Just this summer, my two lilac bushes have accrued what appears to be a disease. One bush has always blossomed really well in direct sunshine and the second bush has always been in the shade and has never blossomed in the 20 years I have lived here. Now, the second bush is getting full sun. The leaves on both bushes are turning brown and curling and crumble in your hand like they are dry. As a result, the leaves are falling from the branches and many of the branches are bare. And, yet, new leaves are beginning to grow back in some areas. We've received sufficient amount of rain this summer, so that should not be a factor, right? Do you know what may have caused this to occur, and what proper action should be taken? Thank you in advance for your help.