Black knot

Apiosporina morbosa

The first symptoms appear as small, light brown swellings of the current or previous season's growth. By the next season the swellings turn olive-green in color with a velvety texture. Over this growing season the knots darken and appear to have a hard, brittle texture. The hard, black knots are the typical symptoms associated with the disease.
Knots vary in size from approximately 1-30 cm (0.5 to 12 in.) in length and from minute measurements to 5 cm (2 in.) in circumference. The infected twigs often appear bent at the tips because of extra cellular growth on one side. Trees with heavy infections may contain numerous knots. Some of the older knots may appear white or pink in color. 

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