Not all plants are dogged by disease problems – a lot of hassle can be avoided by choosing resistant kinds and encouraging good plant health, and by simply not growing the disease-prone kinds. Although the plant defence mechanisms against pests are perhaps more obvious, the defences against fungi are no less effective, and useful to the easy-care gardener. Defences against fungi are mainly barriers of one kind or another.
Leaves of healthy plants are usually coated with wax that prevents the germination of disease spores. Within the sap, there are substances that reduce the growth of fungi, including the tannin mentioned above. The bark of trees and the thick skin of stems, like the waxy coating of leaves, act as a barrier against fungi.
Plants resist diseases
Plants that are attacked by fungi sometimes react by allowing a section of the leaf around the affected area to die. This causes a leaf spot and, occasionally, the plant can react to a disease by dropping all of its leaves, producing a new crop when the worst of the disease attack is over.
As is the case with pests, strong-growing healthy plants are better able to resist diseases. If plants are growing in suitable conditions of soil, site and climate, they will grow well and better resist disease. For example, during dry weather with growth reduced, plants are less able to withstand mildew, watering helps.
Equally, mildew-prone plants should not be grown in overly dry soil. On the other hand, if plants are growing very rapidly in over-rich soil, their tissues will become soft and watery; they are likely to suffer cankers, grey mould and leaf spot diseases.
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