Why are the tree leaves spotted?

Over the past couple weeks, I have received several calls from people telling me about leaf spots or brown, curled leaves on their maple trees. In some cases, the symptoms suggested a common fungal disease called anthracnose.

Anthracnose is caused by several different, but closely related, fungi and may occur on a variety of shade trees. Trees most often affected include maple, sycamore, ash, oak and walnut. Each fungus is specific to the host tree it affects. So, for example, a sycamore anthracnose fungus will not spread to a maple tree.

Anthracnose fungi overwinter on twigs or fallen leaves. Spores are produced during the spring and can be carried by wind-driven rain onto new leaves. Since water is needed for the spores to germinate and infect the tree, the release of spores, together with the infection period, are closely tied to spring weather conditions.

Cool, rainy weather favors its development. Hot, drier conditions slow down anthracnose fungi and lessen the symptoms of the disease over time.

Symptoms of anthracnose infection can vary, depending on the specie of tree affected, severity of infection and type of fungus involved. Therefore, it can sometimes be confused with other foliage diseases. With maple trees, often large brown areas occur near the main veins on the leaf.

Affected tissue begins to turn papery and the leaf may become tattered. Several species of anthracnose fungi can infect maples trees. Many maple varieties are susceptible including the Japanese, Norway, red, silver and sugar.

With anthracnose, once you see the spots on the leaves there is not much you can do as the tree is already infected. However, on established trees that are healthy, damage is usually minimal. Defoliation may occur, but seldom has a long-term impact on the health of the tree. Even in severe cases where the tree loses a large portion of its leaves, it can survive.

New leaves typically emerge during the summer. Tree decline can occur, though, if the tree is subjected to attacks of anthracnose year after year. Repeated defoliation may weaken the tree and make it vulnerable to attacks from insect borers, other fungi or bacterial infections, and winter injury.

Good management practices that promote tree vigor can help trees recover from the disease, and minimize the risk or severity of future infections. Rake and remove fallen leaves, and twigs in the fall, to reduce the overwintering population of anthracnose fungi. Maintain healthy trees with the right amounts of water and fertilizer.

Typically, fungicide control of anthracnose is not warranted, as the fungi do not usually cause serious injury to tree health. For trees that have undergone repeated defoliations because of the disease, or are stressed or recently transplanted, using a fungicide may be justified.

With the use of any chemical control, of course, always follow the label directions and make sure that a proper diagnosis of the problem has been made, with an appropriate treatment indicated. For large trees, sprays may be best applied by a tree care specialist.