In the Garden:
Northern & Central Midwest
August, 2007
Regional Report

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Anthracnose makes the leaves look unsightly, but it's usually not a serious condition.

Anthracnose Disease

If you needed a reason to clean up your leaves this fall, do it to help prevent anthracnose disease from disfiguring the leaves of your favorite trees. Anthracnose is a fungal disease that often appears after a cool, wet spring. It is rampant in my area, and I've received many a call about it this summer.

The good news is that it doesn't usually kill the tree -- just makes it somewhat unsightly. Single attacks don't usually harm the plant, but repeated attacks can weaken a tree and make it susceptible to other problems.

Some types of anthracnose, such as those that affect sycamore and dogwood, can actually be fatal. But the disease-causing organisms are fairly specific, so sycamore anthracnose will not infect maples nor will dogwood anthracnose infect elms. We also see anthracnose in the Midwest on oaks and occasionally on linden, tuliptree, hickory, birch, and walnut.

Symptoms
The main symptoms we see on maples are purple-brown areas on the leaves that eventually turn lighter brown and may crumble away. These spots appear along the veins and between the veins, and the key identifying feature is that they are quite irregular. Scorch, a physiological problem with some maples, such as sugar and Japanese, has similar symptoms, but the browning occurs along the margins and usually looks somewhat regular. With either problem, the leaves may become so damaged that they wither and drop early. Other species than maples may have light brown spots, blackening of the leaves, shortened and dying small twigs.

Disease Life Cycle
The fungus overwinters in leaves left on the ground, although some types can also overwinter in small twigs that remain on the tree. In April and May the fungal spores are active and can be blown or splashed to buds and young developing leaves. A long, cool spring can cause the fungus to spread rampantly.

Control
The other good news about this disease is that it can be controlled without having to resort to fungicides, which must be sprayed as a preventive measure. Simply cleaning up the leaves through the season as the tree sheds them will go a long way toward getting rid of the fungal spores. Be sure to remove them -- don't shred and return them to the same tree bed.

Prune out dead twigs and small branches, and then prune to thin the crown, which improves air movement so the leaves dry faster. Increasing the vigor of the tree will also help. Fertilize in early spring with a general fertilizer made for trees.

With a little attention to culture, you should be able to prevent anthracnose from damaging your trees or making them so unsightly that you cover your eyes when you see them. Just keep in mind that even though you see the disease one year, it may not even appear the next year.


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