There are upwards of 300 species of fungi known to cause powdery mildew over a wide range of host plants. Most species of woody shrubs and trees can be infected. Some species of powdery mildew fungi infect only a few closely related hosts, whereas others have a broad host range.
Powdery mildews are first visible on leaves as a white or dusty grey patches. Flowers and fruit can also be infected. Symptoms appear about midsummer and progress to completely cover the leaves by fall. The overall affect of the powdery mildew can be superficial as is the case in Lilac. There are cases where it can become quite severe distorting, dwarfing and discoloring leaves. Oak can become heavily affected. Early leaf loss can occur as a result of the infection.
Powdery mildew fungi survive the winter on plant tissue and mycelium. Spores are released from the fruiting bodies in the wet spring weather and migrate to unaffected tissue via water or wind. Cycles of infection continue as the days remain warm and dry and the nights are cool and damp.
Most powdery mildew do not have much of an affect on tree health, there are times where some intervention is needed, especially after a plant has had repeated infections. Control can generally be handled culturally. Proper placement of the plants can make a difference. Dense, shady or damp areas favor disease development. Removal of dead leaves around the plants in the fall can really help further inoculation of the plant. . Be sure to avoid excessive fertilization which encourages new growth which tends to be more susceptible to infection. Planting disease resistant varieties is perhaps the best control method out there.
Fungicides can be use to prevent infection, however they must be applied at the first signs of infection otherwise it is just too late.