Black knot causes elongated, rough, black swellings on twigs, branches and trunks. Stems girdled by black knot will die outward from the growth. In spring, fungal spores are released from existing knots and blown in the wind to other trees. Trees will gradually weaken and die if effective control measures are not taken. Do not purchase trees with visible knots or abnormal swellings on the branches.
Resistant plants are available.
A copper fungicide should be applied every two weeks from the time the leaf buds open until three weeks after flower petals fall. Fungicides prevent new infections but do not cure disease already present. There are many formulations of copper fungicides. The fungicide you use must be labeled for black knot fungus on your type of plant. Besides spraying you must prune out all knots to remove old infections.