Durable disease resistance may be achieved by pyramiding multiple qualitative resistance genes in single genotypes and by using quantitative resistance (QR) genes. Quantitative trait locus (QTL) analysis tools can be used to find determinants of QR. Resistance QTL pyramids may also lead to durable resistance, and they provide independent validation of QTL effects and QTL interactions. We used molecular markers to identify allelic architectures at three previously mapped QTL conferring resistance to barley stripe rust (caused by Puccinia striiformis Westend. f. sp. hordei) in a set of barley (Hordeum vulgare L. subsp. vulgare) doubled haploid (DH) lines. The three QTL are located on three different chromosomes. One parent contributed the resistance alleles at two QTL and another parent contributed the resistance allele at the third QTL. In this report, we focus on resistance at the seedling stage; resistance at the adult plant stage will be addressed in a future report. The DH population was phenotyped for resistance by means of four pathogen isolates that show different patterns of virulence on a set of differentials. We used molecular markers to infer the resistance QTL allele architecture of each DH line. There was no significant QTL × race interaction, although some DH lines showed differential responses to isolates. The effects and locations of two QTL, each tracing to a different parent, were validated. The third QTL did not have a significant effect on disease symptom expression. To maximize the probability of recovering the resistant phenotype, resistance alleles are necessary at both QTL.

Two!hundred and thirty!two accessions of barley landraces collected from Tunisia were screened for resistance to powdery mildew[ A number of race!speci_c genes were detected using the detached leaf technique[ Among the 121 accessions tested\ 058 were susceptible to powdery mildew\ 19 were resistant and 32 showed di}erential reactions to the three isolates of powdery mildew used[ An attempt was made to deter! mine the number of genes\ the types of gene\ the types of gene action and the gene loci in 19 resistant accessions[ Three types of cross were made] "0# the accessions were crossed to the susceptible variety {Pallas|\ "1# the accessions were crossed with {Pallas| isolines\ and "2# accessions with identical powdery mildew reaction patterns were intercrossed[ Three isolates of Erysiphe`raminis f[ sp[ hordei were used] Bzm!0\ KM 07Ð64\ R02C[ A number of di}erent resistance genes were detected among the 19 resistant accessions[ Surprisingly\ segregation indicating single genes only were detected with the isolates used[ Some of these genes could be associated with loci already known[ In 08 cases a domi! nant and in one a recessive mode of inheritance was detected[ The recessive gene was not located at the mlo locus[ This investigation represents the _rst systematic study of race!speci_c genes for powdery mildew resistance in Tunisian landraces[ The newly identi_ed sources of resistance may be used in many strategies of breeding for disease resistance[

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North Dakota) cooperated in the development of winter wheat cultivars that resulted in the release of this cultivar. Decade was released for its combination of good winter hardiness, short stature, and high yield potential and for its excellent performance in the eastern Montana and western North Dakota winter-wheat production environments.

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