Black Fungus on Purple Plum Trees

Identification

• Caused by the fungus Dibotryon morbosum, black knot causes black swellings, or knots, on the plum tree's smaller branches. The swellings start out green and are usually hard and elongated. The knots may also appear on the tree's trunk and larger, important branches. Black knot occurs on plum trees throughout the U.S. The fungus lives inside the swellings and in the wood adjacent to the knots. The spores appear in spring on the surface of the knots as small sacs with minute fruiting bodies within. Rain and wind helps push the spores out of the sacs and spread the infection. Dibotron morbusum is also called Apiosporina morbosa. Mild temperatures and spring rains help the spores spread and infect plum trees.

Damage

• Black knot interferes with the plum tree's growth, causing stunted and deformed leaves and twigs. The knots may go unnoticed when they first form on the trees at the end of the growing season. As new growth begins the following spring, the knots also grow, becoming up to 12 inches long. The knots may completely encircle the twig or branch it infects, killing it.

Control

• Prune out infected areas of the plum tree in fall and late winter. Make the cuts 4 to 6 inches beyond the knots, to ensure all nearby infected wood is removed. Gather all plant debris and weeds from around the base of the tree. Bury or burn the plum tree cuttings, weeds and other plant debris to prevent spreading spores inadvertently. Spray the tree with a fungicide formulated to treat black knot and follow label directions for application rates and times. Destroy all wild plum or cherry trees growing within 600 feet of the plum tree.

Prevention

• Plant only resistant varieties, when available. Plant upwind of old orchards or orchards that have black knot problems. Monitor the purple plum tree for signs of black knot and cut away any suspected infections before bud break in spring. Apply a preventive fungicide beginning in early spring, before bud break, and continuing at intervals through summer. Application rates, repetitions and timing vary depending upon the chemical composition of the fungicide. Follow label directions carefully for best results. Disinfect pruning tools and other gardening blades used on and around the plum tree to avoid spreading infection.