Susan,

Thank you for your question and for sending photos.

Your lilacs appear to be suffering from bacterial blight caused by Pseudomonas syringae pv. syringae (Pss) which is typically found on the surface of lilac leaves without doing any harm.  Wet weather (and we've had a lot of that this Spring!) can cause the population to increase and infect tender early leaves or leaves that have been damaged by wind, hail, insects, etc. 

Typically, this will not kill an established bush and they recover in the following year.  (Hopefully your bushes were not impacted by fungal diseases that were wide-spread last summer in VT as this could mean they may already be energy depleted.)  

To deal with bacterial blight, you should prune the bushes (see below for details from the University of Wisconsin Extension) and remove the leaves and twigs to reduce the number of bacteria.  General pruning every year or so to improve air flow will also help prevent both bacterial and fungal diseases.

https://hort.extension.wisc.edu/articles/bacterial-blight/
 Management of bacterial blight includes good fall clean up and pruning in the case of branch dieback. Prune four to six inches below obviously dead areas on branches. Be sure to prune only when it is dry, and be sure to disinfest your pruning tools between cuts by dipping them in 10% bleach or (even better) 70% alcohol (e.g., rubbing alcohol). Spray disinfectants can be used as a source of alcohol as well. Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol. If you use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting that can be caused by the use of bleach. You can dispose of branches by burning (where allowed) or burying them.

Lastly, here is a link to an alert our Plant Pathologist, Ann Hazelrigg, sent out last August on other lilac pests to be aware of - pruning is the best way to prevent these as well.  

https://www.uvm.edu/extension/news/lilac-pests

Hi Susan,

It is recommended that you wait until it's dry - I think this makes it less likely for the bacteria to spread and reproduce in these conditions.  It probably only needs to be a few days without rain as long as the ground is dry.

Best,

Andrea