Leaf mould is a fungal disease that affects the foliage of tomatoes, particularly those grown in greenhouses.  Symptoms are sometimes confused with those of other foliar diseases such as grey mould or tomato blight.

The first symptoms of leaf mould can develop in April or May, but it is more usually found in June or July and can develop quite suddenly. You may see the following symptoms:

• Lower leaves are generally attacked first
• Yellow blotches develop on the upper leaf surface
• A pale, greyish-brown mould growth is found on the corresponding lower surface
• Where the disease is severe the mould growth may also be found on the upper surface
• Heavily infected leaves turn brown and shrivel, but do not fall
• The grey mould fungus, Botrytis cinerea, may colonise affected leaves, masking the leaf mould. The spores of grey mould are more powdery, and the fungal growth more fluffy than that of leaf mould
• Flowers and fruit may be attacked, but this is less common than leaf infection
• Affected plants lack vigour and yield poorly

Non-chemical control

• Provide ample ventilation to avoid an excessively moist atmosphere
• Try to avoid wetting the leaves when watering. This is particularly important when watering in the evening, as the leaves may then stay wet throughout the night
• Avoid an over-dry soil or growing medium (which could also lead to other problems such as blossom end rot)
• Try to avoid temperatures above 21°C (70°F)
• Trim off some of the lower leaves once fruit has set to encourage air circulation
• Pick off infected leaves as soon as they are seen, and dispose of affected plants and all debris at the end of the season. Disinfect the greenhouse structure with a product such as Jeyes Fluid (tar oils). Apply according to the manufacturer’s instructions
• Resistant cultivars: Current resistant cultivars include ‘Shirley’ and ‘Cumulus’. However, the cultivars may not be resistant to all genetic strains ('races') of leaf mould, and the resistance may also be prone to breakdown if new races of the fungus emerge

Chemical control

No fungicides are available to home gardeners for the control of tomato leaf mould.

• The leaf mould fungus produces huge numbers of microscopic spores on affected leaves
• These are spread by air currents, insects or on hands or clothing
• They are resistant to dry conditions and may overwinter on leaf debris and greenhouse structures
• Infection and disease development is greater in warmth and high humidity. Under these conditions the disease can spread very rapidly
• Plants that have been weakened as a result of dryness at the roots may also suffer severely
• The fungus is known to exist as a large number of strains or ‘races’ that differ in the tomato cultivars that they can attack