Disease Potential of Soil- and Tuberborne Inocula of Colletotrichum coccodes and Black Dot Severity on Potato.

Inoculum of Colletotrichum coccodes, the cause of potato black dot, is soil- or tuberborne. Understanding the disease potential of sources of inocula is crucial for developing disease management strategies and resistance screening techniques. Two hypotheses were tested in this study: (i) soilborne inoculum causes more disease than tuberborne inoculum and (ii) black dot severity is related to the concentration of soilborne inoculum. Trials were conducted in the greenhouse with standardized inoculum. Plants grown in infested soil had more sclerotia on roots than plants grown from infected tubers in three of four trials. In general, plants grown in infested soil produced fewer tubers and lower yields than the noninoculated plants. Plants grown from infected tubers produced similar numbers of tubers as the noninoculated plants in all trials, and had reduced yields in one of the four trials. Increasing concentrations of soilborne inoculum had a nonlinear association with disease development. Foliar symptoms, sclerotial density on roots, and sclerotial development on stems did not increase when soil inoculum exceeded 0.5 or 1.7 g/liters of soil. In this study, soilborne inoculum caused more disease than tuberborne inoculum and disease severity remained constant above a threshold of soilborne inoculum.