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Anthracnose Leaf Blight of Shade Trees

Anthracnose Leaf Blight of Shade Trees

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A number of different trees are affected by anthracnose diseases. These fungal diseases can cause severe leaf blighting and deformation, but in many cases damage to plant health is not severe. However, with sycamore anthracnose and dogwood anthracnose, the fungus regularly moves back into stem tissue and causes more significant problems. Following are profiles of some of the more common anthracnose diseases of landscape trees.

Maple Anthracnose. Pathogens: Discula spp., Kabatiella apocrypta Anthracnose diseases are generally not severe on maple, but can cause considerable unsightliness from brownish leaf blotches and some leaf drop when moist weather conditions make the disease particularly severe. Extensive development of stem infections is not common on maples, as it is with sycamore anthracnose and dogwood anthracnose.

The most common symptoms include brownish discoloration along veins, varying from discrete spots to irregular patches of discoloration bordered by 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. Where fungicides are used, applications must be started at bud break and continued during early leaf development. When fungicides are required, use labeled products containing mancozeb, thiophanate-methyl, or chlorothalonil.

Ash Anthracnose. Pathogen: Apiognomonia errabunda This anthracnose disease is primarily a leaf blighting and blotching disease of white ash and, to a lesser extent, green ash. Small twig cankers do occur and the fungus overwinters on these twig lesions, but little damage occurs from this phase of the disease. In wet, cool spring conditions, leaves and sometimes shoots first develop water soaked areas and later large tannish blotches and leaflet distortion.

Considerable leaf drop occurs, especially from lower areas of the canopy. Though this causes concern when leaves litter the ground in late spring, damage to overall plant health is not generally severe and plants typically releaf. As leaves mature they tend to become more resistant to infection. Fungicide applications, if warranted, should be made at bud break, with several repeat applications early in the season. When fungicide is required, use a labeled material containing thiophanate-methyl, chlorothalonil, or mancozeb.

Oak Anthracnose. Pathogen: Apiognomonia quercina White oaks are the most susceptible of many oak species to this leaf blotching disease. Twig infections occur but are not significant except as sources of overwintering fungal inoculum from year to year. Leaves and shoots are infected during cool, wet spring conditions causing leaf blotches that often are strictly delimited by leaf veins. Eventually, lesions become a papery tan color and some leaf shriveling occurs. Multiple cycles of infection can occur. Just as leaves near maturity, lesion size lessens, and once leaves mature they become fully resistant by early to mid summer. Fungicides are generally not recommended. If fungicide is required, use a labeled material containing thiophanate-methyl, chlorothalonil or mancozeb.

Sycamore Anthracnose. Pathogen: Apiognomonia veneta This is a potentially serious disease of American sycamore and to a lesser extent London planetree. Susceptibility of London planetree varies considerably with seed source. Shoot blight, leaf blight and twig and branch cankers and dieback can be severe. The fungus overwinters on twig tissue on the tree with spores splashing to new buds, shoots and leaves in the spring, with disease being enhanced by cool, wet conditions during shoot and leaf development. Considerable defoliation, sometimes with complete leaf loss, occurs on many trees by late spring in some years.

Trees typically releaf by early to mid summer and are less susceptible to continued infections because of warmer, drier conditions. Also, as leaves age they become less susceptible to infection. With repeated infections over the years, cankering of twigs and branches can result in erratic shoot growth that gives an overall distorted appearance to the tree, and also witches-brooming where there are numerous side shoots that develop around a central terminal shoot that was killed by the fungus. Fungicide applications to prevent infections in the spring are sometimes warranted. If fungicide is required, use a labeled material containing thiophanate-methyl, chlorothalonil or mancozeb. Fungicide injections (which are made by tree care professionals) are also used in spring and fall to systemically control the disease.

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Stephen Nameth & Jim Chatfield


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