Cucurbit downy mildew

What you see

Angular yellow patches on the upper leaf surface, sharply squared off by the leaf veins so they look like tiles. Turn the leaf over in the early morning and the matching patch underneath is furred with grey-purple down.

Conditions

Needs liquid water on the leaf and mild nights - heavy dew is enough. It does not overwinter in cold ground; it blows in on the wind from warmer regions each year, which is why it arrives suddenly and always in the same week of the season.

Treatment

Remove the worst leaves. Copper protects the leaves that are still clean. Switch to watering the soil at the base early in the day so foliage is dry by evening.

Prevention

Grow resistant varieties. Space widely, trellis the vines vertically so air moves through, and never water overhead in the evening.

Months: June, July, August, September, October

Plants

Cucumber, Marrow / Courgette, Melon, Pumpkin, Watermelon, Butternut squash

Remedies

Bordeaux mixture (copper), Bacillus subtilis biofungicide

The thing people get wrong

The square-edged patches are the whole diagnosis. Powdery mildew makes round white patches you can rub off with a thumb; downy mildew makes angular yellow tiles that stop dead at a vein, and it will kill a cucumber plant in ten days where powdery mildew merely annoys it. They also want opposite weather - so treating one as the other is the classic way to lose a crop.