Phylogenetic relationships and virulence assays of Fusarium secorum from sugar beet suggests a new look at species designations

Authors: Webb, Kimberly; SHRESTHA, SUBIDHYA - North Dakota State University; Trippe Iii, Richard; RIVERA-VARAS, VIVIANA - North Dakota State University; Covey, Paul; Freeman, Claire; DE JONGE, RONNIE - Utrecht University; SECOR, GARY - North Dakota State University; Bolton, Melvin

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2019
Publication Date: 8/9/2019
Citation: Webb, K.M., Shrestha, S., Trippe III, R.C., Rivera-Varas, V., Covey, P.A., Freeman, C.N., De Jonge, R., Secor, G.A., Bolton, M.D. 2019. Phylogenetic relationships and virulence assays of Fusarium secorum from sugar beet suggests a new look at species designations. Plant Pathology. 68(9):1654-1662. https://doi.org/10.1111/ppa.13082.
DOI: https://doi.org/10.1111/ppa.13082

Interpretive Summary: Many Fusarium spp. cause disease in sugar beet with the most common being F. oxysporum f. sp. betae. A new species, F. secorum was recently reported to also cause disease on sugar beet but little is known about how it compares in virulence or how it is related to the current population of F. oxysporum f. sp. betae. We used gene sequencing of the Tef1-a from 7 F. secorum isolates and used this to add those isolates to a previously described phylogenetic tree. Those findings suggest that some isolates previously identified as F. oxysporum are actually F. secorum (Clade B). When we added these isolates to a second tree representing the entire Fusarium genus, we also discovered that some F. oxysporum isolates may be another species, F. commune. Virulence testing on sugar beet indicates that F. secorum isolates range in aggressiveness to sugar beet from low virulence to high virulence. Taken together, our data suggests a greater range of Fusaria cause disease on sugar beet than previously thought. Consequently, screening sugar beet for disease resistance should rely on isolates representing the diversity of the population.

Technical Abstract: Fusarium spp. are responsible for significant yield losses in sugar beet (Beta vulgaris) in most production regions, with Fusarium oxysporum f. sp. betae most often reported as the primary causal agent. Recently a new species, F. secorum was reported to cause disease in sugar beet but little is known on the range of virulence within F. secorum nor how this relates to the overall Fusarium population previously described. To initiate this study, we obtained Tef1-a sequence from seven isolates of F. secorum and added this data to a phylogenetic tree that includes F. oxysporum f. sp. betae. Unexpectedly, the F. secorum strains nested into a distinct clade (Clade B) that included several isolates previously designated as F. oxysporum f. sp. betae, suggesting those species designations are outdated. These results prompted an expanded phylogenetic analysis of the Tef1-a sequence from genome sequences of publicly-available Fusarium spp. This analysis further designated isolates previously reported as F. oxysporum f. sp. betae from Clade A as F. commune, a species that is not known to be a sugar beet pathogen. Sugar beet isolates within Clade C nested within the Fusarium oxysporum species complex, confirming those isolates as F. oxysporum. Inoculation studies on cultivars with differing genetic backgrounds demonstrate that F. secorum range in virulence from low-highly virulent dependent on cultivar. Taken together, our data suggests a greater range of Fusaria cause disease on sugar beet than previously thought. Consequently, screening sugar beet for disease resistance should rely on isolates representing the diversity of the population.