First Detection in North America of Virulence in Wheat Leaf Rust (Puccinia triticina) to Seedling Plants of Wheat with Lr21.

Leaf rust resistance gene Lr21 is present in hard red spring wheat (Triticum aestivum) cultivars grown in Minnesota, North Dakota, South Dakota, Manitoba, and Saskatchewan. Isolates of Puccinia triticina, the causal organism of wheat leaf rust, with virulence to this gene have not been previously detected in annual virulence surveys in the United States. (2). In 2010, hard red spring wheat cvs. Faller, RB07 (1), and Glenn, all with Lr21, had 0 to 5% levels of leaf rust severity, which was higher than in previous years in research plots in North Dakota and Minnesota. Leaf rust collections from wheat cultivars and germplasm lines with Lr21 at three locations in Minnesota and North Dakota were increased on plants of the leaf rust susceptible wheat Thatcher and the Thatcher line with Lr21. Single uredinia from the collections were isolated and increased on seedlings of Thatcher. The single uredinial isolates were inoculated to 7- to 8-day-old seedling plants of the set of 19 differential lines that are currently used in the leaf rust virulence surveys (2). Thatcher lines with Lr3bg, Lr14b, Lr20, and Lr23 were also tested. The isolates were also inoculated to seedling plants of hard red spring wheat cultivars with Lr21: Glenn, Steele-ND, Faller, RB07, Amidon, AC Cora, and McKenzie (3). Previous standardized methods for growing seedling plants, increase of rust isolates, inoculation, incubation, and evaluation of infection types (IT) were used (2). All tests with the Thatcher differential lines and the cultivars with Lr21 were repeated at least twice. Virulence phenotypes were described based on virulence to the 19 differentials in the P. triticina virulence nomenclature system used in the United States. (2). Two virulence phenotypes, TFBJQ and TFBGQ, with virulence to Lr21 were found at the three locations. TFBJQ is virulent (IT 3 to 4) to genes Lr1, 2a, 2c, 3, 10, 14a, 14b, 20, 21, 24, 26, 28, and avirulent (IT 0 to 2) to genes Lr3ka, Lr3bg, Lr9, Lr11, Lr17, Lr18, Lr30, LrB, and Lr39/41. TFBGQ was avirulent to Lr14a and Lr20, but identical to TFBJQ for virulence and avirulence to the other resistance genes. Isolates of both phenotypes were virulent on seedlings of Faller, Glenn, RB07 (1), Steele-ND, AC Cora, and Amidon. McKenzie had IT of 2 due to the additional presence of Lr16 (3). Both TFBJQ and TFBGQ have intermediate IT of 2 to Lr16; IT 23 to Lr23, and are completely virulent to Lr1, Lr2a, and Lr10 that are present in hard red spring wheat cultivars. Both phenotypes have high IT to Lr24 and Lr26 that are present in soft red winter wheat and hard red winter wheat cultivars. The Lr21 virulent phenotypes likely arose by mutation from the group of P. triticina genotypes in the simple sequence repeat group NA-5 (4) that have intermediate IT of ;2 to ;2 to the Thatcher line with Lr21. P. triticina isolates with virulence to Lr21 are a new threat to wheat production since in 2010 more than 50% of the hard red spring wheat acreage in Minnesota and North Dakota relied on Lr21 for effective resistance to leaf rust. References: (1) J. A. Anderson et al. J. Plant Regist. 3:175, 2009. (2) J. A. Kolmer et al. Plant Dis. 94:775, 2010. (3) B. McCallum and P. Seto-Goh. Can. J. Plant Pathol. 32:387, 2010. (4) M. E. Ordoñez and J. A. Kolmer. Phytopathology 99:750, 2009.