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Powdery mildew is a fungus that produces airborne spores from infected stems or buds. After overwintering on your plants, the disease is most likely to flare up if the roots are in dry soil and the leaves are in humid air - conditions that are often found when plants are grown near or against a wall.
Solution
Organic
Prune out and bin (don't compost) infected leaves or stems. When buying roses, choose varieties with good resistance to powdery mildew - a reputable grower will be able to advise. Keep the soil around the roots moist at all times. When planting roses, prepare the soil well by digging in generous amounts of well-rotted compost or farmyard manure into the planting hole. For established roses, a thick layer of mulch around the base of the plant will help.
Chemical
Before symptoms appear, apply a fungicide containing flutriafol, myclobutanil or penconazole at the recommended intervals from early spring.
