- Winter squash (Cucurbita maxima) grown in Oregon’s Willamette Valley for edible seed, frozen foods, and fresh markets is susceptible to an undiagnosed soilborne disease. Diseased squash fields exhibit symptoms of stunting, root and crown rot, vascular discoloration, and late-season wilt, which in extreme cases can lead to total crop failure. Surveys were conducted in 64 commercial fields from 2014 to 2016. Over 10,000 fungal isolates were identified morphologically, and an additional 1,783 isolates were identified to species by sequencing of ITS and EF1α genomic regions. Fungal communities were analyzed for association with the presence or absence of field symptoms using multivariate community analyses (Indicator Species Analysis, Multi-Response Permutation Procedure, Non-metric Multidimensional Scaling ordination). Although no fungal species were consistently associated with disease in the field, five species were consistently isolated from plants regardless of the presence of symptoms in the field and were capable of causing disease in a greenhouse pathogenicity trial: Fusarium oxysporum, F. solani, F. culmorum, Plectosphaerella cucumerina, and Setophoma terrestris. Results from multivariate community analyses confirmed that some fungi were more common in specific tissues (e.g. P. cucumerina in stems and crown, and 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. Field survey results leave open the question that the five most common fungi may take on greater significance when they co-occur in a host.
A subset of collected fungal isolates (n = 241) were characterized with phylogenetic analyses of the ITS and EF1α genomic regions and by pathogenicity testing in greenhouse and outdoor, large-container trials. F. solani isolates clustered with F. solani f.sp. cucurbitae race 1 and most were virulent on greenhouse-grown squash plants. F. oxysporum isolates were similar to references identified as cucurbit formae speciales but very few isolates caused disease in greenhouse trials. In pathogenicity experiments, virulent F. solani isolates produced the most severe symptoms, followed by F. culmorum, F. oxysporum, P. cucumerina, and S. terrestris. Some treatments of mixed species inoculum produced symptoms above what was expected from individual species. In particular, a mix of F. culmorum, F. oxysporum, and P. cucumerina and a mix of F. culmorum, F. solani, and S. terrestris had higher symptom ratings than F. solani alone. These trials indicate that the soilborne disease is primarily caused by F. solani, but that interactions among the complex of F. solani, F. culmorum, F. oxysporum, and P. cucumerina increases disease incidence and severity.
Cultivars of a range of cucurbit species (cucumbers, melons, and squash) were evaluated for susceptibility to the soilborne pathogen complex that has affected winter squash (Cucurbita maxima). The field trials included seven cucurbit species representing 35 cultivars and were planted at four commercial cucurbit farms in the 2015 and 2017 growing seasons. Fresh market farms with history of cucurbit production had higher symptom ratings than fields with no history of prior cucurbits. Among species, winter squash (C. maxima) and cantaloupe (Cucumis melo) exhibited the highest disease severities. A few winter squash (C. maxima) cultivars (e.g., ‘Winter Sweet’ and ‘Sweet Mama’) were identified as resistant alternatives to current industry standards. A greenhouse pathogenicity trial to evaluate susceptibility of 14 representative cucurbit cultivars to the previously identified members of the soilborne pathogen complex (F. solani, F. oxysporum, F. culmorum, P. cucumerina, and S. terrestris) demonstrated that F. solani was highly virulent. Though C. sativus and C. maxima x C. moschata showed low levels of disease in the field, they were susceptible as seedlings to F. solani inoculation.
Additionally, this project produced first reports of S. terrestris causing pink root rot of winter squash and F. culmorum causing fruit rot of winter squash in Oregon. Overall, this dissertation provides a framework for understanding the soil borne fungal agents that potentially compromise the health of winter squash and provide context for future studies into their management.