Hello,

Common lilacs, (Syringa vulgaris), are susceptible to many diseases including bacterial blight, leaf spots, leaf blight, powdery mildew and there are many insects that feed on its leaves such as lilac borer, lilac miner, caterpillars and scale. Also, lilacs prefer loose well-drained slightly acid soil. Read more here: https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=253923&isprofile=0&

https://extension.psu.edu/lilac-diseases

And here: https://extension.umn.edu/trees-and-shrubs/lilacs#common-problems-1923161

Powdery mildew is present on some of the leaves on your tree. Read about powdery mildew and how to manage it here: https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs

There may be environmental factors that caused the sudden change in your lilacs. Were herbicides sprayed nearby recently? Some herbicides contain a chemical Imazapyr which is water soluble and can affect woody plants. Or it could simply be caused by the drift of the spray onto the lilacs.

It is, therefore, difficult to diagnose the problem from the picture. A proper analysis could be done at the MSU Extension Plant and Pest Diagnostic Lab. To learn how to submit a sample, go to: https://www.canr.msu.edu/pestid/ The diagnosis will include how to treat the problem.

I hope that this helps you with your lilacs.

A certified arborist could assess the lilac. To find an arborist in your area, go to www.treesaregood.org the website of the International Society of Arboriculture.

Thank you for contacting Ask Extension.