Races of Puccinia striiformis f

Stripe rust of barley, caused by Puccinia striiformis f. sp. hordei (PSH), occurred mainly in California, Idaho, Oregon, and Washington and caused localized damage to barley crops. Growing cultivars with high-temperature, adult-plant resistance and the cropping system contributed to the low level of stripe rust. Stripe rust samples were collected from the four states and tested under controlled greenhouse conditions on seedlings of a set of 12 barley genotypes used to differentiate races of the fungal pathogen. A total of 15 races were detected, of which three were new. PSH-72, a new race, was virulent on all 12 differential genotypes. The most predominant race was PSH-71 that was virulent on differential genotypes Topper, Emir, Hiproly, Varunda, Abed Binder 12, Trumpf, Mazurk, Bigo, and Bancroft. Introduction Puccinia striiformis Westend. f. sp. hordei Eriks., the causal agent of barley stripe rust, has established in the United States since it was first reported in southern Texas in 1991 (Marshall and Sutton, 1995; Roelfs et al., 1992). In the early 1990s, the disease occurred in the south central states and later spread to the states west of the Rocky Mountains (Chen et al., 1995). The disease has caused major damage to barley crops in the U.S., mainly in California and the Pacific Northwest (Brown et al., 2001; Chen, 2004). Chen et al. (1995) selected 11 barley genotypes for differentiating races of P. striiformis f. sp. hordei. Later, the barley cultivar ‘Bancroft’ was added to the differential set (Chen and Line, 2001). Using the differential genotypes, 69 races were identified from the early 1990s to 2003 (Chen et al., 1995; Chen and Line, 2001; Chen, 2004). The objectives of this study were to 1) monitor the occurrence and distribution of barley stripe rust and 2) identify races and determine their frequencies and distributions in the United States in 2004. Materials and Methods Monitoring barley stripe rust. Stripe rust was monitored by surveying commercial fields during the growing season. Trap plots consisting of the set of differential genotypes (Table 1) and other cultivars either susceptible or resistant to P. striiformis f. sp. hordei were planted at various locations. For commercial field survey, infection type, severity (percentage of leaf areas infected), prevalence (percentage of plants infected), and information on cultivar, location, and growth stage were recorded. For the trap plots, infection type and severity were recorded one to three times during the growing season. Stripe rust infected leaf samples were collected for race identification. Race identification. Races of stripe rust samples were determined using a set of differential genotypes following the methods described by Chen et al. (1995). In this study, 12 barley