Definition of black knot in English:

black knot

noun

  • 1Irish English rare Irish English a knot which is difficult to undo (now rare).

  • 2A fungal disease of fruit trees characterized by the formation of irregularly shaped rough black swellings on the bark; (also, more fully "black knot fungus") any of several fungi causing this disease, especially Dibotryon morbosum, chiefly affecting plum and cherry trees.

Origin

Mid 19th century; earliest use found in Irish Penny Journal.