Hi Alora,

Thank you for contacting the UVM Master Gardener Helpline with the question about your client’s lilac plants and providing photos. We have received similar questions from people throughout the state whose lilacs are exhibiting the same symptoms. It is probably a matter of time before the third plant becomes infected particularly if the weather continues to be hot and humid.

The following information was recently provided by our UVM Plant Pathologist:

“There is a fungal leafspot common in lilacs especially in rainy years: https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-pseudocercospora-leaf-spot.
The fungus is able to survive two years in the plant debris that is scattered on the ground. The best and easiest strategy to reduce the disease is to rake and destroy leaves in the fall and prune to open up the bush to air and light. Fungicides are rarely warranted because buds are all set for next year.”

Additional articles that you may find helpful:

. “What’s wrong with my lilac? Summer Foliar diseases on lilac” published by the University of Illinois Extension: https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac which explains the importance of consistently pruning your bush.

. “Leaf Spot on Lilac” published by the University of Missouri Extension: https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pro/PlantDiagnosticClinic/Docs/leaf-spot-on-lilac.pdf which includes information about managing the disease.

The University of Maine Extension offers this article “Bulletin #2169, Pruning Woody Landscape Plants” which has a link to their video about how to prune your lilac: https://extension.umaine.edu/publications/2169e/

An excellent general webpage for reference: https://extension.umn.edu/trees-and-shrubs/lilacs

I hope this information is helpful. Although the leaves are unsightly, rarely will the fungal leaf spot become severe enough to cause the decline of the plant.