Have you noticed that many lilac leaves at this time of year are more white than green? 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. Cultivars and varieties are available that have resistance to the disease. On established plantings, it is helpful to remove dead leaves in the fall. This will help reduce the amount overwintering fungal inoculum. 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. When used, fungicides (such Funginex, Cleary's 3336, Strike, or Bayleton) need to be applied when the disease first appears and repeated according to the label.
This article originally appeared in the August 25, 1995 issue, p. 131.