Characterization of resistance to Fusarium head blight in bread and durum wheat

Fusarium head blight (FHB) caused primarily by Fusarium graminearum (Fg) Schwabe (telomorph: Gibberella zeae Schw. [Petch]) in North America, is one of the most devstating diseases of wheat in Canada. An integrated approach to manage this disease is recommended that combines the adoption of cultural practices (tillage and crop rotation), cultivar resistance, and fungicide application at recommended timings. Resistance to FHB in wheat is a quantitatively inherited trait and highly influenced by environmental conditions. Sources of resistance are available in common wheat but not for durum wheat. There are no commercially available durum cultivars which are moderately resistant in North America which in part, can be explained by a lack of resistance in the primary gene pool. The current study was designed to study the effects (on disease suppression and linkage-drag associated with introgressions) of Sumai 3 derived Fhb1, Fhb2, and Fhb5 genes in hard red spring wheat cultivars [near-isogenic lines (NILs) developed in CDC Go and CDC Alsask backgrounds] from western Canada, the interaction of Fhb1 and Fhb5 with metconazole fungicide, and the mapping of quantitative trait loci (QTL) from emmer and durum wheat lines. The last part of the study utilized X-ray computed tomography as a tool to image selected NILs in the CDC Alsask background and focused on identification of key tissues conferring Type-II resistance to Fg. The phenotypic response of NILs carrying combinations of Sumai 3-derived genes suggested non-additive responses and Fhb5 was as effective as Fhb1 in conferring field resistance in both populations. Four to five resistance improving alleles, other than Fhb1, Fhb2, and Fhb5, in both populations were identified and three of five in the CDC Go population were contributed by the susceptible parent. The regions carrying these resistance improving alleles encoded disease resistance proteins, protein kinases, nucleotide-binding and leucine rich repeats’ domains. Complex epistatic gene-gene interactions among marker loci (including Fhb1, Fhb2, Fhb5) explained >20% of the phenotypic variation in FHB infection measurements. For the linkage drag experiment, introgressions resulted in lower thousand kernel weight and increased plant height with Fhb5. Among end-use quality traits, SDS-sedimentation volume and grain protein content were affected. In addition to Fhb1, Fhb2, Fhb5, we identified 10 loci in CDC Alsask NILs and nine in CDC Go NILs that affected the traits measured and none of these additional loci were common in both populations indicating the presence of multiple alleles in exotic sources that can result in linkage drag. Linkage drag is largely dependent on genetic background and the proportion of donor resistance alleles, thus, we observed more adverse effects among CDC Alsask NILs than among CDC Go NILs. Improvements in FHB resistance can still be made by introgressing the major genes examined in this study by using marker-assisted selection and selecting rare segregants with improved agronomy and end-use quality. There was an additive effect of Sumai 3-derived genes with metconazole in suppressing FHB and deoxynivalenol (DON) accumulation in the grain. Despite higher fungicide efficacy on moderately susceptible (MS) genotypes, FHB severity was greater on MS as compared to moderately resistant (MR) genotypes. Application of fungicides is warranted even on MR cultivars under moderate and high FHB disease pressure to reduce the amount of Fusarium damaged kernels (FDKs) and DON accumulation. In the QTL mapping study of tetraploid wheat, fifteen QTL (derived from both parents) for FHB resistance were identified on 11 of the 14 chromosomes using saturated linkage maps and a majority of the QTL were consistently detected in multiple environments. The combination of four relative large-effect and promising QTL reduced field FHB index, severity, incidence and visual rating index by 59%, 48%, 30%, and 29%, respectively. The majority of the QTL reported in the current study are novel and represent narrow intervals between the flanking markers; therefore, marker-assisted selection shoulb be of value in breeding FHB resistant durum wheat cultivars. In the final study of this thesis, as a proof-of-concept, we showcased the successful use of synchrotron-based X-ray imaging techniques to study the wheat-Fg interaction. This work indicated/re-confirmed the structural role of rachilla and rachis nodes in Type-II resistance to Fg in wheat. The results from all these studies will help wheat breeders to make decisions on introgressing exotic FHB resistance genes into common wheat. Additionally, novel QTL identified in tetraploid wheat can be used to enhance resistance in elite durum wheat lines by marker-assisted selection