YIELD LOSSES IN WINTER BARLEY RESULTING FROM A NEW RACE OF PUCCINIA-HORDEI IN NORTH-AMERICA

Leaf rust resistance, derived from the barley cultivar Cebada Capa, has been effective in the southeastern United States since 1950, when it was first used in the Virginia barley breeding program. In 1990, races of Puccinia hordei virulent to barleys that possess the resistance gene Rph7 were identified for the first time in North America. This research assessed the potential impact of Rph7-virulent races of leaf rust on grain yield and quality in winter barley. Natural epidemics of leaf rust occurred in cultivar trials at Painter and Warsaw, Virginia, in 1991 and 1992. Mean leaf rust severities for barley lines observed over three environments ranged from 10% for the moderately resistant line VA 90-42-45 to 76% for the susceptible cultivar Barsoy. Significant negative correlations between grain yield and leaf rust severity were obtained for three of the four environments. Based on regression analysis over barley genotypes, an average grain yield loss of 0.42% (31.3 kg/ha) for each 1% increment of leaf rust severity on the upper two leaves at the early dough stage of plant development was determined. The susceptible cuitivar Barsoy had an average yield loss of 32%, while the average loss for all genotypes was between 6 and 16%. Test weights were reduced by an average of 4.3 kg/hl at Painter in 1991 and 10.5 kg/hl at Warsaw in 1992. Whereas in the past 40 years, barley leaf rust was of little economic importance, it may become a disease of greater importance in regards to losses in grain yield and quality.