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.