Enhancing the genetic diversity and durability of leaf rust resistance in durum wheat

The importance of leaf rust, caused by Puccinia triticina, has increased dramatically in recent years in durum wheat (Triticum turgidum ssp. durum) worldwide. Little is known on the occurrence and nature of resistance in this crop. Thirty durum wheat lines derived from the International Maize and Wheat Improvement Center (CIMMYT) were characterized for their resistance to the Mexican P. triticina race BBG/BN which was identified in 2001 and caused susceptibility of a large number of the world's durum wheat cultivars. Ten genotypes with race-specific resistance displayed low to intermediate seedling reactions to leaf rust. In the field, eight genotypes were immune and two displayed moderate levels of resistance. The slow rusting resistant lines displayed a range of disease severity responses indicating genetic diversity. The yield protection conferred by race-specific and slow rusting resistance was investigated in yield loss trials under high leaf rust pressure in the field. Race-specific resistance provided effective protection against yield losses caused by leaf rust. Yield losses for slow rusting resistant lines were higher than for immune race-specific resistant ones, but significantly lower than for the susceptible checks. Slow rusting lines with high resistance levels and reduced yield losses were identified. The slow rusting components; latent period, uredinium size and receptivity, were determined in greenhouse experiments, and associations of these components with leaf rust progress in the field were calculated. The results indicated that predominantly uredinium size contributed to slow leaf rust progress in durum wheat. The genetic basis and diversity of race-specific resistance was also determined in progenies from crosses of nine durum wheat lines with a leaf rust susceptible parent, and from intercrosses among the resistant parents. Five distinct sources of resistance were identified, four of which involved single partially, or completely, dominant genes, of which two were closely linked, and a pair of partially dominant complementary genes. Using molecular tools, the two linked genes were located on the long arm of chromosome 6B. The best slow rusting resistant lines and the five distinct race-specific resistance sources can be used for enhancing the diversity and durability of leaf rust resistance in durum wheat

Characterization and Mapping of Leaf Rust and Stripe Rust Resistance Loci in Hexaploid Wheat Lines UC1110 and PI610750 under Mexican Environments

Growing resistant wheat varieties is a key method of minimizing the extent of yield losses caused by the globally important wheat leaf rust (LR) and stripe rust (YR) diseases. In this study, a population of 186 F8 recombinant inbred lines (RILs) derived from a cross between a synthetic wheat derivative (PI610750) and an adapted common wheat line (cv. “UC1110”) were phenotyped for LR and YR response at both seedling and adult plant stages over multiple seasons. Using a genetic linkage map consisting of single sequence repeats and diversity arrays technology markers, in combination with inclusive composite interval mapping analysis, we detected a new LR adult plant resistance (APR) locus, QLr.cim-2DS, contributed by UC1110. One co-located resistance locus to both rusts, QLr.cim-3DC/QYr.cim-3DC, and the known seedling resistance gene Lr26 were also mapped. QLr.cim-2DS and QLr.cim-3DC showed a marginally significant interaction for LR resistance in the adult plant stage. In addition, two previously reported YR APR loci, QYr.ucw-3BS and Yr48, were found to exhibit stable performances in rust environments in both Mexico and the United States and showed a highly significant interaction in the field. Yr48 was also observed to confer intermediate seedling resistance against Mexican YR races, thus suggesting it should be re-classified as an all-stage resistance gene. We also identified 5 and 2 RILs that possessed all detected YR and LR resistance loci, respectively. With the closely linked molecular markers reported here, these RILs could be used as donors for multiple resistance loci to both rusts in wheat breeding programs

Association Analysis of Historical Bread Wheat Germplasm Using Additive Genetic Covariance of Relatives and Population Structure

Linkage disequilibrium can be used for identifying associations between traits of interest and genetic markers. This study used mapped diversity array technology (DArT) markers to find associations with resistance to stem rust, leaf rust, yellow rust, and powdery mildew, plus grain yield in five historical wheat international multienvironment trials from the International Maize and Wheat Improvement Center (CIMMYT). Two linear mixed models were used to assess marker–trait associations incorporating information on population structure and covariance between relatives. An integrated map containing 813 DArT markers and 831 other markers was constructed. Several linkage disequilibrium clusters bearing multiple host plant resistance genes were found. Most of the associated markers were found in genomic regions where previous reports had found genes or quantitative trait loci (QTL) influencing the same traits, providing an independent validation of this approach. In addition, many new chromosome regions for disease resistance and grain yield were identified in the wheat genome. Phenotyping across up to 60 environments and years allowed modeling of genotype × environment interaction, thereby making possible the identification of markers contributing to both additive and additive × additive interaction effects of traits