First Detection of Virulence in Puccinia striiformis f. sp. tritici in China to Resistance Genes Yr24 (=Yr26) Present in Wheat Cultivar Chuanmai 42.

Stripe rust disease of wheat caused by Puccinia striiformis f. sp. tritici was observed on previously resistant bread wheat (Triticum aestivum L.) cv. Chuanmai 42 during the 2008-2009 crop season in Pi County, Sichuan Province, China. More than 10 single pustules were isolated from the diseased leaf samples collected in the field and inoculated on 7-day-old susceptible wheat seedlings cv. Mingxian 169. After 18 to 24 h of incubation at 100% relative humidity in darkness, the plants were moved into the greenhouse, maintained at 15 to 18°C, and supplemented with 10,000 lx of fluorescent light for 10 h per day. The second leaves were clipped when chlorotic spots appeared on leaves (~7 days postinoculation), and plants were covered with glass cylinders to prevent cross contamination. Urediniospores of each isolate were collected 16 days after inoculation and temporarily kept in a dryer at low temperature (3 to 4°C). The virulence spectra of the isolates were tested on Chinese differentials and wheat lines with known Yr genes at the seedling stage (1). A new Yr24 (=Yr26) virulent pathotype, different from currently known pathotypes in China, was identified. To our knowledge, this is the first detection of Yr24 virulence in Puccinia striiformis f. sp. tritici populations on Chuanmai 42. In addition, the new pathotype was also virulent to Lantian 17, Guinong 22 (Chinese differential), and 92R137 derived wheat lines Nannong 04Y10 and Nannong 05Y628, known to carry Yr24 (2,3). The avirulence/virulence formula of the new pathotype is Yr1, 3, 4, H46, 5, 6, 15, 17, 18, 32, Sp, Sd/Yr2, 8, 9, 10, 12, 24 (=26), 31, and Su. Wheat cultivars and breeding materials, previously protected by Yr24 gene, are now vulnerable to stripe rust epidemics in the region. Pure isolates of the new pathotype (Accession No. 09-6-16-3) are stored in the Chinese Academy of Agricultural Sciences (CAAS; Beijing) stripe rust collection. References: (1) W. Q. Chen et al. Plant Dis. 93:1093, 2009. (2) G. Q. Li et al. Theor. Appl. Genet. 112:1434, 2006. (3) Z. F. Li et al. Plant Dis. 90:1302, 2006.