Disease Directory

    Black Knot

    Apiosporina morbosa (Dibotryon morbosum)

    Fungus
    Stems

    Overview

    A distinctive fungal disease of plums, cherries, and other Prunus species, causing hard, black, elongated galls on branches that resemble lumps of coal or tar.

    Symptoms

    Hard, black, elongated, rough-textured galls on branches and twigs. Galls start as olive-green swellings and darken to black over two years. Branches may be girdled and die. Severe infections weaken and deform trees.

    Environmental Conditions

    Humid

    Prevention

    Plant resistant varieties. Prune out knots in winter (at least 4 inches below the gall). Remove wild plums and cherries nearby. Ensure good air circulation.

    Management

    Prune infected branches at least 4 inches below visible galls during dormancy. Destroy pruned material. Apply lime-sulfur during dormancy. Remove nearby wild Prunus hosts. Severe cases may require tree removal.