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Leaf Spot Diseases of Shade Trees

May 11, 2005

By now, most of us are convinced that we made it through another winter. Leaves have emerged from the buds and are about half grown. Soon, we may begin to see some brown or black leaf spots and possibly some leaf drop. How do we know what is significant?

First, think about the leaf spots and the fungi that cause many of these spots. There are millions of fungal spores floating around in the air right now, but only a small percentage will cause a problem. Three basic factors are needed to produce disease: a virulent pathogen, a susceptible host, and favorable environmental conditions for infection. Most leaf-spotting diseases of trees are caused by fungal pathogens. The spores are present now, but most of them are harmless. For instance, a scab spore might fall on your pine tree, but it can’t harm the pine. That scab spore must fall on a crabapple to be a potential problem. In addition, the host must be susceptible. Some crabapples are resistant to the scab fungus, while others are susceptible cultivars. Finally, environmental conditions have to be right for infection. Spring weather usually provides ideal conditions for leaf-spotting fungi. This year has been no exception. Have you ever wondered why you see these leaf spots in the spring but not so much later in the season? As new leaves emerge, the tender growth has not developed a thick cuticle (waxy coating), and tissue is more susceptible to infection. As the leaves “toughen up,” infection successes are reduced.

Some of the common leaf-spotting diseases we see in the spring in Illinois include anthracnose of ash, birch, maple, oak, sweetgum, sycamore, and walnut. It is also common to find other leaf spot fungi on species such as ash, catalpa, linden, dogwood, elm, hackberry, honeylocust, magnolia, maple, oak, poplar, redbud, sweetgum, sycamore, and walnut. Some of the fungal species involved might include Phyllosticta, Cercospora, Alternaria, Marssonina, Septoria, Gloeosporium, Rhytisma, Atinopelte, Tubakia, and Taphrina.

As long as the spotting occurs only in the spring and new growth emerging in the summer is healthy, these leaf-spotting fungi probably cause no significant health threat to the tree. Still, repeated infection over many years can cause a tree stress and predispose it to other problems. In such a case, it is advised to get help from a plant lab or otherwise make a positive identification of the causal pathogen. Trees affected by leaf diseases might also need a little extra tender loving care. Water those trees in periods of drought to help with additional leaf production. Also consider fertilizing the tree in the dormant season. Although fungicides are registered for use on various shade trees to prevent leaf-spotting fungi, they are not usually recommended unless the problem is a chronic or the tree is very young and in a focal point in the landscape.

For more information on anthracnose and shade tree leaf spots, refer to issues of Report on Plant Disease, listed alphabetically on the University of Illinois VISTA Web site, http://www.ag.uiuc.edu/%7Evista/horticul.htm.

Author: Nancy Pataky


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