Powdery mildew

There are many different species of the fungal disease powdery mildew, and each species attacks a variety of different plants. Unlike many other fungal diseases, powdery mildew does not require moisture to infect plants. It also survives well in warmth. This means that it can infect your plants under a wide variety of conditions.
When the fungus begins to take over one of your plants, the mildew that forms is made up of many spores. These spores carry the infection to other plants through the wind. Powdery mildew can slow down the growth of your plant. In some cases, if the infection is severe enough, powdery mildew can kill your plants. 

Identification
Plants infected with powdery mildew look as if they have been dusted with flour.
Powdery mildew usually starts off as circular, powdery white spots, which can appear on leaves, stems, and sometimes fruit.
Powdery mildew usually covers the upper part of the leaves and affects the older leaves first; the leaves turn yellow and dry out.
The fungus might cause some leaves to twist, break, or become distorted.
The white spots of powdery mildew will spread to cover most of the leaves or affected areas.
The leaves, buds, and growing tips will become distorted as well. These symptoms usually appear late in the growing season.