Epidemiology of stripe rust, virulence of Puccinia striiformis f.sp. hordei, and yield loss in barley.

Barley stripe rust (caused by Puccinia striiformis f. sp. hordei) was found for the first time in the United States in 1991. This study was conducted from 1991 to 1994 to determine the occurrence and spread of the disease, the identity and relative frequency of races present, and the amount of yield loss attributable to the disease on cultivars having different levels of resistance. Surveys and urediniospore collections were made in commercial barley fields, barley breeding nurseries, and wild Hordeum spp. throughout Texas, southwest Oklahoma, and northeast New Mexico. Commercial fields of the cultivars Post and Tambar 500 in Winters and Era, Texas, were systematically assessed for stripe rust severity. Race determinations were made by inoculating the urediniospore collections onto a set of 18 differential cultivars. To determine yield loss, four fungicide treatments (triadimefon + mancozeb, propiconazole, tebuconazole, and flusilazole) were compared with untreated checks of seven barley cultivars. In 1991, P. s. f. sp. hordei was found only in the breeding nursery at Uvalde, and on H. jubatum and H. leporinum in the Uvalde area. Stripe rust occurred in commercial barley throughout the state in 1992 and 1993, yet was found only on highly susceptible cultivars in nurseries in 1994. The highest severity found in commercial barley was 8% at the soft dough development stage on cv. Post at Winters in 1993. From a total of 273 isolates, 255 were race 24, 14 were race 23, and 4 were of a different race, labeled TXG. The most grain yield loss was about 72% on cv. Perkins at Uvalde in 1992. We found that cv. Tambar 401 was resistant and cvs. Tambar 500 and Kold were moderately resistant to P s. f. sp. hordei, and none of these three cultivars sustained significant yield loss to barley stripe rust. Over cultivars, locations, and years, all of the fungicide-treated plots had significantly less yield loss than untreated checks. Even though barley stripe rust has the potential to become a severe disease in the U.S., host resistance in winter barley and fungicides can effectively minimize yield loss.