Q: My aspen trees have scale insects on the bark and some disease on the leaves that causes them to drop off early. What can I do?
A: The insects are called oystershell scale (Lepidosaphes ulmi) and they feed on the bark and branches. Large populations can reduce the vigor of the trees and make them susceptible to other problems. These insects may be scraped off the trees by hand or if heavily infested, pruning branches might be best. Alternatively, you can spray them with horticultural oil in the spring when the immature crawler stage is active. That should be right about now but you should examine the bark with a magnifier to see if they are out from under their parent’s protective cover and moving about. From your photo, I think your trees are also suffering from Marssonina leaf spot disease. The black spots with yellow halos in the photo you sent fit the description and this is a common leaf disease of aspen trees. The important thing about the common leaf diseases in aspen trees is that they are all managed in a similar fashion. The most important tactic is sanitation because the diseased leaves on the ground are a source of inoculum. If you have a few of these trees, this might not be a big challenge. I’m not sure how practical removing and destroying the diseased leaves is in a large stand of these trees. If you choose to use a fungicide, there are quite a few to choose from that could help. Look for those that are labeled for use on trees and shrubs. Some common active ingredients include propiconazole, chlorothalonil, mancozeb, and captan.
Peter L. Warren is the Forest Health Program Coordinator for the Arizona Department of Forestry and Fire Management. Questions, photos and videos may be emailed to email@example.com