New insights on the biology and host-pathogen interactions of the dollar spot pathogen, Sclerotinia homoeocarpa

Dollar spot, caused by the ascomycetous fungal pathogen Sclerotinia homoeocarpa, is one of the most economically important diseases of amenity turfgrass species worldwide. The research presented in this dissertation explores the basic biology of S. homoeocarpa with the aim of increasing understanding of pathogenesis to enable development of more sustainable, integrated disease management tactics for dollar spot control. Initial research focused on the sources of primary inoculum for dollar spot. A three-year field study assessing in planta survival of S. homoeocarpa revealed that this fungus overwinters predominantly in the shoots of symptomatic and asymptomatic turf and that environmental factors, particularly temperature and precipitation, may influence S. homoeocarpa overwintering. Commercial creeping bentgrass (CRB; Agrostis stolonifera L.) seed contamination assays employing both culture-based and molecular detection techniques found that viable, pathogenic isolates of S. homoeocarpa could be recovered from seed and that S. homoeocarpa DNA was present in over 50% of seeds lots tested, suggesting a high potential for dispersal of this pathogen in seed. Additional research studies addressed development of a model host system for S. homoeocarpa and elucidation of the role of oxalic acid during host infection. Various model hosts and the natural host CRB responded similarly to S. homoeocarpa inoculation; however, symptoms developed more rapidly in plants with high endogenous oxalate content. Investigation of this finding demonstrated that both host oxalate content and pathogen-produced oxalic acid may influence symptom development in response to S. homoeocarpa inoculation and that oxalic acid is likely associated with symptom development but not initial host colonization. Further, host tissue and the plant cell wall component xylan promoted production of oxalic acid by S. homoeocarpa. Additional studies with Brachypodium distachyon suggested potential for development of this host into a model system for elucidating host/pathogen interactions of S. homoeocarpa and two additional major fungal pathogens of turfgrass, Microdochium nivale and Rhizoctonia solani. Together, these studies provide new understanding of S. homoeocarpa epidemiology and biology and will facilitate development integrated disease management programs that minimize the use of fungicides for dollar spot suppression.