Powdery mildews are common plant diseases. They all produce a white or greyish, dusty surface coating on leaves, stems, flowers and sometimes fruit.
Each species of powdery mildew usually has a narrow range of plants it will attack, which is made up of one species or a few closely-related plants. So, rose powdery mildew won’t attack clematis, cucurbit powdery mildew won’t attack rhododendrons, for example. This means each one doesn’t spread as far as many people think.
Commonly affected plants include the following.
Rhododendron and azalea
Courgette, cucumber and marrow