What should I do about the powdery mildew on my lilacs?

Powdery mildew is a common fungal disease that occurs on many different plants, including common lilacs (Syringa vulgaris). Powdery white spots usually appear first on the lower leaves and quickly move their way upward on the plant until the majority of leaves are covered in white or gray fungal growth by late summer or early fall. Fortunately, powdery mildew is more of a nuisance than a serious health threat, and there are a number of management techniques that can be used to combat the issue.

Planting resistant cultivars is one of the best ways to avoid problems with powdery mildew. A few reliable choices are ‘Charles Joly’, ‘Sensation’, and ‘Old Glory’. Once you have selected resistant plants, make sure your lilacs are planted in a sunny location with good air circulation. This will not only help limit disease issues, but also ensure that your lilacs bloom to their fullest potential. Regular pruning will also increase airflow and reduce disease potential. It is best to prune lilacs shortly after they have flowered. 

Powdery mildew on lilacs is usually only an aesthetic issue. It is often most apparent late in the season, not too long before the shrubs prepare to drop their leaves. Chemical control is not typically necessary from the standpoint of plant health. Several fungicides are available for home gardeners to treat powdery mildew, but they need to be applied at the first sign of disease. Repeated applications may be necessary according to the instructions on the product label.

Powdery mildew overwinters in plant debris. If you notice powdery mildew on the foliage of your lilacs, thoroughly rake up the leaves as they fall and dispose of them to help reduce infection next summer.