The Q&A Archives: Plum tree has black growths

Question: This spring I noticed my plum tree has many black, rough growths on the branches. What are they, and what should I do about them?

Answer: The growth is black knot fungus, and it infects plum as well as cherry trees. It will cause dieback and can eventually kill the tree. The knots have been desribed as looking like blackened marshmallows on the twig -- the same rough, blistered, corky appearance.

Remove the black knots on the smaller limbs by pruning out infected branches four inches below the growths.The best time to prune is in late winter, before growth begins. Burn the pruned branches before the spores are released. For larger limbs that you don't want to remove, scrape the black growth off the bark with a knife. Check each winter thereafter for regrowth. To manage severe infections, apply a lime sulfur fungicide spray in early spring before new growth emerges. You may have to spray more frequently if weather is warm and humid.

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