Q. We moved into our house three years ago. The property has a row of lilacs across the back that I just love. But every summer the leaves get this white film on them and they start to look pretty bad. Is this some kind of blight, and can I do anything about it?
A. Common lilac (Syringa vulgaris) is very susceptible to a fungal disease called powdery mildew. It generally appears mid- to late summer as white, powdery patches on the leaves of susceptible plants. Infected leaves may be stunted, turn yellow, curl and drop prematurely. The good news is that powdery mildew is more of an aesthetic problem than truly damaging to your lilacs.
High humidity at night creates a favorable environment for spore formation, and low humidity during the day allows those spores to be blown from host to host. Temperatures between 70 and 80 degrees are ideal for disease development.
Cultural controls include making sure lilacs are exposed to full sun and good air circulation. If the shrubs are planted too close together, you might consider removing every other one to allow good air circulation. Pruning the largest, oldest stems out at ground level reduces the overall size of lilacs and pushes a flush of young stems that will flower better and at a more appropriate height for cutting. Thin the young stems to the sturdiest, most vigorous ones and make sure they are spaced to allow air circulation in the center of the shrub. Rake up and destroy fallen leaves in fall to reduce the amount of spores overwintering near susceptible plants.
There is evidence that powdery mildew is more common on drought-stressed lilacs, so providing supplemental water when the weather is hot and dry can be helpful. Fertilize very moderately, only based on soil test results. Fertilization pushes succulent growth that is more susceptible to infection. Fungicide applications are rarely warranted because the disease is not life-threatening.
Should you choose to plant lilacs in the future, consider planting resistant species such as dwarf Korean (Syringa meyeri), Manchurian (S.pubescens subsp. patula 'Miss Kim') and littleleaf lilac (S.pubescens subsp. microphylla).
Send questions to Sandy Feather by -mail at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.