Disease
Downy 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
Downy mildew is the name for a group of diseases caused by water moulds, organisms that behave much like fungi. Each species is fussy about its host, so the downy mildew on your lettuces will not spread to your onions or poppies. What they share is a love of cool, humid weather and wet leaves.
Symptoms
Pale green to yellow blotches appear on the upper surface of leaves, often angular because they are limited by the veins. Turn the leaf over and in damp weather you will see a grey, white or purplish fuzzy growth beneath the patches. The blotches turn brown and the leaves die back. On onions, leaves develop pale oval patches with a violet bloom and then collapse. Powdery mildew is different: its white coating sits on the upper side and rubs off easily.
Cause
Spores are carried on air currents and splashed by water, and they need a film of moisture on the leaf to infect. Long dewy nights, mild wet spells and still air in greenhouses or crowded beds all favour it, which is why it often peaks in late summer and autumn. Some species form resting spores in dead leaves that carry the disease over to the next season.
Control
Pick off affected leaves promptly and bin badly infected plants. Thin crowded plants and open up the greenhouse to lower humidity. Water at soil level in the morning so leaves dry before evening. Clear away all debris at the end of the season rather than digging it in.
Prevention
Choose resistant varieties, which exist for lettuce, onions and some basils. Space plants generously, rotate crops and avoid overhead watering. Sow basil and lettuce in batches so a fresh crop is always coming on if one succumbs.
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.