Disease
Powdery mildew
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September | Watch for in October |
What it is
Powdery mildew is the name for a large group of fungi that live on the surface of leaves rather than inside them. Each species tends to stick to its own group of hosts, so the mildew on your courgettes will not jump to your roses. It rarely kills an established plant outright, but it weakens growth, spoils the look of ornamentals and can cut yields of peas, squashes and apples noticeably in a dry summer.
Symptoms
Look for white or pale grey patches that look as if someone has dusted the leaves with talc. They usually start as small spots on the upper surface and spread until the whole leaf is coated. Badly affected leaves yellow, curl, turn brown and drop early; new shoots can be stunted or twisted, and apple blossom and rose buds may fail to open properly. Unlike downy mildew, the growth sits on top and rubs off easily with a finger.
Cause
Unusually for a fungus, powdery mildew does not need wet leaves to infect. It does best when plants are short of water at the roots while the air around them stays warm and humid, which is why it flares up from midsummer onwards, against walls, in crowded borders and on pot-grown plants. Spores blow in on the wind, and many types overwinter on fallen leaves or in dormant buds.
Control
Pick off the first infected leaves as soon as you spot them and bin them rather than composting. Give the plant a deep soak at the base and a mulch so the roots stay moist, and thin crowded stems to let air through. On courgettes and squashes, late-season mildew is often just a sign the plant is ageing and can be tolerated. A fungicide is rarely worth it in a garden; if you do use one, treat it as a last resort and follow the label.
Prevention
Choose varieties described as mildew resistant, which are widely available for phlox, asters, courgettes, peas and apples. Space plants generously, avoid planting thirsty perennials in dry rain shadows, and water in the morning during dry spells. Go easy on high-nitrogen feeds, which produce the soft, sappy growth the fungus likes best.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: May, June, July, August, September and October.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.