Management of southern blight on tomato with SDHI fungicides

Southern blight, caused by the soilborne basidiomycete fungus Athelia rolfsii (formerly Sclerotium rolfsii) is a persistent disease affecting tomato throughout the southern United States and other countries. Post-transplant applications of succinate-dehydrogenase inhibitor (SDHI) or quinone-outside inhibitor (QoI) fungicides may be effective alternatives to broad-spectrum fumigation to manage this disease. In vitro, fluxapyroxad + pyraclostrobin applied directly to dormant sclerotia or to tomato stems inoculated with nontreated sclerotia reduced sclerotial germination and colony diameter. Penthiopyrad applied to tomato stems also reduced sclerotial germination and colony diameter. In 2015 and 2016, pyraclostrobin, penthiopyrad, and fluxapyroxad + pyraclostrobin were applied twice post-transplanting and quintozene was applied once at transplanting to tomato in the field. Fluxapyroxad + pyraclostrobin and quintozene reduced southern blight compared to the nonsprayed control in both years, whereas efficacy of pyraclostrobin and penthiopyrad varied. Quintozene, however, stunted 43-75% of the plants. Fruit number and weight did not differ among treatments in either year. Fluxapyroxad + pyraclostrobin or penthiopyrad followed by pyraclostrobin can be applied to reduce incidence of southern blight on tomato.