| Slow-scalding resistance (S-SR), the phenomenon of slow disease development under severe epidemic of scald caused by Rhynchosporium secalis (Oud.) J. J. Davis, has been gaining increasing attention in barley improvement due to concerns regarding the instability of major-gene resistance.; Indoor and field techniques were optimized to ensure consistently uniform development of infection on barely genotypes at both the seedling and adult plant stages.; Using the optimized techniques, the quantitative reaction of 38 western Canadian barley cultivars to mixtures of scald isolates under different field conditions in Canada and Mexico were determined during 1999-2001. Slow-scalding genotypes were defined and identified by the presence of susceptible reactions at the seedling stage and a low to intermediate level of disease progress at the adult plant stage.; Large pathogenic variability was observed among 19 single-spore isolates of R. secalis studied in pathogenicity tests using a set of 73 barley genotypes. It was hypothesized that genotypes with resistance against a higher number of pathogen isolates could perform in a more durable/stable way in the field.; Histological studies revealed that in the slow-scalding cultivars, rates of sporulation were lower compared to those in susceptible genotypes. The changes in levels of calcium, potassium, silicon, and sodium in the superficial layers of leaves suggested the involvement of active mobilization of and/or sequestration of elements in this pathosystem. It was found that severe auricle infections may adversely affect transport of photosynthates from the leaf into the plant.; The genetic basis of S-SR was investigated in several Canadian and ICARDA/CIMMYT derived genetic populations both in Canada (Alberta) and at Toluca (Mexico). The results revealed the existence of 1-4 resistance genes with additive effects and high heritabilities in the slow-scalding parental lines. Reactions and cross-location resistance stability of susceptible, resistant, and S-SR genotypes and segregating populations varied significantly between Mexican and Canadian locations. These differences were attributed to major pathogen differences in the two countries, which were able to reciprocally overcome S-SR expressed in each country. It was concluded that achieving more stable genetic resistance to scald through breeding for S-SR is both practical and feasible. |
