Plant Pathology & Entomology — Advanced
The Disease Triangle
Plant disease only occurs when three conditions align: a susceptible host, a virulent pathogen, and a favourable environment. Removing any one vertex of the triangle breaks the disease cycle — this is the conceptual basis for most IPM strategy.
Major Crop Diseases — Causal Agents and Management
| Disease | Crop | Causal Agent | Type | Key Management |
|---|
|---|---|---|---|---|
| Blast | Rice | *Magnaporthe oryzae* | Fungal | Resistant varieties, tricyclazole |
|---|---|---|---|---|
| Bacterial leaf blight | Rice | Xanthomonas oryzae | Bacterial | Resistant varieties, balanced N |
| Rust (yellow/brown/black) | Wheat | Puccinia spp. | Fungal | Resistant varieties, propiconazole |
| Late blight | Potato | Phytophthora infestans | Oomycete | Mancozeb, resistant varieties |
| Wilt | Cotton/Pulses | Fusarium oxysporum | Fungal | Crop rotation, seed treatment |
| Red rot | Sugarcane | Colletotrichum falcatum | Fungal | Disease-free setts, resistant varieties |
Entomology — Major Insect Pest Groups
•Sucking pests: aphids, jassids, whiteflies, thrips — damage by sap withdrawal and act as virus vectors (e.g., whitefly transmitting cotton leaf curl virus).
•Borers: stem borer (rice), pink bollworm (cotton), fruit and shoot borer (brinjal) — larvae feed internally, making chemical control timing-critical.
•Defoliators: armyworm, cutworm, leaf-eating caterpillars — economically damaging in outbreak years, often correlated with weather.
•Storage pests: rice weevil, pulse beetle — post-harvest losses can rival field losses if storage isn't managed.
Integrated Pest Management (IPM) — Applied Hierarchy
1.Cultural control — crop rotation, resistant varieties, adjusted sowing dates to avoid pest-favourable periods.
2.Mechanical/physical control — pheromone traps, light traps, hand-picking for low-density infestations.
3.Biological control — Trichogramma egg parasitoids, Bacillus thuringiensis (Bt), predatory coccinellids for aphid control.
4.Chemical control — used only after crossing the Economic Threshold Level (ETL), and rotated by mode of action to delay resistance.
Economic Threshold Level (ETL) — Why It Matters
ETL is the pest density at which control action must be taken to prevent losses exceeding the cost of control. Spraying below ETL wastes money and accelerates resistance; the exam-relevant point is that IPM is an economic decision framework, not a zero-tolerance policy for pests.