Hi Angela,

This may be a really bad case of powdery mildew. See this page:

https://extension.umn.edu/plant-diseases/powdery-mildew-trees-and-shrubs

Sometimes plants have more than one problem. In this very wet summer, many lilacs are getting bacterial blight this year. This usually doesn't kill the plants, but they will look bad for awhile and maybe until next year.

Here's some helpful web pages:

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight

http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

Both of the links above suggest ways to manage blight, including raking up and disposing of fallen leaves and, when it is dry, pruning to open up the plants and allow better air circulation. These same techniques can help prevent powdery mildew.

With these very old shrubs it may be a good idea to do rejuvenation pruning in the spring, before leaves sprout. Removing the oldest stems down to the ground would increase air circulation. But be aware that if you shorten branches in the spring, you'll be cutting off next year's flowers. Lilacs set the buds for their flowers in the previous summer.

https://yardandgarden.extension.iastate.edu/how-to/pruning-large-overgrown-shrubs

Hope this helps.