When the slender sections of the fruit trees crack and swell into knotty black outgrowth, even the most recent gardener senses that something isn’t right. The issue is most likely black knot, a bacterial infection that infects plums, cherries andmuch less frequently, apricots and peach trees. The fungus dibotryon morbosum infects the trees in early spring, although you may not notice the outward symptoms until much later. The ascospores overwinter on the blackened growths, spreading to other trees and branches in the end just before the buds open. You may often eradicate this disease by pruning out diseased places; fungicide is a solution of last resort.
Search each of your fruit trees of this genus Prunus in sunlight for signs of this disease. After the tree is first infected, black knot infection presents as little galls or olive-green swellings. The next growing season, the galls turn into big black knots.
Prune out each twig and branch bearing galls or knots, acting while the tree is dormant and prior to early March. The very first ascospore discharge occurs the spring after infection before the buds open. Make the cuts 8 ins into wood. If the swelling is about the Prunus back, use a chisel to dig into the tree and remove tissue 1 inch past the contaminated place. Use denatured alcohol or a solution of one part bleach to nine parts water to clean your tools between cuts to avoid spreading the disease. Burn all pruned material.
Prune back or remove wild Prunus shrubs in hedgerows around your yard. Wild trees and shrubs may harbor black knot and move it around to more delicate cultivated varieties. Burn all trees and branches cut down.
Use a fungicide spray as a last resort if pruning and good cultural practices fail to cure your trees. Use the spray according to label directions in early spring prior to the fruit tree buds open. Choose among the pesticides registered for this use including copper, chlorothalonil, mancozeb and thiophanate methyl. If you would like to keep organic, then pick an organic copper fungicide. If the illness is acute, reapply according to label directions throughout the summer.