Thanks for your question and pictures. Your lilac is experiencing a blight condition. This is now extremely common in the Metro area. We have been getting numerous questions very similar to yours regarding lilacs.  Some comments:

1). It could be a blight caused by the bacterium Pseudomonas syringae. Among other things, it is characterized by leaf spots that can lead to leaf darkening, leaf wilting, and premature leaf dropping. Fortunately, this is largely a cosmetic condition and unless it occurs over several seasons, there are no adverse effects. It is associated with wet conditions, which certainly describes our current 2024 season. Because this is NOT a fungal pathogen, application of fungicides will do no good. It can be transmitted from one season to the next via leaf debris. To prevent transmission from one season to the next, thorough fall cleanup is essential. See the following for further information and remedial procedures:

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs - bacterial-leaf-spots-and-blight-1156513

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight - :~:text=Management of lilac bacterial blight&text=Select varieties that have tolerance,bleach solution between each cut.

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

https://hort.extension.wisc.edu/articles/bacterial-blight/

https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight

2). It could be a blight caused by either Cercospora or Septoria fungi. The symptoms are small, brown to black spots appearing on lilac leaves. The distinguishing feature of these spots is that they are surrounded by a yellow halo. Inspect your lilac leaves to see if this halo is present. The leaves will also become distorted and drop prematurely. See:

https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pro/PlantDiagnosticClinic/Docs/leaf-spot-on-lilac.pdf

https://yardandgarden.extension.iastate.edu/article/2021/08/summer-foliar-lilac-diseases

3). It really is not important which type of blight exists; it should not be a fatal situation for your lilacs. Do not feel that they must be dug up immediately and discarded. They will recover although their blooming may be delayed by a season or two. However if you are curious as to which type of blight exists, consider submitting plant samples to a diagnostic disease laboratory for an analysis. This would be a definitive way of getting to the bottom of things. Both Wisconsin and Minnesota have excellent labs to do this. Before submitting anything to either of them, call first as to what type of samples should be submitted. Both have a nominal charge for their analysis. See

https://pdc.umn.edu/

https://pddc.wisc.edu/clinic-overview/

4). There is not much you can presently do to make your lilacs attain a normal appearance. Your efforts must be more proactive, thinking ahead to the 2025 season to prevent a recurrence of this blight. Besides cleaning up and removing all leaf debris at this time (late July 2024), you could cut out a few stems to open things up. Do this cutting just above ground level. The objective is to improve air movement and circulation throughout the bush. This will help in decreasing the spread of the disease.

5). The sites previously mentioned will describe further proactive approaches. In short, they are:

a). Remove and destroy infected plant parts, including fallen leaves and pruned branches, to reduce further spread of the bacteria or fungus. Lilacs are tough; they should withstand this removal.

b). Prune the lilacs to improve air circulation and remove diseased branches. Make cuts well below the visible symptoms and disinfect (solution of one part commercial bleach to nine parts water) pruning tools between cuts.

c). Avoid overhead watering to minimize leaf wetness and reduce the spread of bacteria. Water at the base of the plants.

d). Going into the fall, keep your lilacs well-watered and be sure to do a thorough cleanup of all plant debris.

Good Luck. Please get back to me if you have further related questions. Thanks for consulting us.

Thanks for all of this information! Now that another 6+ weeks have passed, my lilac has dropped nearly all of its leaves. I saw some sources recommend pruning below the diseased spot, but since my entire plant was thoroughly affected, that would mean essentially cutting it to the ground. Is something like that necessary, or should I just rake up the leaf debris and see how it does next spring?

Thanks for your update.

There are sources out there – you saw some of these – that suggest rejuvenation pruning. This essentially consists of removing about 1/3 of the branches down to soil level. This is an extreme procedure that is usually employed for lilacs that are large, gnarly, and have not been recently pruned. This is normally done in the early spring but not at this time of the year (mid-August 2024).

As I previously suggested, your focus now is to prevent a recurrence of this blight in 2025. Let me be more specific about some of my previous points:

1). Continue to remove dead leaves from the lilacs. On a regular basis, collect fallen leaves. Combine all leaves in a bag and put into the trash. Do this as frequently as possible. What you will be trying to do is preventing blight-causing bacteria and/or fungi on the leaves from entering the soil.

2). It looks as if you do not have any mulch around the lilac. This is not good. This means that whenever it rains, soil particles could splash up onto the lilac’s leaves. This is often how a blight can start. I would suggest that you put about 3 inches of wood mulch all around the lilac out to a distance of at least three feet from the lilac base. You may experience slight difficulty in doing this by the fence. In that case, put the mulch down out to the fence line. This is something you should do yet this fall.

3). On a very selective basis now (mid-August 2024), prune out a few very large branches to “open up” the interior of the lilac. Related to this, prune the ends of the branches close to the fence. You should have at least 18 inches of open space between the lilac and the fence. These efforts will be attempts to increase air circulation both in the lilac’s interior and in the space surrounding the lilac.  Be aware that this pruning may reduce 2025 flowering to some extent.

4). Keep watering on a regular basis (at least twice a week) up until the ground freezes.

5). Next spring (2025), monitor closely the formation of leaf buds on the different branches. My expectation is that you will see many of the branches now having dead leaves developing new, green leaves.

6). Also next spring (2025), be alert to any branches NOT having any leaf buds. These branches could be pruned out at that time.

Good luck. Please get back to us if you have additional questions. Thanks for using our forum.