Hello there, I have a lilac bush, approximately 8 years old, full sun, had blooms this past spring. I recently noticed that some of the leaves are turning dark purple (not brown), not falling off, leaf color is sporadic but becoming more prevalent. Never had this happen before, should I be concerned?

Some stores carry a soil test for NPK and another for soil pH; ck the soil with them. If phosphorus is deficient –possible in sandy soils or if conditions make it difficult for plants to absorb the phosphorus that is there– the older leaves may start turning a dull, dark green, which then turns to purple. Lilacs prefer alkaline soils but not too alkaline.

It's not pathogenic (bugs or disease). That kind of purpling is more often due to weather stress. I wouldn't be concerned.