Genetics and resistance / Génétique et résistance Identification of resistance genes to barley covered smut and mapping of the Ruh1 gene using Ustilago hordei strains with defined avirulence genes

Covered smut of barley (Hordeum vulgare), caused by Ustilago hordei, is a seed-borne disease. To identify and map disease resistance genes, current Canadian barley cultivars, parents of six barley mapping populations, and four differentials namely ‘Hannchen’ (Ruh1), ‘Excelsior’ (Ruh2), ‘Plush’ (Ruh6), and ‘Odessa’ (universal susceptible), were evaluated. Six mated combinations of U. hordei sporidial lines harbouring known alleles of virulence (avr) and avirulence (Avr) genes were used in the evaluation. Most of the barley cultivars carried the Ruh1 resistance gene, but some also had Ruh2 or Ruh6. Many cultivars showed resistance to a U. hordei sporidial mating of genotype (avr1/avr1 avr2/avr2 avr6/avr6), virulent on ‘Hannchen’, ‘Excelsior’, and ‘Plush’. The resistance indicated the presence of novel avirulence gene(s) corresponding to a postulated resistance gene(s) in these cultivars. Fifty-six doubled-haploid lines from the mapping population ‘Harrington’/TR306 were inoculated with a sporidial mating of genotype SM1 (Avr1/Avr1 avr2/avr2 avr6/avr6), avirulent on ‘Hannchen’ (Ruh1) but virulent on ‘Excelsior’ (Ruh2) and ‘Plush’ (Ruh6). ‘Harrington’ (ruh1) was susceptible to this fungal genotype possessing Avr1 and TR306 (Ruh1) was resistant. Based on field and greenhouse tests, barley lines segregated 27 resistant to 29 susceptible, which was not significantly different from a 1:1 ratio (χ2 = 0.07, P = 0.79); this indicated the presence of the single major resistance gene Ruh1 in TR306. Ruh1 mapped to the short arm of chromosome 1 (7H) between markers iPgd1A and BCD129 on the ‘Harrington’/TR306 map.