Mapping of disease resistance loci in barley on the basis of visual assessment of naturally occurring symptoms

Using field-scored data of disease severity under natural infestation, we mapped loci affecting resistance to powdery mildew (Blumeria graminis DC f. sp. hordei Em. Marchal), leaf rust (Puccinia hordei Otth,), stem rust (Puccinia graminis f. sp, tritici Eriks, & E, Henn,), scald [Rhynchosporium secalis (Oudem,) J.J. Davis]. and net blotch (Pyrenophora teres Drechs.), The mapping population included parents and doubled-haploid progeny of the two-row barley cross Harrington/TR306. Resistance was affected by two to five loci, explaining 8 to 45% of the phenotypic variance, per disease. All chromosomes, except chromosome 5 (1H), contained regions with at least one disease resistance locus. One region on chromosome 4 (4H) contributed to resistance to stem rust, scald, and net blotch. This region has previously been reported to affect days to heading and maturity. Two known resistance genes in the population, Rpg1 and Mlg, were mapped to within 3 centimorgans (cM) of their previously estimated genomic locations by simple interval mapping of the field-scored data. This indicates that the genomic positions of disease resistance genes can be estimated accurately with simple interval mapping, even on the basis of field-scored data.