Aphids on Crepe Myrtles
I used to have 5 Crepe Myrtle trees...and I lost three of them in 2016 to what I believe was Aphids, due to neglect...Really didn't notice the problem till it was too late. I assume this is a very common problem. The two that were left I believe I almost lost last year...but having early on noticed the same condition (Leaves turning black), I was able to save them in the 2019 season and they came back nicely in Spring 2020. This year they even bloomed in the summer, the first time in a long while. I saved them by using a pesticide and a surfactant, in early Spring, made a few applications over time. The leaves were nice and green for most of the spring and summer...BUT I just noticed again they are turning black. Wow. See photo taken today. What bothers me is there are a dozen Crepe Myrtle trees in my neighborhood, two of them are even are less than 30 yards from one of mine. They are not affected in any noticeable way. Green beautiful leaves. So, why am I being attacked??? What can cause this localized attack on only two trees? The picture shows the condition of the leaves...and I assume they will be normally falling soon. While I won't lose them this year, is there something I can do to better prepare them for 2021?
Calvert County Maryland
Aphid infestations can be heavy but should not result in the death of a tree, though a heavy scale infestation might. (The latter will probably be more noticeable when you inspect the stems.) Both will create honeydew waste, which is what the sooty mold feeds on; it does not infect the plant itself. While we do not have data on this, it is likely that different cultivars of crepemyrtle (of which there are well over 50 commonly on the market) have different degrees of resistance/susceptibility to these insect pests, just as they do to powdery mildew and other leaf-spotting diseases. The fact that nearby trees aren't as adversely affected - if affected at all - could be due to various factors, including plant stress from drought, the use of other insecticides on those plants, or that they are merely different cultivars that the pests find less appealing for some reason.
Treatment would depend on which pest is present. If you aren't certain it's aphids, check underneath the leaves - this is where they shelter and feed. Many beneficial predators and parasitoids kill aphids, though some pesticides will harm them indiscriminately (including possibly yard mosquito sprays), allowing aphid populations to build up again without these natural checks and balances. Scale insects can take several forms, but the most problematic species that is new to our area clusters together on the bark and is a plump white oval-shaped insect with no wings or obvious legs. If you see what you suspect are scale, please send us photos for confirmation as this novel pest is being tracked for its presence. Though we cannot see it clearly enough, the fine white debris on top of the sooty mold in the photo suggests aphids, as this debris would be their cast skins from molting.
If crepemyrtle aphid is the pest at work here, you do not need to treat the plants now - the damage being done is not as serious as it looks and will end as freezes trigger leaf drop in a few weeks. Since their eggs spend the winter wedged into bark crevices, you can treat the trees with a dormant oil spray this winter (check product label for any temperature restrictions during spraying) to kill them.
As we move into fall, continue to monitor the plants for watering needs, soaking well as necessary. Even though crepemyrtle don't flower until late in the year, the buds for next year's growth are forming now and may suffer if we experience drought (as we did last autumn). Here is our page with watering guidelines for trees and shrubs: https://extension.umd.edu/hgic/topics/watering-trees-and-shrubs
We do not know what caused the loss of your other crepemyrtle. While they are drought-tolerant plants, they can still suffer root loss in extended dry conditions with no supplemental water. Similarly, over-watering can deprive roots of sufficient oxygen for too long, causing dieback and opportunistic rot. Harsh winter weather - more so dramatic temperature swings than persistent low temperatures - can damage tissues. Any tree that is planted too near a lawn without enough of a protective mulch barrier can succumb to lawn-related damage - either trunk wounds from mower or weed-whacker strikes or exposure to certain herbicides which are known to be absorbed by tree and shrub roots/bark. (Such cautions will be listed on its label when applicable.) What few diseases might kill a crepemyrtle would probably cause localized symptoms and patchy dieback first, and we are not aware of any insect (outside of perhaps the crepemyrtle scale mentioned above) that would kill them outright, at least not without first causing visible symptoms.