Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 12, 2010
Publication Date: January 3, 2011
Citation: Gale, L.R., Harrison, S.A., Ward, T.J., O Donnell, K., Milus, E.A., Gale, S.W., Kistler, H.C. 2011. Nivalenol-type populations of Fusarium graminearum and F. asiaticum are prevalent on wheat in Southern Louisiana. Phytopathology. 101(1):124-134. Interpretive Summary: Fusarium head blight (FHB) or scab is an extremely destructive disease on all varieties of wheat and barley. The disease, which is caused by the fungus Fusarium, has been especially destructive in the United States and can account for up to $1 billion in crop losses. These fungal pathogens also produce several compounds that are toxic to humans and animals. In order to assist effective plant breeding, disease management, and toxin reduction programs, it is essential to understand the epidemiology of the disease as well as the extent of variation in the head blight pathogen. Therefore, a large collection of FHB strains was obtained between 2001 - 2007 from production wheat fields in Louisiana and compared to strains from 11 other states in order to determine FHB pathogen diversity and population structure within the United States. A novel population of the pathogen was found in Louisiana, as was a fungal species previously reported only from Asia and South America. In addition, many of the FHB pathogens found in Louisiana wheat fields produced toxins that were previously considered to be rare in the United States. These results are critical to promoting food safety and cereal production through improved detection of mycotoxin-contaminated grain and through variety improvement efforts that account for the entire spectrum of pathogen and toxin types.
Technical Abstract: U.S. populations of the Fusarium graminearum (Fg) clade cause Fusarium head blight (FHB) on wheat and barley and usually produce the trichothecene mycotoxin deoxynivalenol in infected grain. Recently, however, individual nivalenol (NIV)-producing isolates from the U.S. were described, that belonged either to the newly described species F. gerlachii, or to the genetically distinct Gulf Coast population of F. graminearum sensu stricto (Starkey et al., 2007). Here, we describe the discovery of nivalenol-producing strains of F. graminearum that are found in high proportion (78.5%) in small-grain growing regions of southern Louisiana. From collections gathered between 2001 and 2007, we genotyped 237 isolates from Louisiana with newly developed PCR-RFLP markers and multiplex PCR primers that distinguish among trichothecene type. These isolates were analyzed together with 297 isolates from 11 other U.S. states, predominantly from the Midwest. Using Bayesian-model based clustering, we discovered a Southern Louisiana population of F. graminearum that is genetically distinct from the previously recognized pathogen population from the Midwest (MW15ADON population). Population membership is also correlated with trichothecene type. Most isolates from the Southern Louisiana population displayed the nivalenol trichothecene type, while the majority of the isolates from the Midwest were of the 15ADON type. A smaller proportion of isolates from Louisiana belonged to the previously described Gulf Coast population that was mostly a 3ADON type. The nivalenol type was also identified in collections from Arkansas (11.5%), North Carolina (40%) and Missouri (1.9%), with the collections from Arkansas and North Carolina being small and not representative. Members of F. asiaticum, previously only identified from Asian countries and from Brazil were also detected in 17% of Fg clade isolates from the two Southern Louisiana parishes Acadia and Alexandria. All 41 F. asiaticum isolates were of the nivalenol trichothecene type. Greenhouse tests indicated that U.S. nivalenol producers on average accumulated four times less trichothecene toxin than DON-producers on inoculated wheat. This is the first report of the presence of significant nivalenol-producing populations of F. graminearum s. s. and F. asiaticum in the U.S.