Fusarium head blight (FHB) is an important disease of wheat worldwide. Soissons is one of the most resistant varieties grown in UK. The current study was undertaken to identify QTL for FHB resistance in Soissons and to determine whether the semi-dwarfing alleles Rht-B1b and Rht-D1b have a similar influence on susceptibility to FHB. A Soissons (Rht-B1b; Rht-D1a) x Orvantis (Rht-B1a; Rht-D1b) doubled haploid (DH) population was assessed for FHB resistance in three trials. Soissons contributed a single, stable major FHB QTL linked to the Rht-D1 locus. In contrast, the Rht-B1b allele (contributed by Soissons) conferred no negative effect on FHB resistance, even conferring a very minor positive effect in one trial. The influence of the Rht-B1b and Rht-D1b alleles on FHB resistance was further investigated using both Mercia and Maris Huntsman near-isogenic lines. Under high disease pressure both Rht-B1b and Rht-D1b significantly decreased Type 1 resistance (resistance to initial infection). However, whilst Rht-D1b has no effect on Type 2 resistance (resistance to spread of the fungus within the spike), Rht-B1b significantly increased Type 2 resistance. Our study demonstrates that the choice of semi-dwarfing gene used in plant breeding programmes may be a significant consideration where resistance to FHB is an important breeding target.

SUMMARYFor 4 successive years, cultivars of winter wheat differing in resistance to eyespot (Cercosporella herpotrichoides) and to lodging were grown in plots in which different levels of eyespot were induced by inoculating at different times. Lodging was either allowed, or prevented by growing the plants through nets. Yield losses were greatest in the most severely infected plots in which lodging was allowed; losses were reduced where infection was less severe or where lodging was prevented. In the year in which eyespot was most severe, there was a significant regression of yield on disease severity, even for the plots in which lodging was prevented; where lodging was allowed, regression lines were steeper for the lodging susceptible cultivars, but did not differ for the lodging resistant cultivars. Eyespot reduced the number of tillers, measured in spring, but later there was partial compensation for these losses. Grain number per ear and 1000 grain weight of individual tillers were reduced by severe eyespot infection; moderate infection reduced only 1000 grain weight; slight infection caused no reduction. Tillers without severe infection produced extra grains in severely diseased plots, so that there was no overall effect of eyespot on grain number per ear. The results are discussed in relation to the breeding of dwarf cultivars, which may be highly resistant to lodging but can nevertheless suffer severe losses from eyespot.

Chromosome 7D of the wheat line VPMl derived from a cross oi Aegilops ventricosa with wheat confers resistance to the facultative fungal parasite Pseudocercosporella herpotnehoides. To determine the number of genes responsible for this resistance, homozygous recombinant hnes were developed from an F| between the wheat variety 'Hobbit sib' and a substitution line carrymg chromosome 7D of VPMl in a 'Hobbit sib' background.Resistance to Pseudocercosporella herpotnehoides IS shown to be determmed by a single gene located di.stally on tbe long arm of chromosome 7D. EpDlb, a unique alleie of a gene encoding the readily detectable isoenzyme -endopeptidase, maps without recombination to Pchl suggesting for two separate genes a ma.ximum recombination value of 0.03 (P 0.05). Resistance to Pherpotriehoides could alternatively be a product of Ep-Dlh.Pchl is also mapped against a gene for adult plant resistance to brown rust {Pucania recondita), to Rc3 which confers coleoptiie colour, and to a-Amv-D2, an isQzyme that encodes a-atnylase production.

The genetic control of resistance to the fungus, Cercosporella herpotrichoides, causing the eyespot disease of wheat was studied using the Chinese Spring (Cappelle-Desprez) chromosome substitution lines and F 2 monosomic families of hybrids between Cappelle-Desprez and Mara. Chromosome 7A of Cappelle-Desprez was found to increase resistance to eyespot in both types of test. Chromosomes 2B and 5D of CappelleDesprez gave increased resistance compared to their homologues in Mara on an F 2 background. When substituted into Chinese Spring neither of these chromosomes appeared to increase resistance. Chromosome 1A of Cappelle-Desprez in a Chinese Spring background increased the level of infection. Dominance was towards resistance and the presence of between-chromosome interaction could be deduced.

Eyespot is an economically important stem-base disease of wheat caused by two fungal species: Oculimacula yallundae and Oculimacula acuformis. This study investigated the efficacy of two sources of resistance, viz. the genes Pch1, introgressed into hexaploid wheat from Aegilops ventricosa, and Pch2, identified in wheat cv. Cappelle Desprez, against O. yallundae and O. acuformis separately. In a series of seedling bioassays Pch1 was found to be highly effective against both species. Although Pch2 was found to confer resistance against both pathogen species, it was significantly less effective against penetration from O. yallundae than O. acuformis. Furthermore, a quantitative trait locus (QTL) analysis was not able to locate any resistance to O. yallundae on chromosome 7A of Cappelle Desprez. This has important implications for the use of Pch2 in commercial cultivars as it is necessary to have genes that confer resistance to both pathogens for effective eyespot control. In addition, a set of 22 T. monococcum accessions was screened for resistance to both O. yallundae and O. acuformis to identify potentially novel resistances and to assess the accessions for evidence of differential resistance to the eyespot species. Significant differences in resistance to the two pathogens were identified in four of these lines, providing evidence for differential resistance in T. monococcum. This study demonstrates that future screening for novel sources of eyespot resistance should investigate both pathogen species.