Two Major Resistance Quantitative Trait Loci are Required to Counteract the Increased Susceptibility to Fusarium Head Blight of the Rht-D1b Dwarfing Gene in Wheat

Fusarium head blight (FHB) is a destructive wheat (Triticum aestivum L.) disease of global importance. The widely used dwarfing allele Rht-D1b has recently been shown to compromise FHB resistance. The objectives of this study were to investigate the impact of this dwarfing allele in a segregating population with major resistance quantitative trait loci (QTL) derived from 'Sumai-3' and Nobeokabozu, and to determine how many resistance QTL are needed to counteract its negative effect. Fusarium head blight resistance was evaluated in four field trials with spray inoculation and two field trials with point inoculation in a double-haploid (DH) population from a cross between the Swedish cv. Avle (susceptible spring type; wild-type allele Rht-D1a) and Line 685 (resistant winter type; semi-dwarf allele Rht-D1b). The Rht-D1 locus explained up to 38% of the phenotypic variation and was the most important QTL for FHB severity under spray inoculation but did not show any effect after point inoculation. Fhb1 on 3BS was detected with both inoculation methods but was relatively more important after point inoculation. Another two QTL on 5A and 2BL were detected after spray inoculation and a QTL on 2D after point inoculation. Comparison of phenotypic effects of different allele combinations revealed that a combination of both Fhb1 and the 5A QTL was required to counteract the increased susceptibility of Rht-D1b. Although breeding of FHB resistant cultivars with this dwarfing allele is possible, it requires the pyramiding of several resistant QTL to achieve adequate levels of resistance.