Crop Protection: Pests, Diseases and Weed Management
Questions cover damage diagnosis, IPM principles including economic threshold level, herbicide timing and pesticide safety, plus a calculation of spray concentration.
Core rules
- Pests by mouthpart: chewing (caterpillars, beetles) and sucking (aphids, jassids, whitefly, thrips). Sucking pests cause curling, yellowing and often transmit viruses.
- Diseases: fungal (blight, rust, wilt, mildew), bacterial (blight, canker, leaf spot with water-soaked margins), viral (mosaic, leaf curl, yellow vein - mostly spread by insect vectors).
- Weeds compete most severely during the critical period, usually 30–45 days after sowing. Pre-emergence herbicides (pendimethalin, butachlor) act before weeds emerge; post-emergence herbicides (2,4-D on broadleaf weeds in cereals) act on emerged weeds; glyphosate is non-selective.
- IPM sequence: monitoring, cultural methods, mechanical and physical methods, biological control (Trichogramma, NPV, Bt), then need-based chemicals. ETL is the pest population at which control is economically justified - it is not zero pests.
- Safety: use PPE, observe the waiting period before harvest, respect the label dose and do not repeat one mode of action endlessly.
Worked example
A label recommends 500 g of carbendazim per hectare in 200 litres of water. What is the spray concentration for a knapsack sprayer? Reasoning: concentration = 500 g ÷ 200 L = 2.5 g per litre. For a 16-litre tank, add 16 × 2.5 = 40 g. Fill the tank half with water, add the measured product, then top up, and spray within the correct stage.
Traps that cost marks
- Calling a viral mosaic a fungal disease and spraying a fungicide.
- Using 2,4-D as a general weedkiller in dicot crops - it will injure them.
- Treating ETL as complete elimination of the pest.
- Harvesting before the waiting period ends.