Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 19, 2007
Publication Date: October 1, 2007
Citation: Gale, L.R., Ward, T.J., Balmas, V., Kistler, H.C. 2007. Population subdivision of Fusarium graminearum sensu stricto in the Upper Midwestern United States. Phytopathology. 97(11):1434-1439. Interpretive Summary: Fungal pathogens within the Fusarium graminearum species complex cause diseases of cereal crops worldwide, including Fusarium head blight (FHB) of wheat and barley. These diseases result in billion dollar losses to agriculture each year. In addition, these fungi contaminate grain with trichothecene mycotoxins that pose a serious threat to animal health and food safety. As a result of our active molecular monitoring program we recently identified a significant number of FHB isolates from the Upper Midwestern United States with an unusual toxin type (3ADON chemotype). Our analyses indicate that most of these isolates belong to a novel FHB pathogen population closely related to European FHB populations, and may have been recently introduced into the U.S. These results contradict previous claims that FHB pathogen variation in North America can be represented by local FHB diversity. This information is critical to plant breeding and disease control efforts that will benefit cereal producers and consumers.
Technical Abstract: A collection of 712 Fusarium graminearum sensu stricto strains, originating from 52 counties and 83 wheat and barley fields from nine states within the United States and predominantly gathered between 1999-2000 was characterized by RFLPs and PCR-based trichothecene type analyses and compared with 19 strains of F. graminearum s.s. from an Italian collection. The majority of the Fusarium head blight causing strains from the U.S. belonged to a cohesive genetic population characterized by a 15ADON trichothecene type. High levels of gene flow were observed in the entire geographic range. However, using a Bayesian model-based clustering method we also identified genetically divergent groups of strains collected from Minnesota and North Dakota, and therefore rejecting the hypothesis that F. graminearum s.s. in the U.S. consists of a single population. These strains were primarily of a 3ADON trichothecene type and formed a distinct cluster with the majority of Italian strains, which displayed all three trichothecene types (15ADON, 3ADON, nivalenol). The existence of genetically divergent populations of F. graminearum s.s. within the same geographic range suggests that recombination is limited in this homothallic (selfing) fungal species. This inference was paralleled by moderate amounts of significant disequilibria between specific pairs of loci within populations.