Can you help identify the problem with the lilacs on my property?
Question:
Lilacs commonly suffer foliage fungal infestations, especially in hot, wet summers, and it definitely looks as though that’s what’s going on here. Typically, they are not fatal to the shrub and can be mitigated by adjusting the growing conditions and your management practices; fungicides are rarely needed, except in the cases of severe infection on prized specimens. There isn’t much you can do about the appearance this season, so be thinking ahead to next year.
Because fungus spores proliferate in still, moist air, you want to open up the canopy a bit by removing and disposing of (not composting) badly affected branches, cleaning up any fallen plant debris beneath the lilac (many of these fungal pathogens can overwinter on dead tissue), and make sure you properly prune the shrub. If they haven’t been pruned in some time, the shrubs may benefit from what is known as renewal pruning (learn more here), which is basically removing 1/3 or so of the oldest stems right at the base of the plant over 2-3 seasons–this makes the lilac less dense and encourages the crown to put out new shoots. Please keep in mind that lilacs sets flower buds in the current season for next season’s bloom, so pruning at this point may give you a less than optimal show next season, but after that it will bounce back. If you do any supplemental irrigation for them (i.e., any watering other than rainfall), try to direct the water at the soil beneath to avoid water sitting on the leaves.
You might also consider getting your soil tested through the UMaine Soil Analytical Lab and consulting the best practices management recommendations for lilacs here. These suggestions are aimed at making sure the plant is a healthy and vigorous as possible, which helps it weather these kinds of infections. There is a good further discussion of fungal foliage conditions and their treatment in lilacs in this Q&A from Ask Extension.
If the plants don’t improve using the above advice, or if you remain concerned, you can always contact the UMaine Plant Disease Diagnostic Lab with photos and/or physical samples. While there is a small fee for their service, lab testing is inherently more reliable than visual evaluation.
Happy gardening.