pathogen types acting together (e.g., fungal + bacterial) can alter symptomology and management. Partial resistance – quantitative resistance reduces disease severity but does not meet the “immune” criterion. --- 📍 When to Use Which Choose diagnostic method: PCR/RT‑PCR: when high specificity & quantification are needed (lab setting). LAMP: field‑ready, rapid screening, especially for high‑risk crops. ELISA: cost‑effective for large‑scale serological surveys. Select control strategy: Resistant cultivar available? → prioritize genetics. High inoculum pressure & conducive weather? → add targeted fungicide/bactericide. Soil‑borne pathogen & long crop cycle? → implement crop rotation + soil amendments. When to consider abiotic vs. biotic: Sudden, uniform symptoms across a field with recent weather extreme → suspect abiotic. Patchy lesions, presence of spores, or visible vectors → suspect biotic. --- 👀 Patterns to Recognize Spore‑rain pattern → disease spikes after prolonged wet periods (e.g., downy mildew). Ring‑spot lesions → typical of certain fungal toxins. Yellowing from tip to base → often nutrient deficiency or root‑rot (biotic) – check soil moisture and root health. Systemic symptoms following seedling stage → likely seed‑borne pathogen or virus. --- 🗂️ Exam Traps Distractor: “Only the pathogen matters for disease development.” – ignores the essential host and environment components of the triangle. Choice stating “All cell‑wall‑degrading enzymes are pectinases.” – overlooks cellulases, hemicellulases, and ligninases. Option claiming “ELISA detects DNA.” – ELISA detects proteins/antigens, not nucleic acids. Answer that “crop rotation works for all pathogens.” – ineffective against airborne spores or long‑lasting soil inoculum (e.g., some nematodes). Statement that “non‑host‑specific toxins are harmless to non‑plant organisms.” – many have broader toxicity; not a safe assumption. ---