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Anthracnose on Trees

May 8, 2002

Trees commonly infected with anthracnose in Illinois include sycamore, ash, maple, oak, birch, dogwood, and walnut. Other species are affected less frequently.

Each spring, we see spotting and blighting of the leaves, buds, and sometimes stems of these trees. The severity of infection is directly related to weather conditions as leaves emerge. This disease is anthrac- nose, caused by one of many fungi present in cool, wet conditions as tender leaves are first developing from the bud. The disease is most prevalent in the spring because the cool, wet conditions that prevail then are conducive to fungal development, while slowing plant tissue development. Leaves that emerge in warmer, drier conditions usually escape the disease. The exact temperature and moisture conditions necessary for infection vary with the host and the fungus. Still, in most cases, the critical period for primary infection is the 2 weeks after bud break. Most of Illinois had cool, wet conditions the last few weeks, so it is not too difficult to predict that we will see some anthracnose.

Unless anthracnose fungi have repeatedly hit a tree in the past or a very young tree is involved, we do not recommend using fungicides. Dogwood anthracnose is another exception for which fungicides are recommended. That disease will be discussed in a later issue of this newsletter. In general, anthracnose diseases do not kill trees, but repeated infections can weaken trees to other problems. Some defoliation may occur, but refoliation with healthy leaves follows in warmer weather. Concentrate on boosting tree vitality, which promotes new growth. Prune in and around the tree to open it to better air movement. Remove dead or dying branches, water in periods of drought, and mark calendars now to fertilize affected trees in the fall. Watering in summer drought is probably the best advice we can give to help infected trees. It is odd how we quickly forget this early season stress once new leaves form. Don't add to the stress by ignoring these trees in drought.

You might confuse anthracnose with late frost damage. Anthracnose generally causes more discrete spotting on the leaf blade. Anthracnose occurs where air movement is slow and relative humidity high, so we see anthracnose most severely near the bottom and inside the canopy. Frost injury is more likely on branch tips or near the canopy top, which is more exposed to weather conditions. For more on anthrac- nose, consult Report on Plant Disease (RPD), no. 621, "Anthracnose Diseases of Shade Trees," for photos and details to help with disease identification. See the U of I publication site: http://www.ag.uiuc.edu/~vista/horticul.htm.

Author: Nancy Pataky


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