Definition of black knot
1 : a destructive disease of plum and cherry trees characterized by black excrescences on the branches and caused by a fungus (Dibotryon morbosa)
2 : a destructive disease of the gooseberry characterized by black excrescences on the branches and caused by a fungus (Dibotryon ribesia)
3 : a disease of the filbert and hazel characterized by black warty excrescences on the bark and caused by a fungus (Cryptosporella anomala)
4 : a cane infection of the grape caused by the crown-gall bacterium
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