|Black knot||Dark-brown to black, hard swellings form on twigs and branches. At first these galls are small but continue to enlarge each year, becoming very rough. Each spring, galls are covered with dark, olive-green, felt-like growth. Branches may be girdled and die.||Plowrightia morbosa (formerly, Apiosporina)||Prune and destroy galls, cutting several inches below the gall whenever they are found but especially during dormancy. Remove unwanted Prunus species from the area. If the trunk or a large branch is affected, cut out the gall and also remove about 1 inch of wood around the gall. Apply a fungicide just as green tissue is seen in the spring and again just before and just after flowering.|
|Brown rot||Flowers collapse and brown quickly. Small cankers form on twigs and gum oozes out. Ripening fruits brown and shrivel as they become covered with tan to gray masses of spores. While many fruits fall, some shriveled mummies are left on the tree.||Monilinia fructicola||Apply a fungicide when blossoms first open and again at 70 to 90 percent bloom.|
|Coccomyces leaf spot||Circular, purple to reddish-brown spots up to 1/8 inch in diameter form on the leaf early in the summer and more spots develop as the season progresses. Spots may fall away, leaving a shot-hole appearance. Infected leaves yellow and fall prematurely.||Blumeriella jaapii (Coccomyces)||Apply a fungicide as leaves emerge in the spring.|
|Leucostoma canker||Branch dieback and multiple perennial cankers occur on infected trees. A gummy substance accumulates in the inner bark and erupts through cracks or lenticels in the bark. The gum becomes a blackened crust around the canker. Trees with freeze damage and those under drought stress are most susceptible.||Leucostoma cincta or L. persoonii (formerly Cytospora)||Infection can occur at any time of the year. When the tree is growing, it walls off the fungus. When tree growth slows or stops, the fungus continues invading. Prune cankered limbs and promote tree vigor.|
|Necrotic ring spot||Leafing is delayed in the spring on individual branches or the entire tree. Leaves are smaller than normal and fewer in number. Expanding leaves have light green spots up to 1/4 inch in diameter and dark ring and line patterns. Leaf margins are wavy and blades are rough. Spots on the leaf die and fall out. Bark splitting and branch dieback occur on severely affected plants.||Necrotic ring spot virus||This virus can be transmitted mechanically, through grafting, through seed, and in pollen. Destroy infected trees. Plums and other stone fruits are also susceptible.|
Black knot on twig
Crown gall on roots (Photo courtesy of V. Cotrone)
Prepared by Gary W. Moorman, Professor of Plant Pathology
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Issued in furtherance of Cooperative Extension work, Acts of Congress, May 8 and June 30, 1914, in cooperation with the U. S. Department of Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture.
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