Crop Science — Agronomy, Plant Pathology & Entomology
Agronomy Fundamentals
Agronomy is the science of managing field crops for food, feed, fibre, and fuel production.
Soil-Water-Plant Relationship
Soil texture (particle size):
•Clay: < 0.002 mm — highest water holding capacity, sticky, poor drainage
•Silt: 0.002–0.05 mm — moderate properties
•Sand: 0.05–2 mm — low water holding, fast drainage, easy tillage
•Loam: Mix of sand, silt, clay — ideal for most crops
Soil pH and crop preference:
| pH Range | Crops |
|---|
|----------|-------|
| 4.5–5.5 (acidic) | Tea, blueberries, rice (flooded) |
|---|---|
| 5.5–6.5 | Potato, maize, groundnut |
| 6.0–7.0 (optimal for most) | Wheat, soybean, vegetables |
| 7.0–8.5 (alkaline) | Barley, sugarbeet, date palm |
Water holding capacity (WHC): Maximum water a soil can hold against gravity.
•Clay > Loam > Sand (reverse for drainage rate)
•Field capacity: Soil water after gravity drainage
•Permanent wilting point: Water content at which plants wilt permanently (15 bar suction)
•Available water: Field capacity − Permanent wilting point
Major Field Crops — Agronomy
Rice (Oryza sativa)
•Season: Kharif (upland, lowland, deepwater varieties)
•Optimum temperature: 20–35°C
•Water requirement: 1,200–2,000 mm per season
•Major varieties: IR 64, MTU 7029 (Swarna), Pusa Basmati 1, Sona Mahsuri, Jaya
•Key HYV revolution: Green Revolution — IR8 (Miracle Rice, 1966)
•Cultivation: Nursery → transplanting (25–30 days old seedlings) OR direct seeded
•Major diseases: Blast (Magnaporthe oryzae), Brown Spot, Bacterial Leaf Blight (Xanthomonas)
•Major pests: Brown Plant Hopper (BPH), Yellow Stem Borer, Leaf Folder
Wheat (Triticum aestivum)
•Season: Rabi (October–November sowing, March–April harvest)
•Optimum temperature: Cool — 10–15°C growing, 25°C at ripening
•Water requirement: 400–500 mm (5–6 irrigations)
•Varieties: HD 2967, HD 3086, GW 496, PBW 343, Lok 1
•Green Revolution: Norin 10 dwarf genes → Norman Borlaug → Mexican wheats → India's adoption 1966
•Diseases: Rust (stem, leaf, yellow/stripe) — Puccinia spp., Loose smut, Karnal Bunt
•Critical growth stages: Crown Root Initiation (CRI, 21 days), Jointing, Flowering
Cotton (Gossypium)
•Species: G. hirsutum (American upland, 95% of India's cotton), G. arboreum (desi), G. barbadense (extra-long staple)
•Season: Kharif (April–May sowing)
•Temperature: 21–35°C; requires 180–200 frost-free days
•Bt cotton: Transgenic cotton expressing Cry1Ac and Cry2Ab proteins from Bacillus thuringiensis — toxic to bollworms
•Major pest: Bollworms (American, spotted, pink) — Bt cotton controls; sucking pests (whitefly, jassid, thrips) — managed by insecticides
Sugarcane (Saccharum officinarum)
•Perennial crop; ratoon cropping common (3–5 ratoons)
•Temperature: 21–38°C; needs hot dry weather at maturity for sugar accumulation
•Water: Most water-intensive — 1,500–2,500 mm
•Key disease: Red rot (Colletotrichum falcatum) — most destructive in North India
•By-products: Bagasse (fuel, paper), molasses (ethanol, yeast), press mud (compost)
Plant Pathology — Major Diseases
Disease Triangle: Host × Pathogen × Environment
All three must be favourable for disease to occur.
Types of pathogens:
1.Fungi: Largest group. Rust, smut, blight, wilt, rot. Managed by fungicides.
2.Bacteria: Blight, canker, soft rot. Managed by copper fungicides, resistance varieties.
3.Viruses: Mosaic, yellowing, stunting. Transmitted by vectors (aphids, whitefly). No cure — vector management.
4.Nematodes: Root-knot (Meloidogyne), cyst (Heterodera). Managed by nematicides, crop rotation.
5.Phytoplasma: Transmitted by leafhoppers. Little leaf of brinjal, green ear of bajra.
Important Diseases:
| Crop | Disease | Pathogen | Management |
|---|
|------|---------|----------|-----------|
| Wheat | Stem rust | Puccinia graminis tritici | Resistant varieties (Yr32, Sr31) |
|---|---|---|---|
| Wheat | Loose smut | Ustilago tritici | Seed treatment (Carboxin) |
| Rice | Blast | Magnaporthe oryzae | Tricyclazole, resistant varieties |
| Rice | BLB | Xanthomonas oryzae | Copper fungicides, drainage |
| Cotton | Wilt | Fusarium oxysporum | Resistant varieties, soil fumigation |
| Potato | Late blight | Phytophthora infestans | Mancozeb, Metalaxyl (caused Irish famine 1845) |
| Citrus | Canker | Xanthomonas axonopodis | Copper spray, pruning |
Integrated Pest Management (IPM)
IPM principles:
1.Cultural control: Crop rotation, resistant varieties, timing of planting, sanitation
2.Biological control: Natural enemies — predators (ladybird beetle, Chrysoperla), parasitoids (Trichogramma), pathogens (BT, NPV)
3.Physical/mechanical: Light traps, sticky traps, bird perches (T-perches for cotton)
4.Chemical control: Last resort. Selective pesticides, correct dosage, schedule to protect pollinators
Economic Threshold Level (ETL): Pest population density at which control action is justified — damage would exceed cost of control.
Key biological control agents:
•Trichogramma spp.: Egg parasitoid — used against stem borer, bollworm
•BT (Bacillus thuringiensis): Bacterial biopesticide against caterpillars
•Chrysoperla carnea: Predator of aphids, whitefly, mites
•Beauveria bassiana: Fungal biopesticide against sucking pests
Fertiliser Management
NPK — Macronutrients:
•Nitrogen (N): Vegetative growth, chlorophyll, proteins. Deficiency: yellowing (chlorosis) starting from older leaves. Sources: Urea (46% N), DAP, Ammonium Sulphate
•Phosphorus (P): Root development, energy transfer (ATP), flowering. Deficiency: purple coloration. Sources: SSP, DAP, TSP
•Potassium (K): Water regulation, enzyme activation, quality. Deficiency: marginal leaf scorch. Sources: MOP (Muriate of Potash), SOP
Micronutrients: Zinc (most deficient in India — paddy zinc deficiency common), Boron (cotton), Iron, Manganese
Organic manures:
•FYM (Farmyard Manure): 0.5% N, 0.25% P, 0.5% K. Improves soil structure.
•Compost: Higher nutrients than FYM, better microbial activity
•Green manure: Dhaincha (Sesbania), Sunhemp — ploughed in while green. Fixes nitrogen.
•Vermicompost: Earthworm-processed organic matter. 2–3% N. High in growth hormones.
Biofertilisers:
•Rhizobium: Nitrogen fixation in legume root nodules (symbiotic)
•Azotobacter: Free-living N fixation in soil
•Azospirillum: Associative N fixation (wheat, maize)
•Blue-Green Algae (BGA): N fixation in waterlogged paddy
•Mycorrhiza: Phosphorus solubilisation, extended root absorption