Identity and Pathogenicity of Fungi Associated with Root, Crown, and Vascular Symptoms Related to Winter Squash Yield Decline

Abstract

Winter squash (Cucurbita maxima cv. ‘Golden Delicious’) produced in Oregon’s Willamette Valley for edible seed production has experienced significant yield losses due to a soilborne disease. The symptoms associated with this disease problem include root rot, crown rot and vascular discoloration in the stems leading to a severe late season wilt and plant collapse. Through field surveys, Fusarium oxysporum, F. solani, F. culmorum-like fungi, Plectosphaerella cucumerina, and Setophoma terrestris were identified to be associated with diseased tissues, and each produced symptoms of root rot, crown rot or stem discoloration in preliminary pathogenicity trials. In this study, 219 isolates of these species were characterized by molecular identity analyses using BLAST of the ITS and EF1α genomic regions and by pathogenicity testing in outdoor, large-container trials. Molecular identity analyses confirmed the identity of isolates at 99 to 100% similarity to reference isolates in the database. In pathogenicity experiments, F. solani produced the most severe symptoms, followed by F. culmorum-like fungi, F. oxysporum, P. cucumerina, and S. terrestris. Some treatments of mixed species inoculum produced symptoms above what was expected from individual species. In particular, the mixture of F. culmorum-like fungi, F. oxysporum, and P. cucumerina and the mixture of F. culmorum-like fungi, F. solani, and S. terrestris had equally severe symptom ratings than that of F. solani by itself. Results indicate that this soilborne disease is primarily caused by Fusarium solani, but interactions among the complex of F. solani, F. culmorum-like fungi, F. oxysporum, and P. cucumerina, can exacerbate disease severity.