Distribution and Traits of the Fungal Pathogen Fusarium virguliforme that Influence Spread and Survival in Minnesota

Abstract

Fusarium virguliforme is an invasive fungal pathogen responsible for soybean sudden death syndrome (SDS) and root rot of other legumes. Despite a previous study that suggested the pathogen would not survive in Minnesota due to winter cold stress, the pathogen was found in southern Minnesota in 2002 and has continued to spread throughout the state since its discovery. While much has been learned about the SDS pathogen in the United States, more work is needed to better understand the distribution of F. virguliforme in soybean and other crop fields and the strategies the pathogen uses to survive and spread in northern climates. This thesis includes studies of key traits that may aid in the survival of F. virguliforme, including 1) the asymptomatic and symptomatic host range of this fungus in a field setting, 2) the cold temperature limits of the pathogen’s survival, and 3) the unique nutrient use profiles of F. virguliforme. This work sought to expand our knowledge of the distribution of the pathogen within Minnesota. A distribution study in soybean fields was conducted in 2018 and 2019 based on SDS symptoms and the use of a specific qPCR assay. SDS and F. virguliforme were confirmed for the first time in Rice, Isanti, and Stevens counties; and the pathogen was confirmed for the first time in Clay, Douglas, Hubbard, and Pope counties. Cold temperature limits of the pathogen that may influence the spread and survival of F. virguliforme in Minnesota’s current and future climate were explored. It was determined that the pathogen can survive to temperatures of -40°C and thus survival is not likely limited by cold temperatures in Minnesota. The host range of the pathogen was studied to define which plant species may be vulnerable to disease development and potentially help the survival of F. virguliforme. Eleven species of dry edible beans, native legumes, prairie grasses, and prairie flowers common in the Midwestern United States were selected for field and greenhouse studies. It was determined that black bean, pinto bean, kidney bean, and pea are symptomatic hosts, whereas alfalfa, showy tick trefoil, black-eyed Susan, Indiangrass, partridge pea, and white clover are asymptomatic hosts. Lastly, the nutrients that support the growth of F. virguliforme, as well as the relative nutrient preferences and competitive abilities of F. virguliforme compared to other fungal and oomycete species commonly found in soil or soybean roots was explored within this thesis. The results suggest that F. virguliforme utilizes a larger number of carbon and nitrogen sources, both of which are potentially released from germinating seeds and crop residue and are strong stimulants of F. virguliforme growth. Additionally, it was determined that F. virguliforme is more competitive for these nutrients than the other species included in this study.