Colonization of rotation crops and weeds by the potato black dot pathogen Colletotrichum coccodes

The rotation crops wheat, barley, oat, maize, soybean, rye, yellow mustard, alfalfa, and spring canola and weeds eastern black nightshade, velvetleaf, timothy grass, orchard grass, and Giant foxtail common to potato-growing areas in North America were used to study the host range of Colletotrichum coccodes, the causal agent of potato black dot. The fungus was isolated from nine of 14 rotation crops and weeds that were inoculated: yellow mustard, soybean, spring canola, alfalfa, oat, eastern black nightshade, velvetleaf, giant foxtail, and timothy grass. In all, colonization was highest in black nightshades (87%) and velvetleaf (80%). Among the rotation crops, colonization was highest on yellow mustard (59%) followed by spring canola (33%) and soybean (30%). Colletotrichum coccodes was not isolated from wheat, barley, rye, maize, or orchard grass. The results indicated that crops used for rotation with potato should be selected carefully to prevent the increase of C. coccodes inoculum in the soil and that weeds may help maintain viable inoculum of C. coccodes in the absence of potato. Based on these results we recommend that wheat, barley, maize, or rye be used in rotation with potato in areas where C. coccodes is present in high levels in the soil.