Stripe rust (yellow rust) of barley (Hordeum vulgare L.), caused by Puccinia striiformis Westend. f. sp. hordei Eriksson is a new disease in North America. It was observed in Mexico in 1987 and the United States in the state of Texas in 1991. Within five years, the disease spread north and west to the states of Oklahoma, New Mexico, Colorado, Arizona, Utah, Idaho, Montana, California, Oregon, and Washington. The disease, which is now well established in the western United States, has caused severe yield losses in many regions of the West and has become a threat to barley production.

By using virulence and molecular analyses, we have clearly separated P. s. hordei from the wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici) and from stripe rust of some grasses. The barley stripe rust pathogen in North America was initially considered to be Race 24, which was first reported in Europe and subsequently reported in South America and North America. However, the barley stripe rust pathogen is extremely variable, and the population originally identified as Race 24 in North America consists of many races. We have identified 31 races and have evidence that many more races exist in North America.

Based on field and controlled temperature tests, the environmental requirements for epidemics of wheat stripe rust and barley stripe rust are essentially the same. Therefore, like wheat stripe rust, barley stripe rust will remain indigenous in western North America and continue to cause losses when not controlled. We have implemented a monitoring program to forewarn growers and breeders of the prevalence, distribution, and severity of the rusts and to determine the vulnerability of cultivars to the races. Based on the monitoring data and on forecasting models that were developed for wheat, we have annually provided a "rust alert" for growers in the Pacific Northwest. Growing resistant cultivars is the preferred control. There is a major, cooperative effort in searching for resistance to barley stripe rust and in evaluating barley germplasm for resistance to the disease. Sources of resistance have been identified and studies have been conducting to determine what genes provide resistance and how they provide the resistance. Most barley cultivars grown in North America are susceptible. Rust resistance is being incorporated into locally adapted cultivars, but new races may circumvent some types of rust resistance. There appear to be several types of resistance, such as seedling resistance, adult–plant resistance, slow rusting resistance, and the high-temperature, adult plant resistance that is highly effective and durable in wheat. To study the genetics of barley stripe rust resistance, resistant genotypes were crossed with the susceptible barley cultivars Topper and Steptoe and diallel crosses were made among the resistant genotypes. The results of those studies will be reported separately.

Based on field tests, treatment of seed with Baytan provided early but not always adequate control of stripe rust. Foliar fungicides, when timely applied, provided excellent control of stripe rust and stem rust as well as other diseases. Tilt, Folicur, and Quadris were the most effective foliar fungicides. They might be used when resistant cultivars are not available, resistance becomes ineffective, and/or in an integrated control program in combination with resistance.