from Roland Line, 4 Aug 94
 
 
 Virulence, Aggressiveness, Evolution, and Distribution of Races
 of Puccinia striiformis (the Cause of Stripe Rust of Wheat) in North
 America, 1968-87
 
 Roland F. Line and Abdul Qayoum
 
 Line is a plant pathologist for Agricultural Research Service,
 U.S. Department of Agriculture, located at Washington State
 University, Pullman, WA 99164-6430. Qayoum is a research associate at
 Washington State University, Department of Plant Pathology,
 Agricultural Research Center, Pullman, WA 99164-6430.
 
 Abstract
 
 Line, Roland F., and Abdul Qayoum. 1991. Virulence,
 aggressiveness, evolution, and distribution of races of Puccinia
 striiformis (the cause of stripe rust of wheat) in North America,
 1968-87. U.S. Department of Agriculture Technical Bulletin No. 1788, 44
 pp.
 
 Thirty-nine races of Puccinia striiformis, the fungus that causes
 stripe rust of wheat, have been identified in North America,
 beginning with four races present when monitoring began in 1968.
 Additional races were detected as follows: one each in 1968, 1972,
 1975, 1978, 1980, and 1982; two each in 1974, 1984, and 1985; three
 each in 1981 and 1987;  five in 1977; and six each in 1976 and 1983.
 Eleven of the races were first detected on newly released wheat
 cultivars that were resistant to previously known races. Those 11 races
 caused severe epidemics within 3 years after release of the cultivars.
 Within a few years after detection of the new virulences, new
 combinations of virulences appeared. Reassortment of all existing
 virulences occurred in the absence of a sexual cycle.
 
 North America can be divided into seven wheat-growing regions of
 stripe rust occurrence, based on geographic barriers, prevailing winds
 and other weather, cropping methods, and virulence of the races. They
 are region 1: eastern Washington and Oregon, northern Idaho, and
 eastern British Columbia; region 2: western Montana and southern
 Alberta;  region 3: southern Idaho and northern Utah; region 4: western
 Oregon and northern California; region 5: northwestern Washington and
 western British Columbia; region 6: central California; and region 7:
 the area east of the Rocky Mountains.
 
 In region 1, 31 races of P. striiformis have been detected, of
 which 26 were first detected in the region.  The other five races
 probably evolved independently in region 1 after they were
 detected in other regions.  Races in regions 2 and 3 were
 detected either later in the same year that severe epidemics of
 the races occurred in region 1 or within a few years, suggesting
 that races in regions 2 and 3 originated in region 1.  Most new
 virulences were associated with the release of cultivars with
 high race-specific resistance, but some virulences in regions 5
 and 6 were not associated with resistance of cultivars grown in
 the regions.  Other factors, such as wild hosts and the presence
 of indigenous races, appear to contribute to the diversity of
 races in regions 5 and 6.  Region 5 has 18 of the 39 races, of
 which 6 are found in only that region.  Region 6 has 11 races, of
 which 2 are unique for that region.  One race, which is virulent
 on Lemhi but not on other differential cultivars, occurs in only
 regions 5 and 6.  
 
 Three races detected California have a narrow range of virulence
 on the differential wheat cultivars and lack aggressiveness. 
 Races that are viurulent on the cultivar Chinese 166 (gene Yr1)
 were less aggressive than races virulent on Heines VII (gene Yr2)
 and Fielder (gene Yr6).  Races virulent on Chinese 166 are less
 prevalent even in plots that have been inoculated with the races,
 and they develop more slowly and produce fewer spores.  
 
 Races virulent on the differential cultivars Lemhi and Heines VII
 were detected in all regions;  on Druchamp, Produra, Stephens,
 and Fielder in regions 1,2,4,5, and 6;  on Yamhill in regions
 1,4,5, and 6;  on Lee in regions 1,4, and 5;  on Moro in regions
 1,2,4, and 5;  and on Paha (Suwon 92/Omar) and Tyee in only
 region 1.  Races virulent on Triticum spelta album, Compair, and
 Riebesel 47-51 have not been detected in  North America. Genes
 for resistance in the differential cultivars can be postulated,
 based on the virulence of the races on those cultivars.
 
 Keywords: Puccinia striiformis, stripe rust of wheat, yellow rust of
 wheat, race-specific resistance, forecasting, monitoring, disease
 control, epidemiology, differential wheat cultivar