Crop Protection: Pests, Diseases and Weeds
What you must understand
Know the categories: insect pests (chewing, piercing-sucking, boring), diseases (fungal, bacterial, viral, nematode), weeds (grasses, sedges, broadleaves) and vertebrate pests. Understand the disease triangle - susceptible host, virulent pathogen and favourable environment - and the pest management pyramid: prevention, cultural control, biological control, mechanical control, chemical control. Know integrated pest management (IPM): scouting, economic threshold, selective pesticides, rotation of chemical groups to avoid resistance, and safe use (label, protective clothing, pre-harvest interval).
Recognise key symptoms: fungal - rust, mildew, blight, wilts; bacterial - water-soaked lesions, wilting; viral - mosaic, mottling, stunting, spread by vectors. Know weed competition (early season is most damaging), critical weed-free period, and methods: hand weeding, mechanical, mulching, crop rotation, smother crops, chemical herbicides. Know Striga as a parasitic weed of cereals and sedges as indicators of poor drainage.
Traps in questions
- Viruses are spread by vectors such as aphids and whitefly; spraying the plant does not cure a virus.
- Economic threshold means act before the population causes loss, not at zero pests.
- Repeating the same herbicide group selects resistant weeds.
- Weeds compete hardest early in the season, not at maturity.
- Fungicides protect or control fungi, not bacteria or viruses.
Rule of thumb
Use the disease triangle: remove the susceptible stage, break the pathogen cycle by rotation, or change the environment by spacing and drainage. Control the weakest side.
Worked example
A maize field shows purple-brown streaks and stunting with aphid colonies. Reasoning: the aphids are vectors, and the pattern is viral, so spraying insecticide will not cure infected plants. The answer is to rogue infected plants, control vectors to protect healthy plants, and plant resistant varieties next season.