All plants can be affected by viruses, which can cause a variety of symptoms depending on the disease. The leaves of infected plants may become yellow, (either of the whole leaf or in a pattern of blotches or stripes), distorted (curled) and the whole plant may show other growth distortions, such as stunting or abnormalities in flower formation.

Viruses may spread horizontally, (between plants, by wind, insects or basic injury) or vertically (passed down from a parent plant).

Control

As for humans, there is usually no cure for plant viruses, and so prevention is the key. Minimise the risk of horizontal transmission between plants by keeping tools and equipment clean. Greenhouse sap-sucking pests such as aphids are a major carrier of viruses, so introduce suitable pest control measures to reduce their numbers.

Always wash your hands and tools after handling any suspect plants, and never propagate from them. Any plants badly infected by virus should be burned and replacements planted in a new site in the garden. Consider buying certified (virus-free) stock; although these plants may be more expensive to buy, they tend to be more vigorous and well worth any extra money.