Xianming Chen
Wheat stripe rust has been reported in Texas, Louisiana, Arkansas, Oklahoma, Kansas, Alabama, California, Oregon, and Washington. In the first week of April, susceptible entries had severity levels of 100% in stripe rust monitoring and breeding nurseries in Baton Rouge, Winnsboro, and Bossier City, Louisiana and Ennis, McGregor, and Castroville, Texas. Stripe rust infection types and severities on the entries in the stripe rust monitoring nurseries at these locations were generally as expected based on their reactions to stripe rust in last year. Stripe rust was spreading northwards in the Great Plains. By the mid of April, wheat stripe rust developed up to severity levels of near 100% on susceptible entries in nurseries in the southwestern Arkansas. In Kansas, the disease (less than 1% incidence) was found in south central and southwestern regions. In California, wheat stripe rust developed to 100% of prevalence and 60% of severity on susceptible entries in nurseries in the Davis area in early April. In the Pacific Northwest, wheat stripe rust continued developed in western Oregon and northwestern Washington in April. In the first week of April, wheat stripe rust was found on susceptible winter wheat entries in nurseries in south central Washington. The wet weather condition in April has been favorable for stripe rust infection. So far, barley stripe rust has been reported only in California and Oregon.
Stripe Rust of Wheat and Barley
The Background
Stripe rust of wheat is caused by Puccinia striiformis Westend. f. sp. tritici Eriks. (P. s. tritici) and stripe rust of barley is caused by P. striiformis f. sp. hordei (P. s. hordei). The diseases are most prevalent in cool regions of the world. Because stripe rusts are favored by low temperatures, the diseases occur earlier in the growing season than leaf rust and stem rust, and therefore, have potential to cause more damage. Stripe rust is an important disease of wheat in Asia, Europe, Africa, the Middle East, Australia, New Zealand, South America, and North America. In the recent years, wheat stripe rust has caused major damages in Australia, China, Central Asia, Russia, and South Africa, as well as in the U. S.
In the U. S., stripe rust of wheat has existed for more than 100 years. The disease has been most destructive in the Pacific West (California, Oregon, Idaho, and Washington), but severe epidemics have occurred in the south central states and the Great Plains since 2000. In 2000, wheat stripe rust occurred in more than 20 states from the Pacific Northwest (PNW) and California to Virginia and from Texas to North Dakota. Another severe epidemic occurred in the Great Plains in 2001. The 2001 stripe rust epidemics caused about 40 million-bushel losses in the U. S. In 2002, severe stripe rust occurred on spring wheat crops in the PNW and winter wheat in south central U. S. Washington growers spent more than 2 million dollars for fungicide application. Without the use of fungicide, stripe rust could have caused losses from 26 to 32 million dollars in Washington state alone. In 2003, stripe rust struck the wheat production in the Great Plains again and caused yield losses of 88.9 million bushels in the nation, the highest yield loss in the record. In 2004, stripe rust epidemic occurred mainly in California and the PNW; the yield losses was estimated about 12 million bushels in the country. In 2005, stripe rust of wheat has already occurring in Texas, Louisiana, Arkansas, Oklahoma, Alabama, Kansas, California, Oregon, and Washington.
Barley stripe rust is a relatively new disease in the west hemisphere. It has caused severe damage in some locations since it was introduced to Colombia in 1975 from Europe, and spread to Mexico in 1987 and the U. S. in 1991. Barley stripe rust caused 15, 20, 15, 16, and 6% yield losses in 1996, 1997, 1998, 1999, and 2000, respectively, in California. In the PNW, barley stripe rust caused yield losses of 4% in 1997 and 5% in 2000 in Oregon, and 3% in 1998 in Washington. Now barley stripe rust is established in western U.S and continues to be a threat to barley production.
Puccinia striiformis is a fungus in Uredinales of Basidiomycetes. Its lifecycle consists of the dikaryotic uredial and diploid telial stages in the nature. Teliospores can germinate to form haploid basidiospores. Unlike the stem rust and leaf rust pathogens, the stripe rust pathogen does not have known alternate hosts for basidiniospores to infect, and thus, it does not have known sexual pycnial and aecial stages. Therefore, isolates of the fungus cannot be crossed through sexual hybridization, which makes it impossible to study the fungal genes through classic approach and linkage mapping. The fungus reproduces and spreads through urediniospores and survives as mycelium in living hosts. Because urediniospores cannot keep their viability for very long, living plants (volunteers of wheat and barley crops and grasses, or crops and grasses in cool regions in the summer and in warm regions in the winter) are essential to keep the fungus from season to season. Although the pathogen does not have known sexual reproduction, there is a high degree of variation in virulence and DNA polymorphism in the natural populations of the stripe rust pathogens.
Biology of the stripe rust fungus and epidemiology of the disease have been determined. Models for forecasting the disease have been established, especially for the western U.S. Numerous genes for stripe rust resistance have been identified and used in breeding programs. Molecular markers have been developed for some of the genes. Durable type resistance such as high-temperature, adult-plant (HTAP) resistance have been identified and successfully used in the PNW and have been started to use in other regions. Fungicides that are currently labeled for control of stripe rust (i.e. Tilt, Quadris, Quilt, Headline, and Stratego) on wheat and barley are effective when used appropriately.
The infrastructure of stripe rust research and control
Stripe rust is monitored annually through collaborators throughout the U.S. Samples are sent to the disease laboratory of the USDA-ARS Wheat Genetics, Quality, Physiology, and Disease Research Unit at Pullman, WA for race identification. A total of 115 races of the wheat stripe rust pathogen have been identified since 1960s and 72 races of the barley stripe rust pathogen have been identified since 1991. A new group of races that were first detected in south central states and California in 2000 has spread and become predominant throughout the U.S.
Stripe rust research in the U.S. was started in the early of the last century, stopped from later 1930s’ to late 1950’s, and re-started in the early 1960s. Before 2000, stripe rust research and control were mainly in the western U.S. (Washington, Idaho, Oregon, California, and Montana). Resistance to stripe rust has been among the top priorities of wheat breeding programs in these states. Because stripe rust has become increasingly important in the south central, southeastern, and the Great Plain states, a network for monitoring and breeding for resistance to the disease throughout the country has been established. Identification of races, population structure, and genomic studies of the pathogen; germplasm screening and breeding lines evaluation; host resistance including identification of resistance genes and development of molecular markers; and fungicide testing have been conducted in the USDA-ARS Wheat Genetics, Quality, Physiology, and Disease Research Unit at Pullman, Washington through collaborating with breeders and pathologists throughout the country. Research on stripe rust is now conducted almost in every state where stripe rust is a problem. With increased demand for research and service, facilities (such as dew chambers, growth chambers, greenhouse space, and rust storage facilities) need to be improved for the USDA-ARS stripe rust research program at Pullman, WA and other locations. More funding is needed for research towards more effective and sustainable control of stripe rust.
Issues in stripe rust research and control
The following issues are to be solved: 1). It is not clear whether the stripe rust pathogen can over-summer in the vast region east of the Rocky Mountains. Generally, the weather is too hot for the stripe rust to survive the summer in this region, but some niches (such as river banks and forests) with a cool environment may make the rust survival possible. Experiments and detailed surveys are needed to answer the question. 2). Use of all-stage resistance (also called seedling resistance) is still the major approach for developing resistant cultivars in some regions because of the high-level of the resistance. However, this type of resistance is generally race-specific, and therefore, does not last very long. Certain levels of HTAP resistance are not adequate in regions when the rust infects in the seedling stage and continues developing in the early growing season. Combining different sources of HTAP resistance and effective all-stage resistance is time-consuming. More useful molecular markers are needed to increase the breeding efficiency. 3) Little is know about genome and genes of the stripe rust pathogen. Bacterial artificial chromosomal (BAC) and full-length cDNA libraries have been constructed in the USDA-ARS Wheat Genetics, Quality, Physiology, and Disease Research Unit, and a small number of the fungal genes have been identified. More studies in this direction are needed to characterize the genome, identify functional genes of the pathogen, and determine the molecular mechanisms involved in the plant-pathogen interactions. 4) Cultivars with resistance to stripe rust and other diseases, especially leaf rust, are needed to be developed. In the recent years, cultivars with good resistance to stripe rust have become more and more popular in the Great Plains. However, some of these cultivars (such as Jagger and Jagalene) are or have become highly susceptible to leaf rust.