Thanks for the question.

Yours is neither an easy nor simple question to answer. There are many different diseases that can infect lilacs and yield the symptoms you see. It is very difficult to provide an absolute and accurate answer to your question just from pictures. For a definite answer to your question, the facilities of a plant disease laboratory need to be employed. The University of Minnesota has such a laboratory. See:

https://pdc.umn.edu/

The best I can do is to alert you to a few of the most common lilac diseases.

1). Fungal disease (leaf blight). This results in yellowing and dried leaves. It is often seen on lilacs that have been quite healthy in past years. See the following for more symptoms and remedial procedures:

https://www.startribune.com/what-s-wrong-with-the-lilacs-beloved-plants-in-minnesota-threatened-by-leaf-fungus/572649202/

2). Sun leaf scorch. This can be quite common during hot summers such as we saw in 2021 and in 2022 albeit to a lesser extent. See:

https://pnwhandbooks.org/plantdisease/host-disease/lilac-syringa-spp-leaf-scorch - :~:text=Cause OSU Plant Clinic records,excess salt (from fertilizers).

3). Bacterial leaf blight. This was very common during the 2021 summer and we have also been getting reports of it occurring now in 2022. See;

http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

The following is a good summary of some of these diseases:

https://extension.umn.edu/yard-and-garden-news/lilac-issues-season

I fully realize that my answer may not be completely satisfying to you. Perhaps you were hoping for a single diagnosis and remedial procedure? Unfortunately with lilacs that is simply not possible. If you would like to get greatly clarity on your situation, consider having a professional and licensed arborist do an on-site assessment: See:

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good luck!!



Glad to have been of assistance.