As summer wears on, we see more and more damage from diseases accumulating on landscape trees. How much is too much? It depends on the tree, the diseases that it has, the time of season. Although we do not have frequent disease problems in trees in Montana, there are many diseases that can attack trees, and they can be difficult to diagnose. Some common symptoms that can probably be ignored are the powdery, white accumulations on foliage caused by powdery mildew; the small, round spots of orange powder caused by rust fungi, usually found on the undersides of leaves and causing a corresponding yellow or pale spot on top of the leaf. Sooty mold, which is a fungus that forms in association with aphids is usually not a serious problem either. Most of the pathogens that cause brown or black spots on tree leaves are not serious; if the levels are very high, they can sometimes cause twig dieback. In those instances, prune and dispose of the heavily infected branches. Generally speaking, by September pathogens which are found on the leaves are not that important because the tree has already completed a significant amount of nutrients for winter and the leaves are no longer as crucial as they were earlier in the season.
Symptoms which can indicate a serious problem are sunken areas in the bark of trunks and branches, sometimes discolored, which are called canker. Cankers can girdle trunks and branches, causing mortality of the portion of the tree beyond the canker. Bark that is splitting loose from the trunk, foliage that is wilting, decaying wood, or deformed, streaked, or mottled foliage are also symptoms that can indicate more serious problems. If you see any of these symptoms, it is best to get the cause identified if possible, and implement the appropriate control measures.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997