Removal of infected plant material

With rapid-increase problems, such as peach-potato aphid and white rust of chrysanthemum fungus in greenhouses, removal of affected leaves is practicable in the early stages of the problem, but becomes progressively unmanageable after the pest or disease has increased and dispersed throughout the plants. Slow-increase problems, such as Fusarium wilt disease on tomatoes or carnations and vine weevil larvae found damaging roots of plants such as primulas and begonias, may be removed throughout the crop cycle, but the infected roots and soil must be carefully placed in a bag to prevent dispersal of the problem. In commercial outdoor production, labour costs usually prevent such removal during the growing season. However, removal is achieved chemically in some situations. The destruction of blight-infected potato foliage with herbicide such as diquat prior to harvest reduces infection of the tubers. Burning of post-harvest leaf material and lifting of root debris after harvest (against grey mould on strawberries and club root on brassicas respectively) may help prevent problems in the next crop.

In fruit tree species such as apple, routine pruning operations may remove serious pests such as fruit tree red spider mite, and diseases such as canker and powdery mildew. Pruning should also aim to reduce the density of shoots in the centre of the tree. The reduction in humidity provides a microenvironment less favourable to disease increase.

Tree stumps harbouring serious underground diseases such as honey fungus should be removed manually or using a mechanical stump grinder. Making a feature of an infected stump by placing a bird table on it is one of the least recommended activities in gardening.

Whiteflies Apple Trees
Figure 16.4 A sowthistle acting as alternate host to glasshouse whitefly

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