Hispidus Canker Decay of Hardwoods
Forest Health Guide for Georgia Foresters
Written by Terry Price, Georgia Forestry Commission
Adapted for the web by the Bugwood Network
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Hispidus canker, Inonotus hispidus, is abundant across the southeastern United States. Willow, oak, ash, and hickory are species most often infected. Although the disease is more noticeable on large, old growth trees, it is also common of slow growing young trees. Hispidus cankers are usually several feet long with irregular partly callused margins (Figure 136). The dark, soft, applanate, sporophores are found at the top of the canker. In the fall the conks fall to the ground where they may be found for several months following. Decay is often quite extensive within the tree. There is no known cure or control of this disease. Since dead branch stubs serve as entry points for the fungus, trees should be properly pruned. Infected trees
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Figure 136
photo by Edward L. Barnard |
can be removed during timber stand improvement cuttings under forest management conditions, however, trees with cankers are often used by wildlife. There is little the homeowner can do except fertilizing every 3-5 years and watering affected trees during droughts. Severely decayed trees are susceptible to windsnap and should be removed if in danger of falling on people and property.
Nectria Canker
Many species of hardwoods are subject to Nectria canker. The most common hosts in the South are red and sugar maples, red and white oaks, apples and black locust. Infections often occur through branch stubs or wounding such as pruning. Nectria cankers are targetlike in appearance because the canker grows at the same rate as the callused growth (Figure 137). Affected forest trees are of low merchantable value. Yard trees should not be pruned during wet weather since infection is highest at this time. Severely cankered trees are prone to windsnap and might need to be removed.
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The Nectria canker fungus overwinters as small red, lemon-shaped fruiting bodies at the edge of the canker. In the spring, spores are produced that are blown or splashed to branch stubs or other wounds. Spore dissemination occurs in wet weather, primarily in the summer and fall, and it is for this reason pruning should be done during dry periods. Cankers occurring on branches can be controlled by pruning. Trunk cankers can be treated surgically by removing the bark down to the cambium at least one inch beyond the edge all around the canker. Homeowners should seek the advise of a professional arborist or forester before attempting this surgical procedure.
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Figure 137
photo by Robert L. Anderson |
Black Knot of Prunus
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The fungus, Dibotryon morbosum, causes black knot of cherry and plum. The fungus causes irregular shaped, ugly knots (Figure 138). The great majority of infections occur on green shoots, however, older branches can be infected if the bark has been damaged. A major source of infection for ornamental cherries and plums are the native wild cherries that grow along fencerows. Eliminating wild cherries will help reduce the source of inoculum. Diseased branches should be pruned and destroyed. Otherwise, the disease will continue to spread within the tree.
Chemical control can be achieved if trees are sprayed when the buds begin to swell and the temperatures reach 55ยบ or higher. Several applications may be necessary to protect the new growth from disease inoculum.
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Figure 138
photo by Terry Price |
Bacterial Gummosis
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Gummosis is caused by bacteria (Pseudomonas syringae) and is common on various fruit and nut trees as well as flowering cherry. The classic sign of gummosis is oozing of gum (Figure 139). Oozing occurs during the fall, winter or early spring. The bacteria gain entrance into the tree around wounds such as those caused by lawn mowers and string trimmers and pruning wounds. There is no control for stem infections but branches infected with gummosis can be pruned.
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Figure 139
photo by Dan Pusey |
Cytospora Canker
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Several related fungi in the genus Cytospora cause this plant disease. C. chrysosperma causes cankers on hardwoods and C. kunzei causes cankers on conifers. Maples, poplars, and willows are hardwood species most often infected, while blue and Norway spruces are the most susceptible conifers. The fungi produce sunken lesions on trunks and branches (Figure 140). In the spring, black fungal spores develop in the bark and curly orange spore
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Figure 140
photo by Dan Pusey |
chains form. These spores are wind blown or splashed by rain to uninfected trees where they grow on injured or wounded bark tissue. Bark that has been damaged by sunscald, cold, pruning or other means is especially susceptible to infection. Wet mild springs are conducive for infection. Pruning should be done in dry weather to reduce the incidence of infection. There is no control for the disease other than pruning infected branches. Experienced professionals can cut out individual cankers occurring on the trunk but this practice is not advisable for homeowners.
Bot Canker
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Bot canker is caused by Botryosphaeria dothidea. This fungus is opportunistic in that it generally requires a weakened or damaged host. It is usually associated with plants stressed by drought, heat, freezing or defoliation and those damaged by hail and pruning. It is associated with more than 100 genera of plants. The disease is common on apple, dogwood, elm, oak, persimmon, redbud, willow, sycamore, hickory, holly, Leyland cypress and sweetgum.
Lesions vary in size and number depending on the host and its condition at the time of infection. They are often surrounded by callus tissue (Figure 141). Diseased twigs usually die but larger branches may have several cankers without dying. If the cankers are numerous and they encircle the branch then death will result. Bot canker can be confused with gummosis on peach trees.
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Figure 141
photo by Robert L. Anderson |
Things that can be done to manage bot canker are:
- Prune affected branches before bud break and destroy.
- Water plants to prevent drought stress.
- Avoid wounding plants with lawnmowers and weed trimmers.
- Pruning can cause infections of bot canker. Avoid pruning on cool wet days when disease spores are plentiful.
Chestnut Blight
The American chestnut was one of the most valuable trees in the entire eastern hardwood forests until the chestnut blight, Cryphonectria parasitica, was introduced around 1900. By 1940 the disease had ravaged chestnut trees throughout North America. Efforts to eradicate the disease have failed miserably and all that remains of a once majestic forest of chestnuts are stump sprouts that eventually die before producing chestnuts.
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The chestnut blight fungus also infects post and live oak, red maple and shagbark hickory. Damage on post oak can be extensive but is usually insignificant on the other tree species mentioned. Infection occurs through wounds caused by hail, insects, birds, squirrels and rubbing of branches caused by wind. When these wounded areas become inoculated with spores the disease progresses rapidly girdling and killing branches (Figure 142). Chestnut trees can be girdled and killed in a period from 1-10 years.
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Figure 142
photo by Robert L. Anderson |
Current research efforts to control chestnut blight involve backcrossing hybrids of American and Asiatic chestnuts to the American chestnut and the use of hypovirulent strains of the fungus to stricken the virulent strains. Both offer some promise in bringing back the American chestnut.
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