Fusarium Head Blight (FHB) in North America is primarily caused by three Fusarium graminearum populations known as NA1, NA2, and NA3. NA1 mainly produces the trichothecene toxins DON and 15ADON, while NA2 mostly produces 3ADON instead of 15ADON. The 3ADON chemotype reportedly confers higher levels of aggressiveness and toxigenicity than 15ADON, perhaps accounting for the steady expansion in range of the NA2 population from its apparent origin in the Canadian Maritimes. We are using a genetic approach to test this hypothesis. A self-sterile strain of F. graminearum, generated by deleting the MAT1-1-1 locus in the NA1-15ADON isolate PH-1, was crossed with an NA2-3ADON strain that was more aggressive and toxigenic on wheat. A total of 230 single-ascospore progeny were collected from 20 perithecia. Ninety-three progeny were randomly screened with PCR primers targeting the MAT1-1-1 and the TRI loci. The progeny exhibited expected Mendelian segregation for both markers. Eighty of these progeny, including 20 of each parental and recombinant type, were assayed for disease severity and mycotoxin production on Wheaton and Alsen wheat. No significant difference in either trait was observed between progeny with the 15-ADON versus 3-ADON chemotypes on either wheat variety. Twenty-one progeny strains consistently exhibited high or low aggressiveness (more than two standard deviations above or below the mean). After retesting on Wheaton, the strains comprised three groups: the “high” group was not statistically different from the more aggressive 3ADON parent; the “moderate” group was statistically equivalent to the 15ADON parent; and the “low” group was not statistically different from the water control. Each strain was individually genome-sequenced. Interestingly, this revealed that six of the "low" aggressiveness progeny were actually clonal with the 3ADON parent, and had come from a single selfed perithecium. Further investigation revealed that two of the 20 perithecia sampled were selfed.  Thirteen progeny from these perithecia were removed from the original dataset, with no overall effect on the results. Our analysis so far suggests that factors other than chemotype are more important in conferring aggressiveness and toxigenicity. Comparative analysis of high-moderate progeny pools may help identify these factors.

Acknowledgements: This material is based upon work supported by the U.S. Department of Agriculture.  This is a cooperative project with the U.S. Wheat & Barley Scab Initiative. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.