Lettuce downy mildew is a common disease which attacks seedlings in spring and late in the season on mature plants. Attacks can occur on greenhouse crops or plants growing outdoors. This disease is more likely to be seen in wet seasons.
Pale green or yellowish areas develop on the upper leaf surface with corresponding whitish fluffy mould on the underside which sometimes has a purple tinge. The patches turn brown as the leaf quickly dies. The plants are stunted and the leaves die. Remove the affected leaves as soon as the symptoms are first seen or destroy the whole plant if the attack is severe.
Keep water off the leaves of plants grown in the greenhouse and avoid watering late in the day, ventilating whenever possible to ensure good air movement around the plants. Outdoors remove affected plant material and allow as long as possible between lettuce crops.