Downy mildew
Yellow or pale green patches on the upper side of the leaves, bounded by the veins, with grey to purplish fuzz underneath. The patches turn brown and the leaf dies; common on lettuce, onion, spinach, cucumber, basil and grapevine.
Downy mildew appears in cool, damp weather when leaves stay wet for a long time, for example from dew or watering in the evening.
What can you do?
- Remove affected leaves or plants straight away and put them in the green waste bin or general waste, not on your compost heap.
- Water in the morning at the base of the plant and keep the leaves dry.
- Space plants more widely and ventilate the greenhouse or polytunnel well so the leaves dry quickly.
Prevention
Choose resistant varieties (there are many for lettuce and basil), rotate crops and don’t sow too densely.
When to watch out: May–September