Have a 5-year old Purple Persian Lilac off my patio that's looking bad for some reason. It's taking on this brownish color on many of the leaves (dried out), some of the leaves are dying altogether, and some look just fine. See attached image. The summer has been quite wet, but when it is dry it stays dry for a week. This plant has always done well as is generally very healthy. Any ideas what this may be?

Thanks in advance for you help...