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 grami nis 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 (Pyrenoph ora teres Drechs.), The mapping population included parents and double d-haploid progeny of the two-row barley cross Harrington/TR306. Resist ance was affected by two to five loci, explaining 8 to 45% of the phen otypic variance, per disease. All chromosomes, except chromosome 5 (1H ), contained regions with at least one disease resistance locus. One r egion on chromosome 4 (4H) contributed to resistance to stem rust, sca ld, and net blotch. This region has previously been reported to affect days to heading and maturity. Two known resistance genes in the popul ation, Rpg1 and Mlg, were mapped to within 3 centimorgans (cM) of thei r previously estimated genomic locations by simple interval mapping of the field-scored data. This indicates that the genomic positions of d isease resistance genes can be estimated accurately with simple interv al mapping, even on the basis of field-scored data.