Rose downy mildew

What you see

Purple-red to dark brown angular blotches on the upper leaf, bounded by veins, with faint grey down beneath in humid conditions. Massive, sudden leaf drop is the hallmark - a bush can go bare in under a week.

Conditions

Cool nights, warm days and very high humidity - it needs about 85% humidity and free moisture. Worst under glass and in still, sheltered gardens in spring and autumn.

Treatment

Remove fallen and affected leaves immediately. Improve air movement drastically. Copper gives some protection to remaining foliage.

Prevention

Space bushes, prune open centres, water at the base in the morning, and avoid sheltered airless corners.

Months: April, May, September, October, November

Plants

Rose

Remedies

Bordeaux mixture (copper), Mulch

The thing people get wrong

This is routinely misdiagnosed as black spot and treated with the wrong products for weeks. The distinction is the speed and the shape: black spot makes rounded spots with feathery edges and the leaf yellows around them slowly; downy mildew makes angular purple blotches and strips the bush almost overnight. If your rose defoliated in five days, it was not black spot.