VIRULENCE STUDIES OF SWEDISH NET BLOTCH ISOLATES (DRECHSLERA-TERES) AND IDENTIFICATION OF RESISTANT BARLEY LINES

Six Swedish and one Canadian single spore isolate of Drechslera teres f. teres were used to screen 109 barley lines for disease resistance a nd to select a differential set of barley lines for use in assessing p athogen virulence. A large variation for net blotch resistance was fou nd among the 109 barley lines which were classified into four groups, those showing: 1) only resistant reactions; 2) differential reactions; 3) only intermediate reactions and 4) only susceptible reactions. The European commercial varieties included, showed susceptibility to all Swedish isolates, but a few were resistant to the Canadian isolate. Th e 18- member differential set separated 25 Swedish and two Canadian is olates of D. teres into 14 pathotypes, three of which made up 59% of t he isolates. Only one barley differential (CI 9776) was resistant to a ll net form isolates. Host selection on the pathogen seems to be prese nt as all six isolates obtained from cv. Golf belonged to the same pat hotype and 4 of 5 isolates from cv. Karin shared the same virulence pa ttern. The net form of net blotch (D. teres f. teres) predominated in the sampled regions and only one of 26 Swedish isolates was of the spo t form (D. teres f. maculata).