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Late Summer Tree Stress: When Should You Be Concerned?

Updated: Apr 19, 2022

Written by: Certified Arborist Pat Walsh, WI-1108A

As the end of summer approaches you may be wondering why your trees and plants aren't looking as good as they may have earlier in the season. Chances are your trees and plants are just fine - a little ragged around the edges but no worse for wear. Trees do most of the heavy lifting in spring. With the increase in sunlight and temperature, photosynthesis produces sugars which causes a period of rapid growth both above and below ground. As the summer months progress, the temperatures increase and the enzymes that power photosynthesis cease to function during the day. While the trees are in this kind of late summer holding pattern certain environmental factors will affect the look and health of your tree.

Leaf spot diseases are commonplace this time of year. Most leaf spot diseases are caused by fungi; some are due to bacteria or other pathogens. In most cases these spots will not seriously affect the tree. Most of these fungi only affect a small percentage of the tree’s overall leaf area and create only a minor stress on the health of the tree. Foliar fungus issues, such as apple scab and hawthorn rust, should be taken seriously if they result in moderate to complete leaf loss over the course of two to three growing seasons.

Tar spots on maple trees are very common and do not require fungicide treatments. Pruning to increase light and air flow in the canopy can help to reduce the abundance of the fungus. Leaf anthracnose is another fungal disease that typically will not require management unless noticed in abundance over several growing seasons.

A plant that has fed nothing has not done its job. In other words, insect “damage” is perfectly normal. Plants are part of an ecosystem regardless of how we place them in our landscapes. Plants are the first trophic level, herbivores being the second, and all the subsequent levels depend on those first levels. Insects, by a large margin, make up the bulk of herbivores feeding on the plants. Most of the feeding is done by native insects and, like the fungal diseases, lead to only marginal stress. The type of damage you see depends on the type of insect, how it feeds, and the part of the leaf it feeds on.

Leaf miners feed within the tissues of the plants and leave behind a noticeable line within the leaf. Insects with chewing mouthparts will skeletonize leaves, leaving the margins intact, or eat portions of the leaf surface in its entirety. Insects with sucking mouthparts such as aphids and leafhoppers puncture the leaf surface and draw out the fluids. A leaf hopper will leave lightly colored spots or patches on the leaf surfaces.

Other noticeable insect activity may come in the form of galls, which are abnormal plant growths caused by insects, mites, nematodes, fungi, bacteria and viruses. Most galls are formed by a young insect or mite which feeds within the gall. The insect or mite matures, emerges from the gall and moves on. These growths are almost always benign. Fall webworms will start to present themselves this time of year, too. The webworm is a moth which, while in its larval stage, creates a webbed nest at the ends of branches. The webworm nest, albeit unsightly, does not harm otherwise healthy trees.

If you are concerned about the health of your trees, contact Hoppe Tree Service for a tree inspection. Our certified arborists can evaluate the trees and shrubs in your landscape and recommend a course of action, if needed.

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