Hi Abigail, 

Thank you for reaching out to us at the UNH Extension Infoline. This has unfortunately been a hot topic here at the Infoline over the last two weeks. It is difficult to clearly identify the sole problem but it is possible that the lilacs have Lilac Bacterial Blight. This is a useful resource for this: http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

Make sure that if you fertilize your lilacs, don’t overfertilize and stop fertilizing all together around mid-summer because elevated levels of nitrogen in the soil can lead to more favorable conditions for the disease. We recommend testing the soil to check the pH and nutrient balance. The UNH Extension also has these services for $20 per test and you can find the form here.

Infected branches should be pruned 10-12 inches below the visible infection as soon as you see indications of disease. Pruning should be done during dry weather to minimize the chance of spreading the pathogen and if you are pruning your bushes late in the summer, make as few cuts as possible. The act of pruning sends a signal to the plant to grow and at this time of the year that can lead winter damage on tender new shoots. Pruned branches should be destroyed, ideally by burning. Always sterilize pruning tools between cuts to prevent spreading the bacterium to other areas on the tree.

We also are encouraging everyone to submit a sample to the UNH Extension Plant Diagnostic Lab. The fee for this is $20 and will help you get a more definitive determination of the issue and ways to treat it. The submission form with directions is found here. 

Good luck with your lilacs! Please reach out if you have any other questions and have a great weekend!