Anthracnose is a fungal disease that appears each year on select ornamental trees. The extent of infection depends on weather conditions and the host. It has appeared in 2003 and may worsen with wet conditions and new foliage production. Trees commonly infected in Illinois include sycamore, ash, maple, oak, birch, dogwood, and walnut; others, less frequently.
Symptoms include spotting and blighting of leaves, buds, and sometimes stems. Anthracnose is caused by one of many fungi present in cool, wet conditions as tender leaves are first developing. The disease is most prevalent in the spring because conditions are conducive to fungal development, while slowing plant development. Leaves emerging in warmer, drier conditions usually escape the disease. Conditions needed for infection vary with the host and the fungus. For most, the critical period for primary infection is the 2 weeks after budbreak.
Unless anthracnose fungi have repeatedly hit a tree or a very young tree is involved, we do not recommend using fungicides. Dogwood anthracnose is another story altogether: It can cause extensive damage, and fungicides are recommended. Dogwood anthracnose will be discussed in an upcoming issue. In general, anthracnose diseases do not kill trees; but repeated infections can weaken trees, making them susceptible 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. If we see 6 weeks of drought in 2003 as in 2002, these trees will suffer. To help them, water must be provided during drought, not as an afterthought in the fall. 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; it occurs where air movement is slow and relative humidity high--most severe near the bottom and inside of the canopy. Frost injury is more likely on branch tips or near the canopy top--areas more exposed to weather conditions. For more information, see Report on Plant Diseases, no. 621, “Anthracnose Diseases of Shade Trees,” on Extension’s VISTA Web site.