Good afternoon, I hope that someone can help me understand what's happening to our lilac bushes. You should be able to see that all the leaves browned out and dropped off, so I thought they were all dying for some reason. Yet they seem to be re-leafing now. Do you know what has caused this and what, if anything, can/should be done about it? Thank you, Mike Diegel
Montgomery County Maryland
We have seen multiple examples of very stressed lilacs this year. A few issues in combination are the likely causes for their current condition, and plants will shed leaves early that are either too damaged to function well or for which the plant does not have enough moisture to retain.
Late spring frosts this year damaged tender tissues that had already broken dormancy, facilitating infection from bacterial leaf pathogens. These diseases don't always cause symptoms of infection until later in the season, when summer heat stress worsens for the plant. Powdery mildew is relatively common on lilacs and can cause leaf damage beyond just the white/gray coating on the leaves; the damaged leaf portions can "burn" and crisp after summer heat and drought stress them. In addition, old stems are more attractive to wood-boring moth larvae that target lilacs; they also lose vigor over time and their flowering declines. Finally, many lilacs (unless they are bred for warmer zones) need a long period of winter chilling for the buds to mature properly, so our warmer winters are not so ideal for them in general.
The solution is to remove all old growth by cutting the stems down to ground level - or as low as you can go without cutting too many other stems in the process. This focuses the plant's energies on the young, more vigorous sprouts (both existing and those yet to appear in future springs). You can opt to either remove all old stems at once or do so gradually, taking about a third of them each year until the last are removed the third year. The all-at-once approach will delay flowering until those younger stems mature a few more years, but may avoid future borer infestations. On the other hand, the gradual approach might risk borers (at which point you just cut out that stem anyway) but will allow you to enjoy a few flowers each spring in the meantime while allowing those young stems to mature, so the gap of having no flowering-age stems is minimized. Either way, these cutbacks should take place in spring only.
New growth is, unfortunately, a bit more vulnerable to the bacterial leaf infection, but this trade-off can be mitigated by making sure the plant is not crowded, removing some of the excess stems so airflow between branches is improved. Protective sprays with a copper-based fungicide may help prevent infection, but it will not cure existing infection and must be applied before the leaves emerge as well as followed-up by a few applications afterwards; follow label instructions on the product you choose, and note that there is a limit on how many applications can be made in one year. Weather still has influence over disease development in the end, so some years the protective sprays may not be fully successful; still, disease will hopefully be suppressed where it won't develop into as much of an eyesore or premature leaf loss for the plant.
For right now, nothing needs to be done with regards to any sort of treatment or trimming. You can clean up any fallen leaves that haven't blown away and dispose of them, though this isn't critical; it's more useful in-season as newly-infected leaves may shed when the plant is under stress. To minimize general stress, monitor the plant for watering needs in summer and autumn, soaking well but only as needed when the top several inches of soil has dried somewhat. Plants that flower in the spring start developing their leaf and flower buds now in late summer, and dehydration stress can abort this process, resulting in poorer or weakened bloom and growth the following year. We've had autumn droughts in the past - just last year in fact - so continuing to monitor them until the frosts arrive would be beneficial.
Bacterial infection info: https://extension.umd.edu/hgic/topics/bacterial-blights-lilac
General borer info, with a mention of Lilac borers: https://extension.umd.edu/hgic/topics/borers-shrubs
General powdery mildew info: https://extension.umd.edu/hgic/topics/powdery-mildew-trees-and-shrubs
Watering guidelines for trees and shrubs: https://extension.umd.edu/hgic/topics/watering-trees-and-shrubs