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Identifying leaf rust resistance in diverse accessions and cultivars of wheat Kathryn Turner Leaf rust, caused by Puccina triticina Eriks, is one of the most common diseases affecting wheat, consistently reducing yields by 5-15% with higher losses occurring in some years (Kolmer, 1996). In the hard red spring wheat growing regions of the United States, farmers are spraying fungicides annually to mitigate crop losses, but genetic resistance can provide protection at lower cost to farmers and risk to the environment. Breeding for resistance to leaf rust is the most cost effective method with an estimated 27:1 benefit to cost ratio (Marasas et al., 2004). Race specific resistance genes have been readily defeated when released individually, while combinations of race specific and race non-specific resistance genes have been more durable (McIntosh p25). The race nonspecific resistance gene Lr34 has been highly effective throughout wheat-growing regions of the world, increasing the level of resistance when combined with race specific genes (Kolmer, 1996). In Minnesota and the greater hard red spring wheat growing region of the United States, the combination of Lr16, Lr23, and Lr34 has been highly resistant (Kolmer et al. 2007) for more than 15 years. By identifying new sources of genetic resistance and mapping the location of resistance genes, we can effectively incorporate these genes in breeding efforts. By identifying genes in cultivars that have historically been highly resistant, we can better understand and utilize durable genetic resources for leaf rust resistance. Two projects were developed to identify, map, and effectively utilize leaf rust resistance genes. The first compares biparental and association mapping methods to identify resistance in diverse germplasm. The objectives are to identify and map new leaf rust resistance genes, better characterize and map known genes, and determine whether association or biparental mapping approaches are more effective for identifying leaf rust resistance loci in a diverse set of germplasm. Association mapping has several advantages over biparental mapping, including increased mapping resolution and reduced research time by utilizing historic recombination rather than developing new mapping populations, and the ability to detect a greater number of alleles at a particular locus (Yu and Buckler, 2006). However, with the exception of studies by Crossa et al. in 2007 and Maccaferri et al. in 2010, association mapping has not been widely applied for mapping leaf rust resistance. In 2011 and 2012, 3,200 diverse accessions from the National Small Grains Collection in Aberdeen, Idaho were rated for resistance in leaf rust screening nurseries in St. Paul and Crookston, MN. One replication and repeated checks every 100 experimental lines were used. Thirty accessions were selected on field resistance and maturity to develop biparental mapping populations by crossing to the susceptible cultivar ‘Thatcher’. From fall 2011 to spring 2012, these 30 selected accessions were screened in the greenhouse as seedlings with 10 leaf rust races and as adults using a field mixture of isolates and a single race virulent in the seedling screenings. Pedigree information and marker screening was also used to identify possible known genes present in the accessions. From the results of these screenings, nine recombinant inbred line mapping populations were selected for potential to have novel race specific or race non-specific genes. Biparental populations with candidate novel race specific genes will be mapped initially using bulk segregant analysis, using at least 10 homozygous resistant and 10 homozygous susceptible F2:3 families. Candidate race non-specific resistance genes will be mapped using at least 150 F6 lines. For association mapping analysis, selected lines with similar maturity will be planted in St. Paul and Crookston in 2013. Association analysis will use data collected as a part of the Triticeae Coordinated Agriculture Project (TCAP). Data will include greenhouse seedling screening with Race 1, a field mixture, and a race with specificity to common durum resistance and field severity response ratings in 2011, 2012, and 2013. All 3,200 lines will be genotyped using the Illumina 9,000 SNP chip for wheat as a part of the larger TCAP. The effectiveness of association mapping will be compared to the biparental mapping populations in terms of the number and identity of resistance loci detected. We are also interested in whether one method may be more useful for identifying novel as well as race specific or race non-specific genes. The second project involves identifying leaf rust resistance genes in historically resistant hard red spring wheat cultivars to better characterize the durable resistance present in our breeding germplasm. Our objectives are to determine the number and identity of genes in ‘Ulen’, ‘RB07’, and ‘AC Taber’ and to identify markers associated with the resistance genes to be used for marker assisted selection. Ulen was postulated to have Lr23, Lr10, and an additional unknown resistance gene (Anderson et al., 2006). RB07 and Faller both have the cloned gene Lr21 which conferred near immunity in the field until 2010 when a new race of leaf rust was detected with virulence to Lr21 (Kolmer et al., 2011). Surprisingly, Faller had higher levels of susceptibility than RB07, which remained highly resistant. AC Taber is a widely grown Canadian wheat cultivar, released in 1990 with Lr14a, Lr13, and an uncharacterized resistance gene “LrTb” showing resistance in adult plants (Liu and Kolmer, 1997). Biparental populations have been developed to evaluate RB07 x Faller, Ulen x Thatcher, and AC Taber x Thatcher through seedling resistance screening and field trials in order to map unknown genes. References Anderson, J.A., R.H. Busch, D.V. McVey, J.A. Kolmer, Y. Jin, G.L. Linkert, J.V. Wiersma, R. Dill-Macky, J.J. Wiersma, AND G.A. Hareland. 2006. Registration of ‘Ulen’ Wheat. Crop Science 46: 979-980. Crossa, J., Burgueño, J., Dreisigacker, S., Vargas, M., Herrera-Foessel, S. A., Lillemo, M., Singh, R. P., Trethowan, R., Warburton, M., Franco, J., Reynolds, M., Crouch, J. H., and Ortiz, R. (2007). Association analysis of historical bread wheat germplasm using additive genetic covariance of relatives and population structure. Genetics, 177(3), 1889-1913. Kolmer, J. A. (1996). Genetics of resistance to wheat leaf rust 1. Annual Review of Phytopathology, 34(1), 435-455. Kolmer, J. A., Jin, Y., and Long, D. L. (2007). Wheat leaf and stem rust in the United States. Crop and Pasture Science, 58(6), 631-638. Kolmer, J. A. and J. A. Anderson. 2011. First Detection in North America of Virulence in Wheat Leaf Rust (Puccinia triticina) to Seedling Plants of Wheat with Lr21. Plant Disease. 95(8):1032. Liu, J. Q., and Kolmer, J. A. (1997). Genetics of leaf rust resistance in Canadian spring wheats AC Domain and AC Taber. Plant disease, 81(7), 757-760. Maccaferri, M., Sanguineti, M. C., Mantovani, P., Demontis, A., Massi, A., Ammar, K., Kolmer, J., Czembor, J., Breiman, A., and Tuberosa, R. (2010). Association mapping of leaf rust response in durum wheat. Molecular Breeding, 26(2), 189-228. Marasas, C.N., M. Smale, and R.P. Singh. 2004. The Economic Impact in Developing Countries of Leaf Rust Resistance Breeding in CIMMYT-Related Spring Bread Wheat. Economics Program Paper 04-01. Mexico, D.F.: CIMMYT. McIntosh, R. A., Wellings, C. R., & Park, R. F. (1995). Wheat rusts: an atlas of resistance genes. CSIRO PUBLISHING. Yu, J., & Buckler, E. S. (2006). Genetic association mapping and genome organization of maize. Current Opinion in Biotechnology, 17(2), 155-160.