What is the grayish white growth on my lilac trees?
Question:
I have two 20-year-old lilac trees that have a gray-white fungus all over it’s bark, both old and new growth. It is silky to touch. I have taken out some dead branches and it seems as though some of the foliage is dying in places. Is this something I should be concerned about? Is there a remedy?
Answer:
When I initially read your question, I assumed the problem would be powdery mildew (a very common lilac ailment), but the photos didn’t bear that out–the growth is too “fluffy” and it would be very strange for powdery mildew to be affecting your stem this much without affecting the leaves. So, the two hypotheses I have going at the moment are prunicola scale (I’m leaning toward this, you can see an affected lilac here) or woolly aphids (possible, but less likely). I’m betting if you get a very close look at it, and possibly some photos, you will be able to see the insects and that will help narrow it down to which of these you have. Both pests would be feeding on the sap of the lilac, diminishing its overall vigor, and causing the foliage dieback at the top.
If it’s prunicola, you’ll see in that parenthetical link that there are a couple of options for treatment. I would first try to simply remove the insects with a scrub brush and follow it up with horticultural oil. If that doesn’t seriously mitigate the problem, you could consider consulting a ME-licensed pesticide applicator for other options, but you could also give the shrub a hard cut back, taking out the affected stems at the base–lilac is a perennial with a vigorous crown, so as long as it’s healthy it will sprout new growth next season. You might want to do this anyway for the ones that are exhibiting dieback in the foliage, as the insects may have irreversibly compromised those particular stems. I don’t think you’ll need to remove the whole bush, and I’m optimistic that it will recover after treatment even if you have to give it a serious hard pruning–I had a snowfall flatten and kill basically the entire above ground part of a beautiful lilac I have, and while it’s taken several years to recover, it now looks about the same as before.
I will also mention that the lilac would benefit from pruning–you don’t want to trim the branch tips, because you’ll be removing flower buds that are already setting now for next season (not the end of the world, but you’d have reduced bloom for a season), but you do want to thin the density in the middle by removing up to a third or so of the older stems right at the base. Good air movement through the base of the lilac mitigates all sorts of fungal and pest conditions that prefer close, moist, still area in overgrown shrubs, and as mentioned above removing some stems will promote new growth from the base. It’s possible that overgrowth has stressed the plant, as both scale and aphids are drawn to trees and shrubs that are stressed.
Happy gardening.