Sunday, May 11, 2008

Nursery and Landscape - Anthracnose Diseases in Trees

The cool, wet weather we have had recently will favor the fungi that cause anthracnose diseases in landscape trees. The following is information on several of these anthracnose diseases.

Anthracnose diseases are not all caused by the same fungus. Each host plant has its own anthracnose fungus, so, for example, don't assume that anthracnose of sycamore or grape is a threat to nearby dogwoods. The incidence and severity of anthracnose diseases of landscape trees varies with the season, and this year, symptoms appear to be at moderate levels.

Ash anthracnose. Brown blotches along leaflet edges have been visible for the past week or so on new ash foliage. Many of these infected leaflets will begin to drop soon and carpet the walks and lawns nearby. Ash anthracnose is not normally a threat to ash tree survival, however, and the ash trees will simply put out a new set of leaves. The ash anthracnose fungus is a species of Discula.

Dogwood anthracnose. Caused by the fungus Discula destructiva, dogwood anthracnose is appearing this spring in many flowering dogwoods (Cornus florida). Dogwood anthracnose causes leaf spots, leaf blight, and lower branch dieback and is most commonly observed in forested regions where native understory dogwood trees are threatened. This spring, anthracnose is also present in landscape trees, especially those growing in shaded locations.

Maple anthracnose. Symptoms can range from leaf spots to shoot blight and shoot cankers. Maple anthracnose may be caused by Discula sp. or by Kabatiella apocrypta. There is considerable variation in maple susceptibility to anthracnose. In some cases, sugar maple leaf spotting may be heavy on one tree while the adjacent tree is barely affected.

Oak anthracnose. This disease is less common and is caused by the fungus Apiognomonia quercina.

Sycamore anthracnose. Anthracnose symptoms on infected green, expanding leaves, look for irregular dark, necrotic blotching centered along the leaf veins or leaf edges. These dark blotches may turn a tan color as the diseased areas of the leaves dry out. In the same trees, tips of young shoots with newly expanding leaves are wilting and dying because of twig or shoot infection. With continued rainy weather, the disease should continue to spread in the foliage. Symptoms are not as severe as we see some years when trees are heavily defoliated by now. As the weather gets warmer and drier, sycamores normally put out new, healthy foliage. However, the legacy of crooked branches (because lateral shoots take over when terminals are killed by anthracnose) and multiple shoots arising from the base of a killed branch may still be visible many years later. Sycamore anthracnose is caused by the fungus Apiognomonia veneta, and the fungus attacks both sycamore and London plane.

Delaware growers and gardeners should know how to grow and maintain healthy landscape trees. For most trees, anthracnose disease is not lethal, but it can be for dogwoods. Good growing practices are important in reducing the effects of anthracnose and in preventing loss of dogwoods from anthracnose. Consider the following:

Rake up and compost fallen leaves. Leaves can be a source of inoculum.

Prune out and destroy dead twigs and branches, because for many of the anthracnose fungi, branches harbor fungal inoculum. Although it is difficult to prune large trees, small trees are at greater risk, so prune out dead twigs and branches from them. For dogwoods, pruning out dead branches and water sprouts is especially important where anthracnose might be a threat.

Avoid unnecessary wounding and avoid construction or other activities which could injure the roots or the branches.

Provide mulch as needed. Maintain a 2-3 inch layer of mulch over the root zone of the tree (but not against the trunk) to help maintain soil moisture and to protect trees from lawnmower injury.

Protect trees from drought by watering at least once a week during dry periods. Do not use overhead sprinklers for watering; wet foliage favors infection.
Do not transplant dogwood trees from the wild. Purchase healthy trees from a reputable nursery.

Anthracnose is favored by a moist environment. Select a planting site with a sunny eastern exposure to promote rapid foliage drying early in the day.

Diagnose and treat insect and disease problems appropriately.

Plant disease resistant dogwoods such as C. florida 'Appalachian Spring' or oriental dogwoods (Cornus kousa) for high risk sites such as those with heavy shade and nearby diseased trees.

Although most anthracnose diseases can be controlled using fungicides, the attempt is usually more costly than the benefit. Dogwoods which are threatened by anthracnose may benefit from early spring fungicide applications.

Adapted for Delaware from "ANTHRACNOSE DISEASE SYMPTOMS APPEARING IN LANDSCAPE TREES" By John Hartman in the May 22, 2006 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.

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