Thanks for your question.

Thanks for your question and pictures. Your lilac is experiencing a blight condition that seems to have persisted for a few years.. This is now extremely common in the Metro area. 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. This is probably why your fungicide treatment in 2023 did not produce satisfactory results. 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. Given the ineffectiveness of your 2023 fungicide treatment, I suspect that you do not have fungal blight. 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. You will need to be patient! 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 (early August 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.

With respect to cutting everything back, I believe that is a bit too extreme at this point. I would suggest instead that you follow the steps that I have described. Alternatively, if your lilacs are quite old and you have not done annual pruning on them, consider rejuvenation pruning early in spring 2025, before the leaves sprout. If possible, attempt to do this in late March to early April. Remove the oldest stems down to the ground but not for more than 1/3 of the stems. If you do this, be prepared for a reduction in the amount of flowering you’ll see in 2025. Continue this 1/3 removal in early spring 2026 and early 2027. After that point, you will have essentially removed most of the stems you are now seeing in 2024. See:

https://yardandgarden.extension.iastate.edu/how-to/pruning-large-overgrown-shrubs

https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs#hedges-and-shrubs-1336763

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