Powdery mildew

Podosphaera, Erysiphe and Blumeria spp.

also called White mildew, Mildew

A white dusty coating over leaves and shoots when roots are dry but the air is humid. Keep plants watered and mulched, prune for airflow, and remove the worst leaves.

Fungus Disfiguring Airborne sporesSpread by contact Spreads to neighbors

What you'll see

A white to grayish felty bloom sits on the upper leaf surface and rubs off with a finger.
Young shoots, buds and stalks are coated as well as the leaves, and may be distorted.
Affected leaves yellow, curl at the edges and drop early, often from the base of the plant upward.
Fruit and flowers are undersized, and on apples and gooseberries the skin may be russeted.
Late in the season, tiny black specks (the overwintering fruiting bodies) appear in the white patches.

Why it happens

A family of surface-dwelling fungi that feed through short pegs pushed into the outer leaf cells rather than growing inside the plant. Most are fussy about their host, so the mildew on your courgettes is not the same species as the one on the roses. Spores blow in on the wind and germinate without needing a film of water on the leaf.

It thrives in warm, still, humid air over dry roots — a shaded wall, a crowded border, a greenhouse shut up overnight, or a dry spell in mid to late summer. Plants under drought stress, or lush with too much nitrogen, are the first to show it.

Treating it

Water the roots thoroughly and mulch, so the plant stops drought-stressing; much of the damage is water shortage rather than the fungus.
Pick off and bin the worst leaves as they appear, and thin congested growth to open the plant up.
Move pots into brighter, breezier positions and ventilate greenhouses and frames day and night.
On persistent cases a sulfur or potassium bicarbonate spray gives some control, though sulfur scorches gooseberries and some apples.
Accept a degree of it on late-season courgettes and Michaelmas daisies — by then the crop is largely made.

Keeping it away

Choose resistant cultivars, which now exist for courgette, cucumber, apple, gooseberry and phlox.
Space plants at the distance on the label rather than the one that looks generous at planting time.
Mulch in spring while the ground is damp and keep watering steady rather than feast-and-famine.
Avoid heavy nitrogen feeds, which push soft growth that mildew colonizes first.
Clear and bin infected leaves and prunings in autumn instead of composting them.

Who gets it

Almost every garden plant has its own mildew, but courgettes, cucumbers, roses, apples, gooseberries, peas, grapes and phlox suffer most visibly.

and 272 more in the catalog