Host resistance and fungicide control of leaf rust (Puccinia hordei) in barley (Hordeum vulgare) and effects on grain yield and yield components

The effectiveness of varying levels of host resistance in five barley (Hordeum vulgare L.) genotypes and three fungicide application protocols on the control of leaf rust (caused by Puccinia hordei G. Otth) and losses in grain yield, test weight, and 1000-kernel weight was evaluated in field experiments at two locations over four crop years. Fungicide treatments applied to all barley entries at each location and each year included a non-treated control, a single threshold-based application of triadimefon (140 g a.i. ha−1), and a complete control treatment comprised of multiple applications of triadimefon to emulate a disease-free environment. Leaf rust severity was assessed using the James [1971. A Manual of Assessment Keys for Plant Diseases. Canada Dep. Agric. Publ. 1458. The American Phytopathological Society, St. Paul, MN.] scale (0–25%) at three to four plant growth stages and data were used to calculate mean rust severity (MRS) and area under the disease progress curve (AUDPC). Leaf rust epidemics were severe in 1 (1995) of the 4 years of the study, and the treatment effect on grain yield was significant at one location (Warsaw). The susceptible cv. Wysor had a grain yield loss of 33% and MRS was 9.25% (34.23% in modified Cobb scale) at this location. In 2 (1995 and 1997) of 4 years of study, significant (P⩽0.05P⩽0.05) negative correlations were obtained between MRS and grain yield, test weight, and 1000-kernel weight of cv. Wysor at both locations. In the other 2 years (1994 and 1996) when leaf rust epidemics were mild, these correlations were mostly non-significant. In general, fungicide treatment significantly (P⩽0.05P⩽0.05) reduced MRS and AUDPC and reduced losses in grain yield, test weight, and 1000-kernel weight for the susceptible and moderately susceptible cultivars compared with the non-treated control. The cv. Starling, possessing adult-plant resistance, did not consistently benefit from fungicide application. There was no benefit from fungicide application for control of leaf rust in the resistant line VA92-42-46. In general, the complete-control fungicide treatment did not result in further significant reduction in leaf rust or losses in grain yield compared to threshold-level fungicide applications.