Plant Protection: Pests, Diseases and Weeds
Core concepts
Learn to distinguish pests (insects, mites, molluscs, nematodes), diseases (fungi, bacteria, viruses) and weeds. Know signs versus symptoms: signs are visible pest or pathogen structures such as webbing or mould; symptoms are plant responses such as yellowing, wilting or lesions. Understand the disease triangle: a disease develops when a susceptible host, a virulent pathogen and favourable environment coincide.
Know control methods and integrate them. Cultural control includes rotation, sanitation, resistant cultivars and correct spacing. Physical control includes barriers, traps and hand removal. Biological control uses natural enemies. Chemical control uses pesticides, which must be used according to label instructions. Understand integrated pest management (IPM): monitor, identify, set thresholds, and use the least harmful effective method first.
Traps in questions
A common trap is treating a virus with fungicide; viruses are not controlled by fungicides. Another is assuming any spotted leaf is fungal; nutrient problems and spray damage can mimic disease. Candidates also forget that broad-spectrum pesticides can kill natural enemies and worsen pests.
Worked example
A crop shows mottled, distorted leaves and stunted growth with no mould. This suggests a virus, spread by vectors such as aphids. The correct approach is to identify, remove infected plants, and control vectors, not to spray fungicide. Rule of thumb: identify before you treat.