Stable resistance in barley to Pyrenophora teres f. teres isolates from the Nordic-Baltic region after increase on standard host genotypes

Results from tests of a mixture of Finnish net blotch, Pyrenophora teres Dr echs. f. teres Smedeg., isolates on a differential series of barley seedlin gs, comprising 17 genotypes, indicated that patterns of infection response (IR) and percentage leaf area damaged (PLAD) were unaffected by differences in seedling size. Variation of the concentration of inoculum between 1,250 conidia ml(-1) and 20,000 conidia ml(-1) produced similar patterns of IR a nd PLAD on the differential series. IR and PLAD scored on the second seedli ng leaf differentiated resistance to P. teres f. teres among the genotypes better than on the first seedling leaf. In a second experiment, 120 single- spore P. teres f. teres isolates from Finland, Sweden, Norway, Latvia, Esto nia and Ireland were used in tests conducted in the greenhouse to different iate them in terms of virulence reaction on seedlings of six differential b arley genotypes. Each isolate was tested directly following isolation from the leaf material and after having passaged each through barley cvs. Arve o r Pohto, to produce 360 isolates in total. Virulence of the isolates differ ed significantly on the members of the differential series, but differences associated with country of origin and passaging, and interactions, were sm all. It is concluded that little variation between virulence of P. teres f. teres isolates is evident over a large geographic area, incorporating Nord ic and Baltic countries, and Ireland. Barley genotype response to P. teres f. teres appeared to be of more significance than relative virulence of the pathogen isolates. This could simplify breeding barley for improved resist ance to this phytopathogen.