Lilacs seem to have had a rough year, as we have seen numerous examples of premature leaf loss, leaf damage, and occasional dieback. Stressed plants and those suffering from infections will shed leaves early, though in and of itself early leaf shed this time of year is not a concern and won't detriment the plant.

Multiple factors have played a role in this, from several late-spring frosts (damaging tender growth which had already broken dormancy) to various pathogens that commonly infect vulnerable lilac tissues, especially in cooler and wetter spring weather. Bacteria and fungi can cause leaf symptoms that occur sometimes months past infection, after the cumulative stresses of hot and dry summer weather push the tissues to their tipping point. Once symptoms manifest, treatment with a fungicide spray is impractical at best and ineffective at worst. To encourage overall plant vigor so they can rebound when ready, make sure Lilacs are sited in full sun (6+ hours daily in summer), have their oldest, thickest stems removed entirely every few years, and receive good air circulation by not being over-crowded between other large shrubs or their own dense branching.

Drastic pruning in winter or after blooming via removal of the oldest stems stimulates the plant to produce new, more-vigorous shoots from the root system which will be a bit more resilient and will flower more profusely as soon as they're old enough. While it's true these newer growths can be a bit more blight-prone, they are needed to rejuvenate older, tired shrubs that otherwise will only continue to decline over time. Preventative fungicide use can help protect new growth, but must be applied early enough in the season (when infection is actually taking place) and repeated as needed (as indicated by the product in question, on it's label instructions). A copper-based product may be the best choice as they have some impact on bacteria, whereas most other fungicides do not. Nursery growers of lilac even start their preventative spraying just before leaves appear, to try to eliminate any spores present on the outside of the branches. Copper sprays have a limit to how many applications can safely be made per year (to avoid toxicity to the plant tissues), so as before, follow label instructions if you try them.

For now, just discard fallen leaves and continue to monitor the root zone for watering needs if we experience dry spells this autumn. (We did last year, for example.) Drought stress can also cause leaf desiccation, usually beginning on the tips or edges. (Frustratingly, the same symptoms can appear on over-watered plants, so this is why checking the soil first is useful.) Here are general tree/shrub watering guidelines: https://extension.umd.edu/hgic/topics/watering-trees-and-shrubs

Here is information on some Lilac ailments:
https://extension.umd.edu/hgic/topics/bacterial-blights-lilac
https://extension.umd.edu/hgic/topics/powdery-mildew-trees-and-shrubs
https://extension.umd.edu/hgic/topics/borers-shrubs


Miri