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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois
Shade Tree

Anthracnose of Shade Trees

Posted by Ron Wolford -

Spring weather promotes growth of trees, flowers, and lawns, but also spurs many diseases that attack the plants, said a University of Illinois Extension horticulture educator.

"None of these diseases is probably more noticeable and frustrating than shade tree anthracnose," said David Robson. "Anthracnose affects most of the major shade trees. However, while symptoms appear similar, different fungal organisms affect different plants."

Anthracnose thrives and develops when spring temperatures are on the cool side and moisture is adequate. The spring of 2008 has been ideal for this problem. The good news is that while disease symptoms increase as temperatures become warmer, when temperatures move into the 80s consistently the infections become less and less.

"The disease causes spots or lesions on leaves, flowers, twigs, and branches," said Robson. "Lesion colors range from tan to brown to olive green to black. Leaves curl, die, and fall off. Many trees appear defoliated or sparsely leafed out. Symptoms are noticeable during the latter part of May and into June.

"Trees usually affected include ash, birch, catalpa, dogwood, elm, hickory, linden, maple, oak, poplar (including cottonwood and aspen), tulip tree, and walnut."

However, he added, no tree gets hit as much as the sycamore, which has lived with the disease since records have been kept.

"Just looking at sycamore should give the gardener a clue as to the amount of damage to expect on other plants," he said.

In most cases, the disease starts out as small, irregular-shaped spots on the leaf, usually at the leaf margins or tips. Spots enlarge and leaves may curl. The oldest leaves appear infected first, but newer leaves can show symptoms if the weather is ideal.

"The color of the lesions varies," he said. "Dogwood exhibits a dark purple color. Elms show a gray to black color while lindens are light brown with some yellow."

Ash, maple, and sycamores are most often the hardest hit. Ash trees start with tan to brown lesions on the leaflets, usually at the margins or along the veins. Leaves curl, become distorted, and fall.

"Maple infections vary," Robson said. "Japanese maple shoots may turn black and shrivel. Purple or brown streaks develop along the veins of Norway maples, including the 'Crimson King' varieties. Greenish brown to reddish brown spots form between the veins of sugar maples. Spots merge as they get larger and leaves curl."

Sycamore anthracnose appears the most serious, though the trees recover easier than any others. Leaves develop brown lesions along the veins. Entire leaves then turn brown, curl, and fall. Sycamore anthracnose also attacks twigs and young shoots, girdling them, and causing them to die back and drop.

"Trees seldom die as a result of anthracnose," he said. "Most have the ability to produce new sets of leaves. Since environmental conditions are different in June and July, anthracnose fungal spores seldom infect new leaves and twigs.

"However, producing new leaves does limit the tree's growth and reduces the food reserves. Repeated infections over the years can reduce the plant's vigor and make it more susceptible to other disease and insect problems."

Control, Robson noted, is difficult. Protective fungicide sprays are usually too late and too costly to provide any acceptable control except on small trees.

"Sanitation is crucial for good control," he said. "Rake up fallen leaves and branches. Leaves may be composted to kill the disease spores if the compost pile reaches 140 degrees or more."

Trees should be fertilized in the late fall to maintain the plant's vigor. Watering may be necessary throughout the summer to reduce the tree's stress.

"Water thoroughly and deeply as opposed to small amounts daily," he said. "Make sure the tree is mulched with two to four inches of wood chips."

Source: David J. Robson, Extension Educator, Horticulture, drobson@uiuc.edu




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