The leaves on my lilac are covered with a white substance. Is this a serious problem?
The white substance is likely powdery mildew. Powdery mildew is a fungal disease. It appears as a white, dusty growth on plant foliage. In the home landscape, powdery mildew is commonly found on lilac, phlox, monarda, zinnia, and turfgrass. Amongst lilacs, the common lilac (Syringa vulgaris) is highly susceptible to powdery mildew, while the Preston lilacs (Syringa x prestoniae) and dwarf Korean lilac (Syringa meyeri) are resistant.
Powdery mildew is favored by high humidity, cool nights, and warm days. Plants growing in partial to heavy shade are most susceptible to powdery mildew.
Powdery mildew does not cause serious harm to lilacs. The damage is mainly aesthetic. Spraying with a fungicide is not warranted. When planting lilacs, select a site that receives at least 6 hours of direct sun each day. Powdery mildew will not be a serious problem in sunny areas. Judicious pruning of nearby trees (to increase the amount of sunlight) should help reduce the severity of powdery mildew on lilacs growing in shady locations. Another option would be to transplant the lilacs to a sunny site.