Development and Genetic Diversity of Sclerotinia Sclerotiorum on Potato in the Columbia Basin

By Zahi Kanaan-Atallah, Ph.D. Washington State University May 2003 Chair: Dennis A. Johnson Sclerotinia sclerotiorum (Lib.) de Bary is a cosmopolitan, homothallic and necrotrophic ascomycetous fungus dispersed by airborne ascospores and soilborne sclerotia. It causes disease in over 400 host species. A significant correlation between potato yield losses and stem rot incidence has not been observed, while 1-3 fungicide applications are made to manage the disease. In the Pacific Northwest, iprodione, dichloran, quintozene and fluazinam are registered for stem rot control; labels recommend fungicide applications be made prior to row closure. Reduced control could be caused by mistimed applications, inadequate tissue coverage or a selection for fungicide resistant isolates. Ascospores were captured and disease incidence was monitored in ten potato fields in the Columbia Basin to determine when infections occur. Peak ascospore release corresponded with initial full bloom (7 to 10 days after row closure) and disease onset occurred 10-14 days following row closure. Airborne S. sclerotiorum ascospores impacted potato blossoms attached to plants. Blossoms fell to the ground and onto stems, senesced and were colonized by the fungus. Potato vines became infected after dropping to the ground and coming in contact with the fungus. Flower removal and fungicide applications at initial full bloom reduced disease incidence, in comparison to untreated controls. These observations indicate that fungicide label recommendations need to be based on initial full bloom, instead of row closure, to provide