Diversity, Geographic Distribution, and Pathogenicity of Root-associated Fusarium spp. in Nebraska Soybeans

Authors

• Kelvin Kagondu Muchiri, University of Nebraska-LincolnFollow

First Advisor

Teddy Garcia-Aroca

Committee Members

Dylan Mangel, Clemencia Rojas, Stephen N. Wegulo

Date of this Version

8-2026

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Plant Pathology

Under the supervision of Professor Teddy Garcia-Aroca

Lincoln, Nebraska, August 2026

Comments

Copyright 2026, Kelvin Kagondu Muchiri. Used by permission

Abstract

Soybean (Glycine max [L.] Merr.) is a major crop in Nebraska and the U.S. Midwest. Fusarium includes economically important soilborne pathogens of soybean. Despite their known presence in agricultural systems, the diversity, geographic distribution, and pathogenicity of root-associated Fusarium spp. in Nebraska remain poorly documented.

Symptomatic soybean plants were collected from 23 Nebraska counties mainly during the 2024 growing season and partly in 2025. Fungi were isolated from diseased roots, grouped by colony morphology and screened using the internal transcribed spacer region. Isolates identified within the genus Fusarium were further sequenced for partial translation elongation factor 1-alpha (tef1-α) and RNA polymerase II second-largest subunit (rpb2) loci. Newly generated sequences were analyzed with reference sequences using Maximum-likelihood phylogenetic analysis and bootstrap support to assign isolates to species and species complexes.

Fusarium was the most frequently recovered fungal genus. Ninety-seven isolates were assigned to seven species complexes: Fusarium incarnatum-equiseti (FIESC), F. oxysporum (FOSC), F. graminearum/F. sambucinum (FGSC/FSAMSC), F. fujikuroi (FFSC), F. solani (FSSC), F. tricinctum (FTSC), and F. nisikadoi (FNSC). FIESC and FOSC were the most frequently recovered, whereas FNSC was the least frequently recovered. Pathogenicity and virulence of 36 isolates were evaluated on two soybean cultivars AG27xF3 and NK28P6xF1 under controlled conditions. Eight isolates caused significantly greater severity than the control on AG27xF3, while only F. vanettenii FELCC712 differed from the control on NK28P6xF1.

In a separate greenhouse experiment, six standardized five-component Fusarium species-complex consortia were evaluated using colonized wheat-kernel inoculum. The assay was repeated twice, and biomass measurements were averaged across runs. Consortium treatment significantly affected fresh and dry biomass. FFSC, FGSC/FSAMSC, FOSC, FSSC, and FTSC reduced biomass relative to the uninoculated wheat kernel control, whereas FIESC did not differ from the control.

Overall, soybean root-associated Fusarium populations in Nebraska were phylogenetically diverse and geographically variable. Disease responses differed among isolates and species complex consortia. These findings demonstrate the importance of integrating multi-locus identification with controlled pathogenicity testing to determine disease relevance of recovered Fusarium taxa.

Advisor: Teddy Garcia-Aroca