Determining quantitative resistance of barley cultivars at the seedling stage to net blotch caused by Pyrenophora teres

Four barley (Hordeum vulgare L.) cultivars, Heartland, Harrington, Johnston , and Kasota, were compared at the seedling stage for reactions to the net and spot forms of net blotch caused by Pyrenohora teres f. teres and P. ter es f. maculata, respectively. Effects of leaf wetness period, leaf position , and inoculum concentration on lesion development were investigated. The l inear relationship between inoculum concentration and diseased leaf area wa s used to estimate inoculum concentrations causing 50% diseased leaf area ( ED50) on leaf 2 (the second seedling leaf) and causing 15% diseased leaf ar ea (ED15) on leaf 3 (third seedling leaf). Resistance of cultivars was comp ared on the basis of ED50 and ED15. The linear relationship of diseased lea f area with time was used to estimate the rate of lesion spread and to comp are resistance. Lack of a significant linear relationship over time on leaf 3 for cvs. Heartland and Kasota inoculated with P. teres f. teres reflecte d resistance in these two cultivars. Both pathogens caused less than half t he diseased area on leaf 3 as on leaf 2 in all cultivars. A mist period of 48 h tended to obscure the differences on leaf 2 between resistance and sus ceptibility. in general, there were positive correlations between the two p athogens based on ED50 or ED15 Pyrenophora teres f. maculata resulted in su bstantially higher ED50 and ED15 values on cvs. Johnston and Heartland, ind icating that P. teres f. maculara was less virulent than P. teres f. teres on these cultivars. ED50 and ED15 values could be used to quantitatively di fferentiate moderately resistant cultivars from susceptible cultivars at th e seedling stage.