Topic summary
Black knot
Extracted from the Wikipedia article Dibotryon morbosum.
Black knot occurs only on the woody parts of trees, primarily on twigs and branches, but can spread to larger limbs and even the trunk. Olive-green swellings from the disease are visible in the late spring; as it spreads and matures, typically by autumn, rough black knots circle and kill affected parts. The knots vary in diameter from one inch to one foot (2.5–30 cm). Older knots can kill trees by promoting insect infestations.