Reduction in Yield Loss Using Incomplete Resistance to Pyrenophora teres f. teres in Barley. Brian J. Steffenson, Former Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616. R. K. Webster, and L. F. Jackson. Professor, Department of Plant Pathology, and Extension Agronomist, Department of Agronomy and Range Science, University of California, Davis 95616. Plant Dis. 75:96-100. Accepted for publication 5 July 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0096.

Disease development and yield loss due to the net blotch pathogen (Pyrenophora teres f. teres) were investigated in two barley genotypes, UC 603 and Kombar. These genotypes show moderately high to high (moderately susceptible to susceptible) infection responses (based on lesion size and degree of associated chlorosis) to P. t. f. teres in both the seedling (growth chamber study) and adult plant (field study) stages, but disease development is much reduced on UC 603 in the field. During the severe epidemic season of 1985–1986, terminal severity and area under the disease progress curve values were high (100% and 4,153, respectively) for plots of Kombar not protected with a fungicide and moderate (29.6% and 934, respectively) for unprotected plots of UC 603. Yield losses in plots where net blotch was not controlled with the fungicide propiconazole were 35.3% for Kombar and 3.2% for UC 603. Net blotch severity was lower during the second season (1986–1987); nevertheless, yield loss was 31% in unprotected plots of Kombar compared with 5.3% for UC 603. Kernel weight and size were the yield components most affected by P. t. f. teres. Propiconazole effectively controlled net blotch in the sprayed treatments; disease severities were all less than 2%. The data indicate that the incomplete resistance of UC 603 can be highly effective in reducing yield loss due to P. t. f. teres.