The effect of combining scald resistance genes on disease levels, yield and quality traits in barley.

Pairwise combinations of genes for resistance to scald in barley were developed using linked isozyme markers to test whether such combinations conferred improved resistance to the pathogen, Rhynchosporium secalis. The resistance genes originally derived from Hordeum vulgare ssp. spontaneum. The combinations were bred into an essentially similar genetic background because the scald-susceptible, Australian barley cultivar 'Clipper' was the recurrent backcross parent in their ancestry. In field tests of the recombinants over 2 years, disease levels were lower in three of six doubly resistant lines than in backcross lines carrying a single resistance gene, which in turn were less diseased than either 'Clipper' or recombinants that lacked the marked resistance genes. All resistant lines significantly outyielded 'Clipper' but did not themselves differ significantly. Lines resistant to scald had significantly higher grain size and grain weight. Gains for malt yield of about 1 % were detected in the higher disease environment. Resistance was not accompanied by any obvious "cost" in terms of yield or quality. Protection against scald is therefore a significant requirement for new malting barley cultivars in scald-prone areas.