Mildew on lilacs

Credit: Brand X Pictures/Home and Garden/Alison Miksch

Powdery mildew is a fungal organism (Microsphaera spp.) that appears as a white powdery growth on the leaves of lilacs (Syringa vulgaris). The symptoms tend to get noticed in July on older leaves at the plant’s base, and the fungus eventually spreads throughout the lilac by mid August, continuing until leaf fall in October. Because powdery mildew appears late in the season, it is more of an aesthetic problem than a serious health issue for the plant.

Microsphaera thrives in warm, humid air as opposed to developing as a result of moisture on the leaves. It is spread mainly by wind and can survive in leaf debris from year to year.

To control the fungus, try the following measures:
1. Rake up and dispose of leaves.
2. Ensure good air circulation around and through the lilac canopy. Avoid planting in shaded areas.
3. When watering, ensure deep, even soil penetration
4. Most Syringa vulgaris cultivars are susceptible, but the following Syringa species show some measure of disease resistance: S. diversifolia, S. emodii, S. pubescens ssp. julianae, S. meyeri, S. persica and S. yunnanensis.