Powdery Mildew on Lilac
The sight of a lilac bush laden with flowers is a delight to the eyes, and just the fragrance of lilac on a spring breeze is enough to warm our hearts and put a smile on our faces. But that smile can quickly turn to a frown when we first see a white powdery substance growing on the upper leaf surfaces of the lower leaves. These leaves and then others, may become twisted, distorted, then wilt and die; they have been infected with powdery mildew.
Powdery mildew is favored by high relative humidity at night (which favors fungal spore formation), low relative humidity during the day (which favors spore dispersal), and temperatures of 70 to 80 degrees. Powdery mildews are parasitic fungi that can only utilize the nutrients of a live host plant. Although the powdery mildews seldom kill their hosts, the fungi reduce the amount of photosynthesis taking place, increase respiration and transpiration, and cause slower growth.
If powdery mildew occurs in the late summer or fall, it is usually not necessary to apply a fungicide, since the plant will have stored sufficient energy to flower and put on foliage the following spring.
But when powdery mildew attacks in the spring or early summer, it may be necessary to spray an appropriate fungicide in order to control the disease.
A fungicide can be applied as soon as the first symptoms appear, with follow-up sprays every 7 to 14 days while conditions are favorable for growth and spread of this disease. Another approach is to use antitranspirants, which can be applied to the leaves to prevent infection for up to 30 days. When applying antitranspirants or fungicides, read the label carefully to make sure you are applying the right product at the right time and under the right environmental conditions.
It is important to properly space lilac plantings so as to maximize air circulation. Avoid planting lilac in very protected areas, a walled corner, for instance, where airflow would be reduced. To prevent the occurrence of powdery mildew, prune lilacs regularly to promote good air circulation. The Extension Office can send you information on proper pruning techniques for lilac, techniques that will not only increase air circulation in the lilac, but will help you create the conditions in which the plant can continually rejuvenate itself and delight our senses for many springs to come.