This looks like a high population of White Prunicola Scale, a sap-sucking insect that prefers to feed on cherrylaurel and lilac, among other shrub species. Scale can be difficult to manage, and might take over a year using various methods (including insecticide) to suppress their numbers. Not all of the insecticide options can be applied by the general public, so you might need to hire a certified pesticide applicator for that type of treatment.

Depending on the type of lilac (the common or "French" type versus one of the tree types that are East Asian in origin), they are best maintained with occasional drastic pruning to remove old wood, so that can work in your favor in the approach to treat the scale outbreak. Old stems on common lilacs tend to flower less prolifically with age and are much more vulnerable to attack by Lilac Borer than younger vigorous growth, and so are best removed completely every few years. The pictured plant's stems are so thick that we can see they haven't been renewal-pruned in some time, which is one reason we were wondering if this is one of the tree types instead (they mature around 20 feet tall and only bloom white). Those tree types are not pruned the same way because they are intended to reach tree size and look distinctly treelike, but since they usually don't have multiple trunks, we weren't sure which type of Lilac this is.

If you can remove most or all of this old growth by cutting the plant down to almost the ground, that will immediately remove the vast majority of the scale, simplifying any treatment of what remains. Granted, the regrowth won't be old enough to bloom for the next several years, but the plant should be in better health overall afterwards. As it stands now, the scale population is taxing the plant's energy reserves, so might still impact its blooming and overall growth as their infestation progresses, even if not cut back.

Miri

Hello Jenny,

Unfortunately the lilac photo didn't come through for some reason, but that's ok because yes, purple blooms suggest it's a Common/French Lilac, not one of the "tree" types as they only come in an off-white color.

We understand that a drastic cut-back is alarming, but it's the standard care advice for maintaining vigorous lilac shrubs for the long term. This process is usually started at a much younger age, though, so it's less drastic at that point. Even so, if your plant is still blooming well as-is compared to when it was younger, and has never had problems with borers causing branch dieback, then there's no requirement to cut it back if you wanted to take another approach to scale management.

These do look like White Prunicola Scale, but it's hard to ID them concretely from photos, especially once detached from their covers due to physical removal. This is one of the only scale species with this appearance that regularly occurs on Lilac, however. Even though the potential host range for this scale is relatively broad, as it is for other scale species as well, we mostly see White Prunicola Scale on cherrylaurel and lilac, aside from certain fruit trees or their flowering cherry relatives.

Scale do travel, but via using wind or possibly hitch-hiking on bird feet or other animals, since they move relatively tiny distances under their own power. (This is how they wound-up on the lilac in the first place, whenever they first arrived a year or, more likely, several years ago.) The "crawlers" are the aptly-named mobile life stage, and they are barely visible to the naked eye. This species of scale has three generations per year, so that means crawlers have the potential to disperse a bit three times per year, with the last generation of this year having gone through this phase back in earlier September. Any young scale present on those other plants now would be small and hard to see unless more than one generation has been present on those plants for some time already. You can send us photos of any suspicious symptoms on those plants if you're concerned about scale. Around a dozen or more scale species have the potential to regularly occur in home gardens, so it's entirely possible that if other plants in the yard have a scale outbreak, that it's a different scale species.

You can certainly attempt to use other management methods before resorting to a drastic cut-back. Physical removal via a light scrubbing might take off a bunch of individuals, though it can't eradicate a population because there are too many nooks and crannies for scale to hide in, and you don't want to abrade away bark in the process. Treatment during the crawler periods with horticultural oil might help to kill crawlers before they settle and begin to secrete the waxy "shell" that protects scale from most pesticides. You would also consider hiring a certified pesticide applicator to treat the tree with something longer-lasting once the blooms have finished next year, since we don't want to risk pollinator health by making an application of plant-absorbed chemical too early.

Miri

You can discard the infested branches with either trash or yard waste; this pest is non-native but has been long-established in our region and occurs throughout Maryland, so there is little risk of having it spread to areas it is not already present. If you prefer to compost the branches, that's also fine; giving the bark a mild to modest scrubbing to abrade off the scale before putting the wood in a compost pile would remove and kill most of the scale.

Miri

Spring is probably the best time to do pruning at this point. While any trimming between now and spring will remove flower buds, and in that regard won't matter much in terms of timing, we do want the plant to seal-over the pruning cuts relatively quickly to lessen the risk of wood decay, which won't happen if trimmed in autumn or winter.

Miri