LINKAGE STUDIES OF BARLEY POWDERY MILDEW VIRULENCE LOCI

Two isolates of the barley powdery mildew fungus were crossed and 165 progeny isolates were tested for virulence on 12 differential barley l ines representing eight sources of resistance important in European ba rley production. The progeny displayed segregation in 17 different vir ulence loci. Ten loci belong to one linkage group about 200 centimorga ns (cM) long, four belong to a second linkage group, and three were in dependently inherited. Evidence was found that the previously reported virulence loci V(Ru3), V(Mu2), V(No3) and V(LG2) are different; howev er, we will not exclude the possibility that locus V(Mu2) is identical to locus V(Ru3) or to the previously established locus Val3. We found segregation for four new virulence loci tentatively designated V(MC3) , V(MC4), V(No4), and V(LG3). This suggests the existence of four new resistance genes corresponding to the new virulence loci. The powdery mildew resistance genes occurring together with resistance gene Mla7 d erived from three different sources are different.