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Sycamore Anthracnose

May 2, 2006
We tend to assume that anthracnose refers only to leaf-spotting diseases on shade trees, but that is not the case. There are many different anthracnose fungi on a diversity of plants, including strawberry, corn, dogwood, and turfgrasses, to name only a few. The name “anthracnose” refers to a disease that has a leaf, stem, or fruit lesion and forms spores in a specific type of fruiting body called an acervulus. Anthracnose on shade trees in Illinois appears in the spring, most commonly on sycamore, ash, oak, maple, and walnut. The diseases require cool, wet conditions for infection. How cool does it need to be? For sycamore anthracnose, it has been determined that for the 2 weeks following bud break an average temperature lower than 55ºF will result in a serious infection. Of course, that assumes the presence of rain (water). If the temperature is 55º to 60ºF, the infection will be less serious. If the average temperature at that time is greater than 60ºF, you won't see much sycamore anthracnose. The 2-week period following bud growth is critical because succulent new growth is most susceptible. Older leaves, drier conditions, and warm temperatures usually retard disease development. The weather has been ideal for the development of sycamore anthracnose, and it is obvious now on sycamores in central Illinois.

As long as I have lived in Illinois (since the 70s), we have seen conditions cool and moist enough to have some anthracnose each spring. If you are patient and can wait for the second flush of new leaves on infected sycamore, ash, oak, and maple, you probably will not see any more anthracnose that year. Walnut trees vary in susceptibility and may show much more injury from anthracnose. We see anthracnose on stressed birch trees, especially those with chlorosis. Leaf drop may seem excessive but is not solely due to the anthracnose. One other tree species that can host anthracnose in Illinois is dogwood. Dogwood anthracnose appears in June and will be discussed in a later newsletter. That disease can be quite serious.

Symptoms of anthracnose include brown to black leaf spots, brown to black blotches, and sometimes (as with sycamore anthracnose) death of entire, young leaves, as seen in image 060304. Rarely does anthracnose cause all of the foliage to die. Sycamore anthracnose also causes small stem cankers. Anthracnose of shade trees is usually worse in the lower or inner canopy of the tree where leaves stay moist longer.

As a reminder, fungicides are not recommended for these early anthracnose diseases of shade trees. Help the tree produce a new flush of foliage by providing water in periods of drought. Stressed trees can be fertilized in early spring or in the fall. A fact sheet on anthracnose of shade trees can be found at http://www.ag.uiuc.edu/%7Evista/horticul.htm.

Author: Nancy Pataky


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