Answer: The disease black knot is caused by the fungus Apiosporina morbosa (sym. Dibotryon morbosum). Black knot can occur on both sweet and sour cherry, Damson, American, European, and Japanese varieties of cultivated plums and prunes as well as wild cherries and plums. The disease is common throughout much of North America occurring on many wild plum and cherry trees. These serve as an annual source of infection for cultivated varieties. Black knot occurs on the woody parts of the tree such as twigs, limbs and sometimes the trunk. The knot is unsightly and often becomes infested by borer-type insects. The disease may become very destructive causing death of effected twigs and limbs, and occasionally entire trees. There is great variation in level of susceptibility in wild cherry.
Control consists of two steps: During fall and winter, or before new growth starts in the spring, prune out and destroy all visible knots. On large, main branches and trunks, knots should be cut out with a knife or chisel. One inch of healthy bark around the knot should also be removed. Fungicide sprays applied in a timely, regular manner can prevent most new infections. Use Benomyl (Benlate 50% wettable powder) at the rate of 1/2 tablespoon/gallon of water.
Spray schedule: Apply first spray in the spring just as green tissue begins to appear. Make additional applications just before and just after bloom. Spray at 2-week intervals until terminal growth stops, usually early to mid-June.
Hope this information helps.
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