Downy mildew (Peronosporaceae family)A range of species of fungus, each of which may only have a limited range of hosts, but which manage to infect a very wide range of edible and ornamental plants. The fungus spreads by airborne spores and thrives in conditions when leaves are damp. It is most serious on lettuce, where it can kill whole leaves, and onions, where the leaves die back and the bulbs can rot, preventing storage.
- Discoloured, often yellow (but also brown, purple or pale green) patches on the surface of leaves. Infections on other parts of plants are less common but pea pods are sometimes affected
- Corresponding grey (lettuce), white (brassicas) or purple (legumes) mouldy/furry patch on the underside.
- Weak or stunted growth
- Leaf die-back or fall
Prevention & control
- Grow resistant varieties
- Practise good rotation as spores survive for 5 years or more
- Avoid warm, damp, poorly ventilated conditions:
- avoid over-crowding
- keep weeds under control
- avoid wetting leaves, particularly in the evening when they won't dry out until morning
- Remove and burn infected parts/plants immediately