Plant diseases and defences
Biology · GCSE · The School
Plants get diseases too, from similar causes
Plant pathogens include fungi, bacteria, viruses and insects. These are the same broad categories that cause disease in animals. They spread by similar routes too: through soil and water, carried on the wind, or by an insect vector — an aphid, say, carrying a virus from plant to plant.
A worked example: tobacco mosaic virus
Tobacco mosaic virus (TMV) infects many plant species. It produces a "mosaic" pattern of light and dark patches on leaves, and stunts growth. It spreads by direct contact between plants, and on tools. There is no cure once a plant is infected. Control relies entirely on stopping its spread and removing infected plants.
Physical defences: barriers that stop entry
A waxy CUTICLE on the leaf surface stops water and pathogens getting in. Cell walls made of cellulose form a barrier, tougher than an animal cell's membrane alone. Bark on a woody plant is a physical layer against insects and fungi. Some plants even close their stomata when a threat is detected, to limit entry points.
Chemical defences: substances that deter or kill
Many plants produce ANTIBACTERIAL chemicals, which kill or inhibit bacteria directly. Others produce POISONS, to deter animals from eating them. Some substances simply taste unpleasant. Tannins, found in tea and many plant tissues, deter insect and animal feeding. These are the plant's own version of an immune response — produced, not triggered from outside.
Reading symptoms to identify a problem
Stunted growth can mean a nutrient deficiency, a virus, or pest damage to the roots. Spots on leaves usually mean a fungal or bacterial infection. Yellowing can mean a mineral deficiency instead of disease — nitrogen deficiency specifically causes it. Malformed growth or holes usually mean an insect pest. Telling these apart from symptoms alone is a real, examined skill.
Why identification has to come before treatment
Fungicide does nothing for a nutrient deficiency. Fertiliser can make a fungal infection worse, by encouraging soft, disease-prone growth. So the identification has to come first. Is it disease, deficiency, or pest damage? Reading the PATTERN of symptoms, not just noticing they exist, is what decides which treatment will actually help.