Answer: The whitish appearance is caused by the powdery mildew fungus. The white "powder" is composed of fungal structures (mycelium and spores).
White spots on leaves usually start to develop in mid-summer and enlarge as the summer progresses. By late summer or fall entire leaves may appear white. Also in the fall, tiny specs (cleistothecia) appear on leaves. These fungal structures are especially evident on lower leaf surfaces.
Powdery mildew is seldom serious, causing more of an aesthetic problem than harm to lilac plants. As with many diseases, the best way to control powdery mildew is to prevent its occurrence. Rake up and remove dead leaves in the fall to help reduce the amount of overwintering fungal spores. Also, improve air circulation and sunlight as feasible. Dense, shady, and damp conditions favor disease development.
Because the disease is seldom serious, chemical control measures are not usually warranted.
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