Characterization of the Fungal Community Associated with Root, Crown, and Vascular Symptoms in an Undiagnosed Yield Decline of Winter Squash

Abstract

Winter squash (Cucurbita maxima) is produced in Oregon’s Willamette Valley for edible seeds, processing, and fresh markets. Recently, prominent cultivar Golden Delicious has experienced significant yield losses due to a soilborne disease. Symptoms include stunting, root and crown rot, vascular discoloration, and late-season vine collapse. To identify potential causal pathogens, 64 fields were surveyed during 2014 to 2016 to characterize the fungal community associated with surface-disinfested root, crown, and stem tissue of diseased and healthy squash. Over 10,000 fungal isolates were identified morphologically, and 1,783 isolates were identified to species by sequencing of internal transcribed spacer and translation elongation factor 1 alpha genomic regions. Fungal communities were analyzed for association with the presence or absence of field symptoms using multivariate community analyses (indicator species analysis, multiresponse permutation procedure, and nonmetric multidimensional scaling). Although no fungal species were consistently associated with disease, five species were consistently isolated from plants regardless of the presence of symptoms and were capable of causing disease in a greenhouse pathogenicity trial: Fusarium oxysporum, F. solani, F. culmorum, Plectosphaerella cucumerina, and Setophoma terrestris. Results from community analyses confirmed that some fungi were more common in specific tissues (e.g., P. cucumerina in stems and crown, F. solani in roots and crown). Symptom severity tended to be greater in fields with a prior history of squash production, although a few fields with no prior history of squash had above average symptom ratings. The results from this study suggest that the five most common fungi may take on greater disease significance when they co-occur in a host.