Morten Lillemo

Spot blotch caused by Bipolaris sorokiniana is a major disease of wheat in warm and humid wheat growing regions of the world including south Asian countries such as India, Nepal and Bangladesh. The CIMMYT bread wheat line Saar which carries the leaf tip necrosis (LTN)-associated rust resistance genes Lr34 and Lr46 has exhibited a low level of spot blotch disease in field trials conducted in Asia and South America. One hundred and fourteen recombinant inbred lines (RILs) of Avocet (Susceptible) × Saar, were evaluated along with parents in two dates of sowing in India for 3 years (2007–2008 to 2009–2010) to identify quantitative trait loci (QTL) associated with spot blotch resistance, and to determine the potential association of Lr34 and Lr46 with resistance to this disease. Lr34 was found to constitute the main locus for spot blotch resistance, and explained as much as 55 % of the phenotypic variation in the mean disease data across the six environments. Based on the large effect, the spot blotch resistance at this locus has been given the gene designation Sb1. Two further, minor QTL were detected in the sub-population of RILs not containing Lr34. The first of these was located about 40 cM distal to Lr34 on 7DS, and the other corresponded to Lr46 on 1BL. A major implication for wheat breeding is that Lr34 and Lr46, which are widely used in wheat breeding to improve resistance to rust diseases and powdery mildew, also have a beneficial effect on spot blotch.

Fusarium head blight (FHB) is a destructive wheat disease of global importance. Resistance breeding depends heavily on the Fhb1 gene. The CIMMYT line Shanghai-3/Catbird (SHA3/CBRD) is a promising source without this gene. A recombinant inbred line (RIL) population from the cross of SHA3/CBRD with the German spring wheat cv. Naxos was evaluated for FHB resistance and related traits in field trials using spray and spawn inoculation in Norway and point inoculation in China. After spray and spawn inoculation, FHB severities were negatively correlated with both anther extrusion (AE) and plant height (PH). The QTL analysis showed that the Rht-B1b dwarfing allele co-localized with a QTL for low AE and increased susceptibility after spawn and spray inoculation. In general, SHA3/CBRD contributed most of the favorable alleles for resistance to severity after spray and spawn inoculation, while Naxos contributed more favorable alleles for reduction in FDK and DON content and resistance to severity after point inoculation. SHA3/CBRD contributed a major resistance QTL close to the centromere on 2DLc affecting FHB severity and DON after all inoculation methods. This QTL was also associated with AE and PH, with high AE and tall alleles contributed by SHA3/CBRD. Several QTL for AE and PH were detected, and low AE or reduced PH was always associated with increased susceptibility after spawn and spray inoculation. Most of the other minor FHB resistance QTL from SHA3/CBRD were associated with AE or PH, while the QTL from Naxos were mostly not. After point inoculation, no other QTL for FHB traits was associated with AE or PH, except the 2DLc QTL which was common across all inoculation methods. Marker-assisted selection based on the 2DLc QTL from SHA3/CBRD combined with phenotypic selection for AE is recommended for resistance breeding based on this valuable source of resistance.

Powdery mildew is one of the most important wheat diseases in temperate regions of the world. Resistance breeding is considered to be an economical and environmentally benign way to control this disease. The German spring wheat cv. ‘Naxos’ exhibits high levels of partial and race non-specific resistance to powdery mildew in the field and is a valuable source in resistance breeding. The main objective of the present study was to map the genetic factors behind the resistance in Naxos, based on a population of recombinant inbred lines (RIL) from a cross with the susceptible CIMMYT breeding line SHA3/CBRD. Powdery mildew severity was evaluated in six field trials in Norway and four field trials in China. The major quantitative trait locus (QTL) with resistance from Naxos was detected close to the Pm3 locus on 1AS in all environments, and explained up to 35% of the phenotypic variation. Naxos was shown to carry another major QTL on 2DL and minor ones on 2BL and 7DS. QTL with resistance from SHA3/CBRD were detected on 1RS, 2DLc, 6BL and 7AL. The QTL on the 1B/1R translocation showed highly variable effects across environments corresponding to known virulence differences against Pm8. SHA3/CBRD was shown to possess the Pm3 haplotype on 1AS, but none of the known Pm3a-g alleles. The RIL population did not provide any evidence to suggest that the Pm3 allele of SHA3/CBRD acted as a suppressor of Pm8.

With 4 tables Powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important bread wheat (Triticum aestivum L.) disease that is favoured by the temperate Scandinavian climate. A total of 38 lines, comprising the current spring and winter wheat cultivars in Norway, various breeding lines and introduced sources of partial resistance were evaluated for race‐specific resistance at the seedling stage. Various resistance genes were postulated from the reaction patterns of lines with known genes tested against 20 differential isolates in detached leaf assays. Detection of the Pm8 gene and alleles at the Pm3 locus were further aided by specific PCR primers. The most frequently present resistance genes were Pm8, followed by Pm5a, Pm3d, Pm3f, Pm3b and Pm2. Several lines were susceptible to all isolates at the seedling stage, but were known to possess high levels of partial resistance in the field; these included the German spring wheats ‘Naxos’ and ‘Paros’ and the Swedish winter wheats ‘Folke’ and ‘Mjølner’. These lines represent highly valuable sources of partial and potentially durable resistance to powdery mildew.

Powdery mildew, caused by Blumeria graminis f. sp. tritici is a major disease of wheat (Triticum aestivum L.) that can be controlled by resistance breeding. The CIMMYT bread wheat line Saar is known for its good level of partial and race non-specific resistance, and the aim of this study was to map QTLs for resistance to powdery mildew in a population of 113 recombinant inbred lines from a cross between Saar and the susceptible line Avocet. The population was tested over 2 years in field trials at two locations in southeastern Norway and once in Beijing, China. SSR markers were screened for association with powdery mildew resistance in a bulked segregant analysis, and linkage maps were created based on selected SSR markers and supplemented with DArT genotyping. The most important QTLs for powdery mildew resistance derived from Saar were located on chromosomes 7DS and 1BL and corresponded to the adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29. A major QTL was also located on 4BL with resistance contributed by Avocet. Additional QTLs were detected at 3AS and 5AL in the Norwegian testing environments and at 5BS in Beijing. The population was also tested for leaf rust (caused by Puccinia triticina) and stripe rust (caused by P. striiformis f. sp. tritici) resistance and leaf tip necrosis in Mexico. QTLs for these traits were detected on 7DS and 1BL at the same positions as the QTLs for powdery mildew resistance, and confirmed the presence of Lr34/Yr18 and Lr46/Yr29 in Saar. The powdery mildew resistance gene at the Lr34/Yr18 locus has recently been named Pm38. The powdery mildew resistance gene at the Lr46/Yr29 locus is designated as Pm39.

Powdery mildew, caused by Blumeria graminis (syn. Erysiphe graminis) f. sp. tritici, is an important disease of bread wheat (Triticum aestivum) in many countries. The CIMMYT bread wheat line Saar has exhibited a high level of partial resistance to powdery mildew in field trials conducted in Europe, Asia, and South America, and represents a valuable source of resistance in wheat breeding. A set of 114 random F₅ inbred lines from the cross Saar × Avocet-YrA (susceptible) were evaluated in replicated field trials at two locations in southeastern Norway to determine the number of genes involved in partial resistance to powdery mildew. Narrow-sense heritability estimates were high (0.83 to 0.92). Based on both quantitative and qualitative genetic analyses, the minimum number of genes with additive effects segregating for powdery mildew resistance in the population was four. Transgressive segregation indicated that Avocet-YrA might have contributed one minor gene for resistance. It is concluded that partial resistance to powdery mildew in Saar is controlled by at least three genes. Such resistance conferred by multiple genes having additive effects is expected to be durable.

A good understanding of the target environment and the extent of genotype × environment (G × E) interaction is essential for all cereal breeding programs. Differential adaptation of bread wheat (Triticum aestivum L.) to various heat‐stressed environments around the world was analyzed by cumulative cluster analysis of locations and genotypes in 9 yr of CIMMYT's High Temperature Wheat Yield Trial (HTWYT). The grouping pattern of yield‐testing environments could largely be explained by the temperature at different growth stages and relative humidity at booting. A clear distinction was observed between sites with heat stress and more temperate locations, and the heat‐stressed environments could be grouped into sites experiencing high temperature throughout the season and sites with more specific terminal heat stress. In addition, dry and humid heat‐stressed locations tended to differentiate. The ability of individual locations to predict yield in different heat‐stressed environments was studied by the shifted multiplicative model (SHMM) site clustering method, and identified locations like Tandojam (Pakistan), which associated well with both heat‐stressed and temperate environments. The good ability of the January planting date in Ciudad Obregon (Mexico) to predict yield performance in many heat‐stressed environments was also confirmed. Genotypes grouped according to their relative performance in different locations, and specific adaptation to the various types of heat‐stressed environments was apparent. However, a subset of genotypes was identified that showed stable, and high yield across all types of environments, both heat‐stressed and temperate.