Anthracnose Diseases of Shade Trees

Anthracnose affects a wide variety of plants. Many popular shade trees, including sycamore, ash, oak, maple and walnut, commonly exhibit symptoms each spring. Although symptoms observed on different tree species may be similar, the various anthracnose pathogens are host-specific, infecting only one tree species.

Disease Development:

Anthracnose fungi overwinter in twigs and small branches on the tree and on fallen leaves. The fungi are active during periods of mild winter weather, forming cankers and causing twig dieback. In spring, fruiting bodies form on dead twigs and fallen leaves. Spores are released and disseminated by wind and rain to the young expanding shoots and leaves. Cool, rainy spring weather provides optimal conditions for infection. Repeating cycles of the disease may occur throughout the summer when weather conditions are favorable. Hot, dry weather halts the progress of the disease.

Sycamore anthracnose:

Pathogen: Apiognomonia veneta

Sycamore anthracnose results in twig cankers, shoot blight and leaf blight--in that order. Substantial defoliation often results. The fungus actively colonizes buds and twigs when the plant is dormant and susceptible during mild weather in autumn, winter and early spring. Twig dieback occurs when a canker enlarges and girdles the twig. Trees with high bud and twig mortality produce only thin crowns by late spring. In early spring, spores are produced on dead one-year-old twigs which infect expanding shoots and leaves. Infected shoots suddenly wilt and appear scorched, commonly following spring rains. Leaf blight of newly expanded leaves also occurs, especially on the lower branches. Infected leaves develop tan to reddish-brown lesions that typically center on and extend along the leaf veins. This disease tends to be worse along the coast due to high moisture levels.

The Oriental plane tree has developed resistance to this disease and the London plane tree (a cross between the highly susceptible American sycamore and the Oriental sycamore) is partially resistant. Select a cultivar of London plane tree (such as 'Bloodgood', 'Colombia', or 'Liberty') rather than a seedling to ensure resistance is maintained through vegetative propagation.

Ash anthracnose:

Pathogen: Apiognomonia errabunda

Ash anthracnose is a common problem on white ash and, to a lesser extent, green ash. Symptoms first appear as water-soaked spots on the expanding shoots and leaves when cool, wet weather prevails in spring. The spots enlarge and develop into brownish-green to dark brown lesions along leaf veins and margins. Growth of the infected tissue slows or stops as the rest of the leaf continues to expand. As a result, the leaf tissue around the lesion becomes twisted and wrinkled. As leaves mature, they tend to become more resistant to infection. Considerable leaf drop may occur, particularly from lower areas of the canopy. The fungus can also grow into twigs and develop as cankers, where it will overwinter, but little damage occurs from this phase of the disease. Green ash, which possesses some resistance to anthracnose, is a better landscape selection than white ash.

Oak Anthracnose:

Pathogen: Apiognomonia quercina

Oak anthracnose attacks many species of oaks including bur, white, swamp white, English, pin, northern red and chestnut oak. Susceptibility varies among species, with white and bur oaks usually most severely affected. When outbreaks are promoted by rainy weather in spring, leaf and shoot blight become prominent on low branches, then spread upward. The first symptom is usually shoot blight, which can develop suddenly following a rainstorm. Young leaves and shoots appear brown and shriveled. Leaves that have already expanded may become cupped and distorted with large areas of dead tissue. Small, brown, spore-bearing structures may become visible on the lower surface of dead leaves. Necrotic spots result from infection of mature leaves, which are fairly resistant. The fungus also infects twigs, producing cankers and causing twig dieback during the winter and early spring.

Maple anthracnose:

Pathogens: Gloeosporium spp.

Several species of fungi cause maple anthracnose. K. apocrypta causes necrotic spots or scorch-like blight on leaves of Japanese, Norway, red, silver and sugar maples as well as boxelder. Symptoms, which develop during wet weather in late spring and early summer, begin as discrete reddish-brown lesions (tan on Japanese maples). Lesions soon coalesce and kill large areas of the leaf. Young shoots and leaves may shrivel and blacken following infection. Severe infection may lead to defoliation. Discula sp. affects sugar and striped maples, causing brown to reddish brown lesions along or between leaf veins. Spore masses of the fungus can sometimes be found on lower leaf surfaces along veins during extended moist conditions. The fungus spreads from previously infected tissue in spring to new growth. This disease can be serious in rainy seasons.

Walnut anthracnose:

Pathogen: Gnomonia leptostyla

Black walnut is most severly affected by walnut anthracnose under natural conditions. Symptoms first appear in late spring as small, brown, circular lesions, typically surrounded by a yellow margin, on the under side of leaves which have attained full size. Lesions are soon apparent from both the lower and upper leaf surfaces. Extensive leaf spotting causes yellowing, curling and premature defoliation. Lesions may also appear as dark, sunken spots on the nut husks. Affected nuts may have reduced meat quality and drop prematurely. The severity of walnut anthracnose tends to increase as the summer progresses, especially if rainfall is frequent. This disease usually does not significantly affect tree growth, however, as most of the year's growth is complete by the time symptoms become severe.

Control:

Rake and remove fallen leaves and twigs from the site to reduce the overwintering population of anthracnose. Prune out dead twigs and branches Wet leaf surfaces promote establishment of this disease--improve air circulation and sunlight penetration by thinning crowded stands and pruning. Maintain health of trees by providing adequate water and fertilizer. Healthy trees are more likely to recover from a severe anthracnose infection than stressed trees. Chemical sprays to control anthracnose are rarely justified except when the disease occurs in stressed or recently transplanted trees, or when the disease causes repeated defoliations.

Adapted from the University of Nebraska Cooperative Extension, 1999