Evaluation of an inoculation method and quantitative trait loci for fusarium head blight resistance in wheat

Fusarium head blight (FHB), which is principally caused in the U.S. by Fusarium graminearum Schwabe [telemorph: Gibberella zeae Schw. (Petch)], is a devastating disease of wheat (Triticum aestivum L.) and barley (Horderum vulgare L.). Losses inflicted by this disease are the result of reductions in grain yield and test weight, as well as the contamination of grain with the mycotoxin deoxynivalenol (DON). In the first study, the objective was to identify quantitative trait loci (QTL) for resistance to FHB traits in the resistant line IL97-1828. Resistance to FHB in IL97-1828 is independent of the well characterized and widely used Asian sources. A population consisting of 242 wheat recombinant inbred lines (RILs), developed from a cross between IL97-1828 and the FHB susceptible line Clark, were evaluated for FHB resistance in the field at Urbana, IL in 2009 and 2010, and at Wooster, OH in 2010. The population had broad continuous distributions for all measured disease parameters (incidence, severity, FHB index, Fusarium damage kernels (FDK) percentage, DON concentration, and ISK index, which is a disease index that incorporates disease incidence, severity, and % FDK. Significantly less disease incidence was observed in 2009 than at both locations in 2010, and significantly lower FDK percentage was observed in Ohio compared to Urbana in 2010. Correlations among environments for disease measurements, while significant, were moderate to moderately weak (0.20< r <0.54). Within an environment, disease measurements were also significantly correlated in all cases. QTL for resistance to FHB were identified on seven linkage groups that mapped to six different chromosomes. In all cases, QTL were minor explaining between 2.9% and 8.7% of the phenotypic variance. No QTL were consistently identified across all three environments, and three of the detected QTL were only identified in a single environment for a single trait. A region on chromosome 1B was the only region identified for Type I resistance. A QTL detected on chromosome 2B was significant for severity, FHB index, FDK percentage, and ISK index, and explained 8.7% of the phenotypic variance for the mean of ISK index averaged over environments. The region on 2B was also identified for reduction in DON concentration in both years in Urbana, IL. A QTL on the long arm of chromosome 3B was identified for severity, FHB index, FDK percentage, and ISK index, explaining the most phenotypic variance for ISK index at Urbana in 2009 (R2=7.72%). Previous reports have identified QTL for FHB resistance in similar genomic regions as those identified in this study. The results of this study indicate several regions contributing small effects are important for resistance to FHB in IL97-1828. In the second study, two different inoculation methods for the evaluation of FHB resistance were compared. In one method, the spray and bag method, a bag was placed over wheat heads for 24 h that had been inoculated using a conidia suspension. The second method, a grain spawn method, was used in which F. graminearum infested maize kernels were disseminated, and irrigation was used to provide favorable disease conditions. The latter is a widely used and accepted method, which served as the standard with which the spray and bag method was compared. Plots for the spray and bag method were evaluated for incidence, severity, and FHB index at a single location in Illinois in 2008, and at two locations (Urbana and Brownstown) in Illinois in 2010. Plots inoculated with grain spawn were evaluated at a single location for both years for disease incidence, severity, FHB index, FDK percentage, and ISK index. A subset of plots in 2010 were inoculated using only a conidia spray to evaluate the effect on FHB measurements of placing a bag over inoculated heads. Spearman correlation coefficients in 2008 for entry means were significant (0.0018 < P < 0.0001) between the two methods in all cases with coefficients ranging from 0.41(incidence) to 0.83 (severity). In 2010, correlations were not as strong and ranged between 0.22 (incidence) and 0.54 (FHB index) for the two location mean of the spray and bag method and the grain spawn method. Incidence was extremely high in 2010, especially in Brownstown (97.6%), possibly contributing to the weaker correlations between methods. High disease pressure can potentially overwhelm resistance, making all lines appear susceptible regardless of resistance level. Plots spray-inoculated without a bag correlated poorly with the grain spawn method, possibly due to disease escape. Bagging of inoculated heads appears ideal in order to provide high relative humidity for disease development, similar to irrigation in the grain spawn method. Decreasing the inoculum concentration and possibly the duration of bagging would potentially better separate breeding lines for FHB symptoms, especially incidence. Additionally, it could possibly improve correlations between methods. The results from this study indicate the spray and bag method could be used to screen advanced wheat breeding lines for FHB resistance