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Garden: Black knot disease prevalent on cherry and plum trees

A common fungal disease of cherry and plum trees is widespread in Greater Columbus, likely worsened by the abnormally wet conditions experienced locally both this spring and last spring. Black knot, caused by the fungus Apiosporina morbosa, has been identified in several cherry trees in Columbus. While the fungus primarily attacks cherry and plum trees, it can also infect apricots, peaches and any tree or shrub in the Prunus species, including ornamental cherry species. It has been detected primarily on both sweet cherry and ornamental cherry trees locally this year. The black knot fungus attacks twigs and branches with infections starting on the youngest growth on the tree. Infected trees will have brown gall-like structures called knots enveloping twigs and branches. In late spring, these rapidly growing knots have a soft, pulpy texture and become covered with a velvety olive-green growth of the fungus. In summer, the young knots turn darker and elongate along twigs and branches. Garden: Here's what to know about plant life cycles As the fungus grows, it releases chemicals that make the tree grow extra plant cells that are unusually large. This unusual growth results in the swollen knots. By fall, the knots become hard, brittle, rough and black. During the following growing season, the knots enlarge and gradually encircle the twig or branch. The gall can completely encircle and girdle (or strangle) a branch. When this happens, the leaves beyond the gall wilt and die. Several of the samples which we have received in the spring appear to contain knots which developed on the tree last year. Abnormally wet weather the past two springs would have favored the spread and growth of this disease. The fungus which causes black knot overwinters in knots on twigs and branches and in the infected wood immediately surrounding the knots. In the spring, the fungus produces spores which are ejected into the air during rainy periods and are blown great distances by the wind. Only succulent green twigs of the current season's growth are susceptible to infection. Spores which land on the current season's growth will germinate and cause infection if the twigs remain wet for a long period of time. Normal growth is disrupted in the infected regions, and a knot is formed as the fungus causes the plant to produce tumor-like growths. Knots may become visible by late summer of the year of infection but often are not detected until the following spring, when they begin to enlarge rapidly. Cherry and plum trees should be inspected each year for the presence of black knot, and infected twigs should pruned out before the tree flowers in order to reduce the potential reservoir of fungal spores. If you first detect knots later in the season, infected twigs and branches can be pruned out at any time during the season. Diseased trimmings should be disposed of in household trash and not added to compost piles. It is important to prune at least 2 to 4 inches above and below each knot, as the fungus grows beyond the edge of the knot. Applications of fungicide can reduce the spread of the fungus but should not be a substitute for pruning infected branches. Application of fungicides should begin in spring at bud break and continue into early July. Frequent rainfall events will require reapplication of fungicides. Garden: Are you wanting to be a master gardener? For many years, the fungicide captan has been used to effectively control the spread of black knot. Production of captan has ceased permanently recently, but existing stocks of this fungicide can be used according to label directions. Supplies of captan have been depleted at most area garden centers. Alternatively, any fungicide containing chlorothalonil will be effective in controlling the spread of black knot. Always follow all label instructions when using any pesticide. Fungicides are most effective in reducing the spread of the fungus when they are applied early in the spring, so if you detect the presence of knots at this time of the growing season, the best approach right now may be to prune out and dispose of infected branches and begin application of a fungicide early next spring. The application of a fungicide at this time will have little effect on existing knots. Mike Hogan is Extension Educator, Agriculture and Natural Resources and associate professor with Ohio State University Extension. hogan.1@ This article originally appeared on The Columbus Dispatch: Black knot disease on trees