Anthracnose Disease On Shade Trees

It’s springtime. The bulbs are blooming, the birds are singing, and from a plant pathologist’s point of view, the fungi are growing.

Early spring is the time of the year when new tree growth begins to break bud and expand, and it’s when most foliar tree diseases occur. The cooler, wet conditions of spring favor fungi growth and infection of the newly developing leaf and stem tissues.

The most common foliar disease occurring on numerous species of shade trees across the country is collectively called “Anthracnose.” Anthracnose disease is most severe on ash, oak, dogwood and sycamore. However, other trees also can be affected including elm, birch, redbud, willow and maple.

Anthracnose is a fungal disease, but the same fungus does not cause disease on all trees. Although the fungi causing anthracnose are similar, most belong in the genus of Discula, (conidial state of Apiognomonia). Anthracnose-causing fungi are very host specific meaning that the disease on ash will not spread to sycamore, oak, maple, dogwood or any other tree species.

Anthracnose is leaf and stem disease. The most common symptoms of infection include light tan to brown leaf spots that develop along leaf margins and veins, wilting and shoot blight of new developing twigs, branch death, and branch or stem cankers. Early spring infection of developing buds can result in death of the bud. Infection during shoot elongation results in wilting and death of the expanding shoot and immature leaves.

These symptoms can sometimes be confused with frost injury. After shoot elongation, more mature, but still developing leaves develop leaf spots that progressively kill the leaf along the midrib or a vein. This pattern of leaf death is generally indicative of anthracnose diseases.

The earlier the tree becomes infected in the spring, the more severe are the symptoms. In some cases, severe defoliation can occur. Anthracnose disease development is very environmentally dependent. Prolonged cool (60-65 F), wet weather favors infection. Usually trees that are defoliated by anthracnose in early spring will refoliate and look normal by mid-summer because the regrowth usually occurs when the environmental conditions do not favor re-infection.

DISEASE SURVIVAL. Disease development is minor in the summer and into the fall. Anthracnose-causing fungi overwinter (survive) in infected buds and fruiting structures on dead leaves and twigs on the ground and on dead twigs and canker margins in the tree. The fungal fruiting structures look like small, black, pimple-like structures that erupt from branch or leaf tissues. Thousands of spores are produced within the structures that are discharged in the spring and spread by wind and water-splash to developing bud tissues where they germinate and infect.

Another important aspect of anthracnose is the development of cankers that can girdle and kill branches. A canker is defined as a dead, often sunken area on a stem, branch or twig. Anthracnose cankers develop in late winter and early spring and kill buds and twigs. Following infection of new growth in the spring and early summer, the anthracnose-fungi remain relatively quiescent until the infected tree enters dormancy. In the dormant tissue, the fungus may begin to grow and kill bark and cambial tissues. Cankers that develop on small twigs can kill the twig within one dormant season.

On larger branches, the canker expands during dormancy but it does not kill the branch. The tree may completely enclose the canker under callus tissue or produce callus along the edge of the canker during the current growing season. The fungus then invades and kills the callus roll plus some surrounding cambium and bark when the tree enters dormancy. The cycle can continue for years. Resulting cankers will have rolls on top of rolls of killed callus tissue. Branch death may eventually occur following years of canker growth.

GROWTH HABITS. Twig death and canker development caused by anthracnose can alter the growth of some trees, most notably sycamore and white oak species.

Chronic infection and death of terminal twigs results in a lateral twig replacing the former leader. Affected branches look crooked with excessive and angular branching because of the repeated changes in direction of the leader. On sycamore, there also may be a development of a cluster of new twigs around a central point because of the repeated killing of terminals.

Anthracnose diseases are difficult to control because of the abundance of inoculum sources (fungal fruiting structures) that may exist on dead twigs and branches within a tree. However, the disease isn’t always severe enough to warrant fungicide control measures. One year of defoliation will not affect the tree’s health. While two or more years of successive defoliation may decrease the health of the tree and make it more susceptible to other diseases, insect pests, and environmental stresses, the environmental conditions that favor severe anthracnose infection usually do not occur in two successive years.

Trees that are stressed from other factors, such as poor planting site conditions or water and temperature stresses, are more susceptible to anthracnose infection because of the reduced growth exhibited by trees under stress compared to non-stressed trees. So, often it is the planting site that is slowly killing the tree rather than the anthracnose that is infecting the stressed tree. For example, these conditions often exist in parking lot islands and along road corridors.

The cost of establishing a fungicide control program to reduce anthracnose may need to be weighed against whether the tree would survive in the given location with or without anthracnose. When fungicide control measures are warranted or desired because of the ornamental value of the tree, it must be a part of a total control program that includes cultural and physical control measures.

Avoiding plant stress and providing adequate irrigation to landscape trees can also reduce a tree’s susceptibility to disease. Anthracnose control should begin with removal and destruction of dead twigs and branches during dormancy. Pruning cuts should be made at least 4 to 6 inches below the visible canker.

Raking and removing fallen leaf litter may or may not reduce disease development because it is questionable whether the fungus surviving on the leaf debris significantly contributes to anthracnose infection of new growth. But, it can’t hurt, and often it is the only thing a homeowner can do to participate in the disease management program.

Anthracnose symptoms can be reduced through a yearly fungicide control program. Typically, three fungicide applications are needed each spring. The first is applied when the buds are about to break in early spring. The second is applied approximately two weeks after the first. And, the third is applied when the leaves are about one-half mature size.

The author is extension plant pathologist, University of Georgia, Athens.