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Black Growths on Tree

These black spots or growths are on my tree in the front yard. How do I treat them?

The black growths on the stems are caused by black knot fungal disease. This disease attacks 24 species of Prunus, but is most often found on wild and cultivated varieties of plum and cherry.

The stem swellings, or 'knots', can girdle (stop flow of water and nutrients) limbs and stunt the plants growth. The fungus overwinters in these knots, then after a period of warm and wet weather in the spring the disease spores are released to infect other plant tissue or susceptible plants. This usually happens about the time of bud break.

Prune out and destroy limbs bearing knots now, before bud break, to prevent new infections. Cuts should be made at least 6-8 inches BELOW the knots. Disinfect your pruning tools after every cut to prevent spreading the disease. Rubbing alcohol or a solution of one part bleach to nine parts water will work too. Clean your tools after pruning to minimize the adverse effects of bleach. This is the best control THOUGH NOT 100% EFFECTIVE since there are likely other diseased plants in nearby natural and cultivated landscapes that serve as a source of future infection.


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