Gall maker: Dibotryon morbosum, a fungus.

Shape and Size: Knotty, irregular growth. Appears as a black swelling encircling the branch, soft in the spring and becoming brittle and crystalline in the fall. Size ranges from 1-30 cm or more in length, and up to 5 cm in diameter.

Location: Affects branches and twigs of Prunus genus.

Color: Usually dark to black; darker towards summer and fall.

This odd but common gall is not caused by an insect, but rather by an infection of Dibotryon morbosum fungus. Upon infecting the tree, the fungus causes the plant to create a tumor-like growth.

Felt, P. E. Plant Galls and Gall Makers.
Jodhpur, India: Agrobios, 2001.
p 270

And

Ellis, Michael A. "Black Knot of Plums and Cherries."
http://ohioline.osu.edu/hyg-fact/3000/3011.html.
Last accessed 11/22/03