Emerging new virulence gene combinations in the Mexican Pst population
Yellow (stripe) rust continues to be an important disease of wheat in the irrigated EL Bajio region and northwestern Mexico, and in the High Plateau of Central Mexico. Isolate MEX 96.11, virulent to race-specific resistance genes Yr2, Yr3, Yr6, Yr7, Yr9 and Yr27, represented the most prevalent Pst race until 2001 and evolution of new virulences was slow. Several additional R-genes, including a gene in Pollmer triticale, are now defeated. The aggressive Pst race with Yr8 virulence, first detected in the United States in 2000, not only became widespread in Mexico by 2003 but continued to evolve at a more rapid rate with virulence to Yr1 occurring in 2003. However these races did not cause crop losses other than increased levels of head infections in some cultivars. Variants with virulences to Yr17 and Yr31 were detected in 2007 and 2008, respectively. These resistance genes are known to occur in some wheat varieties and breeding materials. 2010 disease data from trap nurseries that included the Avocet isolines and other varieties indicated the existence of virulence for Yr1, 2 (Siete Cerros), 3 (Tatara), 6, 7, 8, 9, 17, 27 and 31 (Rebeca F2000) in different Pst isolates. During the 2014 crop season, an epidemic occurred in farm fields and samples were collected and analyzed in greenhouse tests. Virulence combination V2, 3, 6, 7, 8, 9, 17, 27 and 31 first identified and represented in isolates CEVAMEX14.25 and MEX 14.141, and a similar isolate MEX14.146 virulent to Yr1 were the most frequent. These new virulence combinations caused yield losses in cultivar Nana F2007 grown in the Mexican highlands and Luminaria F2012, released for the irrigated areas of Bajio. Our results indicate a continuing evolution and accumulation of virulences in the aggressive Pst lineage. Determination of the defeated genes in Nana F2007 and Luminaria F2012 is underway.