DIVERSITY AMONG FINNISH NET BLOTCH ISOLATES AND RESISTANCE IN BARLEY

Seedlings of a differential barley (Hordeum vulgare L.) series (21 gen otypes) and of six check genotypes were used in the greenhouse to asse ss variation in virulence among 20 single-spore isolates of the net bl otch pathogen, Pyrenophora teres Drechs. f. teres Smedeg., collected f rom various sites in Finland. The experiment was run twice and symptom expression was recorded on the first three leaves. Analysis of second leaf symptom scores from Run 1 indicated differences in virulence bet ween isolates, all of which were pathogenic, and differential resistan ce among the barleys. The virulence of P. teres isolates appeared to b e conditioned by the host barley from which the isolate derived; the a verage virulence of isolates collected from a susceptible host was gre ater than that of isolates collected from a less susceptible hot. Resu lts from Run 2 were similar regarding resistance within the barleys, b ut variation in virulence among the P. teres isolates was not consiste nt with that from Run 1. CI 9819 carries duplicate genes for resistanc e to P. teres (Rpt1b and Rpt1c), and CI 7548 possesses Rpt3d. Both gen otypes were highly resistant to all isolates. The Rpt1a gene of Tifang (CI 4407) conferred resistance to all isolates in Run 2, but only to about half of the isolates in Run 1. The checks, including two of the symptomatically most resistant Nordic barley genotypes, were universal ly susceptible during these stringent tests. No selective pressure has been placed on Finnish isolates of P. teres through previous deployme nt of major resistance genes, and it is speculated that any variation in virulence among isolates is likely to be due to a combination of ev olutionary forces including, natural selection, random genetic drift a nd gene flow.