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 Gary Gao reported received a call about black knot fungus on fruit plums. The black knots resulted from last years' infection by the fungus Apiosporina morbosa (syn.= Dibotryon morbosum) which is indigenous to North America, but not found in Europe. The black knot fungus mainly affects twigs, branches, and fruit spurs. On infected plant parts, abnormal growth of bark and wood tissues produce small, light brown swellings that eventually rupture as they enlarge. In late spring, the rapidly growing young 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. By fall, they become hard, brittle, rough, and black.
During the following growing season, the knots enlarge and gradually encircle the twig or branch. The cylindrical or spindle shaped knots may vary from 1/ 2" to 1' or more in length, and up to 2" in diameter. Small knots may emerge from larger knots forming extensive galling. After the second year, the black knot fungus usually dies and the gall is invaded by secondary fungi that give old knots a white or pinkish color during the summer. Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus. The entire tree may gradually weaken and die if the severity of infection increases and effective control measures are not taken.
To control this fungal disease, infected twigs must be pruned out and destroyed, or removed before bud break. It is important to prune at least 2 4" (5 10 cm) below each knot because the fungus grows beyond the edge of the knot itself. Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. Where infectious spore concentrations are high because of an established black knot problem, or a neighboring abandoned orchard, protection may be needed from bud break until early summer. For the most current fungicide recommendations and spray schedules, backyard growers are referred to Bulletin 780 "Controlling Diseases and Insects in Home Fruit Plantings," and commercial growers are referred to Bulletin 506 A2 "Ohio Commercial Tree Fruit Spray Guide."
For more information, see:
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