| Stripe rust, caused by the fungus Puccinia striiformis f. sp. tritici, is an important disease of wheat on all continents where wheat is grown. In the Unites States, stripe rust occurred infrequently east of the Rocky Mountains before 2000. Since 2000, however, epidemics occurred every year and yield losses increased dramatically in this region. The first objective of this study was to characterize the "old" (before 2000) and "new" (since 2000) populations of P. striiformis f. sp. tritici in eastern United States by determining the virulence phenotype and amplified fragment length polymorphism (AFLP) fingerprints of representative isolates. All isolates of the old population were avirulent on wheat lines with resistance genes Yr8 and Yr9, whereas all isolates of the new population were virulent on Yr8 and Yr9. Cluster analysis of AFLP data showed all isolates of the old population and all isolates of the new population were at least 92% and 96% similar, respectively. However, similarity between isolates of the old and new populations was only 50%. This study indicated the new population was an exotic introduction that replaced the old population. The second objective was to characterize resistance in three soft red winter wheat lines (McCormick, VA96W-270, and VA96W-270V). Crosses with a susceptible cultivar were made, and parental and F1 populations were characterized in growth chambers using races PST-3 ('old' race) and PST-100 ('new' race) of Puccinia striiformis f. sp. tritici. Parental, F 1, F2, F3, F4, and backcross populations were evaluated infield plots inoculated with PST-100. Resistance in McCormick was race specific adult-plant resistance controlled by two or three recessive effective factors (genes). Resistance in VA96W-270 was adult-plant resistance conferred by two recessive genes. Resistance in VA96W-270Variant was conferred by a dominant gene for adult-plant resistance and a recessive gene for all-stage resistance. Estimates of narrow-sense heritability for resistance in these lines was moderate (0.29-0.51), indicating the resistance could be incorporated into wheat cultivars. This study was the first to report the inheritance of resistance to the new stripe rust population, and to characterize the molecular diversity of the Puccinia striiformis f. sp. tritici populations in North America. |
