Advice
RHS Help & Advice
Lettuce downy mildew (Bremia lactucae)
Symptoms
Pale green or yellow areas develop on the upper leaf surface, later becoming brown and angular as the tissues die. Under humid conditions a whitish mould develops beneath these affected areas.
Severely affected plants also show reduced vigour, and the parts of the leaf affected by downy mildew may eventually become colonised by the grey mould fungus Botrytis cinerea, causing extensive rotting.
Cause
Downy mildew is a fungal disease that can devastate crops of lettuce during prolonged periods of cool, wet weather.
The white mould on the undersurface of the lesions consists of large numbers of spores which are dispersed by wind or water splash to spread the disease. The fungus can also produce long-lived resting spores which contaminate the soil.
Non-chemical control
- Limit damage by increasing space between plants to improve air circulation and reduce humidity. Keep water off the leaves of glasshouse-grown plants.
- Remove and dispose of affected leaves and any severely affected plants.
- Remove all crop debris at the end of the season.
- Rotate crops, so lettuce is not grown on the same land for at least three years.
- Check your seed catalogues and grow resistant varieties such as 'Avondefiance', 'Beatrice', 'Musette' and 'Valmaine'.
Chemical control
Spray plants with mancozeb (Dithane). Ensure that you comply with the harvest interval.