Lilac New Growth or Disease?
Genesee County New York
Hello and thank you for contacting us about your lilac. You did not provide a photo of the entire tree, but from your email, it sounds as though part of the tree is healthy while dieback is occurring in other parts.
The irregular growth on the bare branches indicates some problem but I cannot diagnose the cause. These branches are not dead but diseased and dying. You should prune them back to the source of the branch, disinfecting your tools after each cut.
To maintain the health of the tree, mulch with 2 to 3-inches of woodchip mulch, and irrigate deeply once a week in the absence of rain. Long, slow drip irrigation is best. This should moisten the soil 2 inches down across the entire root zone.