Powdery mildew on upper surface of grape leaf.
Image by University of Minnesota

Maple leaves

Late Summer Tree Problems Q&A

Late Summer Tree Problems

By Pat Curran, Horticulture Educator, Tompkins County Cooperative Extension

Question: The leaves on my lilac have some white smudges; my maple and oak leaves have funny bumps; and some of my trees have grayish webs at the tips of the branches. What's going on?

A number of problems appear in the landscape late in summer. The good news is that most of them are merely cosmetic problems, which are not dangerous to the plant. The white stuff on the lilac leaves is powdery mildew. The funny bumps on many leaves, including maples and oaks, are galls. They often house little insects, which somehow cause the leaf tissue to grow the distorted bumps. The grayish webs at the tips of tree branches are produced by a native insect called the fall webworm.

Usually, none of these problems is serious. This late in the growing season, healthy plants (if they were not overly stressed by the heat and drought of July) have made enough food through photosynthesis. Losing a little leaf surface now to the fall webworm will not make a significant difference. The leaves with galls are often still all green around the galls. The mildew on the common lilac often appears every year and doesn't seem to affect the shrub's vigor (there are other kinds of lilacs that don't get powdery mildew, however).

Treating these and many other late season problems is usually an exercise in futility - ineffective as well as an unnecessary use of chemicals and a waste of time and money.

For more information on gardening, including plant diseases, consult the Cornell gardening website or call the horticultural hotline at Cornell Cooperative Extension of Tompkins County at 272-2292.

Last updated October 26, 2014