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Downy Mildew: how to recognise and treat it

Disease

Downy Mildew — Photo: Christian Hummert (Ixitixel), CC BY-SA 3.0, Wikimedia Commons

Symptoms

Pale green, yellow, purple or brown blotches on the upper side of leaves, often angular where they are bordered by the veins.
A white, grey or purple mould on the underside of the leaf, directly below each blotch, in damp weather.
Badly affected leaves shrivel and turn brown or yellow and fall early; plants become stunted and some may die.

Organic treatment

Pick off and dispose of affected leaves as soon as symptoms appear (burn, bury deeply or use the council green-waste bin), and remove badly affected plants.
Open greenhouse doors and vents to move the air, and water early in the morning so leaves dry quickly.
Space plants more widely and keep weeds down so air can circulate.

Chemical treatment

In the UK no effective fungicides are available to home gardeners; where a copper fungicide is approved for the crop, it can only protect healthy leaves before the disease spreads — follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Prevention

Avoid overhead watering and long periods of wet leaves or high humidity, especially overnight.
Rotate vegetable crops, and do not replant the same ornamental in an affected spot for at least a year.
Grow resistant cultivars where they exist, for example of lettuce and onions; each downy mildew attacks only a narrow group of related plants.

Plants it affects

Sources

Figures checked against these sources.