I have a row of standard purple lilac bushes in my yard and some on the opposite side of my yard. Their leaves are turning brown. It started on the one side of my yard and now I see some starting on the opposite side. I've had them for many years without issue. What is it and what can I do to stop it from getting worse? They have very dear sentimental value as they came from my grandmother's lilac rows. I'll do anything to save them. There is new growth but even those are starting to get spots. I don't know of anyone in my area to come diagnose/cure them.