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'Sick' Trees Becoming Common

By Mike Boersma
Extension Educator, Pipestone and Murray Counties

Over the past several weeks, many homeowners have witnessed a number of problems with their trees. Some abnormalities this spring may include bare branches that failed to leaf out, seemingly healthy leaves falling from trees, small or under-developed leaves, curled leaves, or brown to black spots on otherwise normal leaves. While these symptoms can be a result of several things, many of them have likely been caused by one sort of environmental stress or another.

The dry fall, lack of snow cover, unseasonable winter temperatures, warm March, cool April, late frost, and wet May and June have all contributed to some extent. Dry conditions last fall and winter certainly caused stress to trees - especially young trees - that weren't properly watered. The warm March followed by a late frost also caused damage to leaf buds or very young leaves. But the most common cause for concern lately has been the brown or black patches developing on leaves of many species of shade trees. These patches are likely caused by a fungus, as the frequent rainfall during May and June provided ideal conditions for fungal growth.

Fungal diseases are most common in late spring and early summer and tend to follow stretches of cool, wet weather conditions. These diseases can take different forms and affect different species of trees, but they commonly appear as brown or black patches on the leaves of shade trees. Trees that are affected in early spring will often develop large patches that can cause the leaves to curl and fall from the tree. However, trees affected later in the growing season will likely only show small spots of black or brown on otherwise normal leaves.

While the signs and symptoms can vary significantly, the causes and effects of these fungal diseases are usually quite similar. In addition to cool, wet weather, dense trees or multiple trees planted too close together are often more vulnerable since the dense vegetation remains protected from sun and wind. This causes trees to remain cooler and wetter than ideal and allows fungi to grow and spread more readily. Fungal diseases often start on the lower, inner branches and work their way up and out.

Unfortunately, there is not much that we can do to help these trees right now. Simply watering, feeding, fertilizing, and keeping the trees as healthy as possible may be the best thing for them since fungal infections are not usually detrimental to the long-term health of the tree. Also, collecting and disposing of any fallen leaves will help prevent the fungus from spreading and re-infecting trees in the following growing season. Pruning dense growth to allow for more air circulation and sunlight penetration will also help to prevent the growth and spread of the fungus.

If the problem persists for multiple years or if the tree is under other stresses caused by root restrictions, insect pressure, or drought, homeowners may want to treat the infected tree with a fungicide as a means of prevention. Fungicide applications need to start in early spring, at bud break, and continue according to label recommendations. Unfortunately, while this process would help prevent future infection, it is not likely cost-effective until it is absolutely necessary - especially for large mature trees.

Visit the What's Wrong with My Plant Website to help you diagnose your tree problems: The web page is designed to help gardeners in Minnesota diagnose problems in the yard and garden caused by insects, diseases, and nonliving factors.