Lilacs are currently losing their leaves due to a disease called lilac leaf blight, caused by the fungus Pseudocercospora spp. Common throughout Vermont and the Northeast region, this disease also occurred in 2024 because of the wet weather in the spring and early summer.

The fungus overwinters on the fallen leaves and infects the emerging leaves in the spring during wet weather. As wet weather continues, more leaves become infected. Lilac leaf blight symptoms start as brown spots on the foliage that begin at the leaf edges.

As the infection progresses, the spots grow together and cause leaves to curl and eventually drop. The fungus is favored by rainy or humid weather, and I suspect the recent hot dry weather is accelerating the defoliation.

Although it may look like your lilacs are dying, if you scratch just under the bark near the end of a twig and still see green, and if the terminal buds for next year are obvious, the plant will be fine in the spring. Over several years of defoliation, however, the plant may be weakened.

Anything you can do to remove further stress in the bush this season will be helpful, such as watering at the base of the plant during hot dry weather and mulching plants to conserve water.

Rake and destroy the infected fallen leaves this autumn, and thin the plant during the winter to improve air circulation and light penetration. Be sure plants are not crowded.

Fertilizing lightly in early spring may help improve vigor, especially if the plant has been defoliated from the disease over the past two years. Fungicides can be used as the leaves are emerging and expanding in the spring and early summer but are not typically warranted or necessary. Fungicides at this point in the season would have no effect on the disease. 

If questions about this or other disease and insect pest problems, you can contact me, Ann Hazelrigg, director of the University of Vermont Plant Diagnostic Clinic at ann.hazelrigg@uvm.edu.