Powdery mildew: how to recognise and treat it

Disease

Symptoms

White, powdery patches on the upper or lower leaf surfaces, shoots, flowers and sometimes fruit; they spread and merge until whole leaves are covered.
Leaves may twist or become distorted, or turn yellow, die and drop, exposing fruit to sunburn; on tomatoes, peppers and artichokes one type causes yellow patches with little white growth.
Badly infected plants give lower yields over a shorter season, and fruit may have little flavour.

Prune out infected shoots promptly and destroy fallen infected leaves in autumn.
On susceptible plants such as courgettes, squash and melons, spray wettable sulphur before or at the first sign of disease, every 7–10 days while conditions favour it; not above about 32 °C and not within 2 weeks of an oil spray (sulphur can damage some squash and melon varieties).
For mild infections, horticultural oil or neem oil; potassium bicarbonate also kills the fungus but can scorch badly infected plants; Bacillus subtilis products help prevent infection but work less well.

Myclobutanil is sold for home gardens as a protectant and eradicant; check that the label lists your plant (especially edibles) and follow its dose and pre-harvest interval. The RHS advises gardeners not to use fungicides at all.

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

Choose resistant varieties where offered (cucumbers, melons, squash, pumpkins, peas, cabbages and many ornamentals).
Plant in full sun, away from still air against walls and fences; space plants out and prune shrubs to an open shape.
Water regularly in dry spells and mulch to hold moisture — drought-stressed plants are more susceptible.
Do not overfeed with nitrogen; soft, lush growth is especially susceptible.

Figures checked against these sources.