Genetics and mapping of seedling resistance to Ug99 stem rust in winter wheat cultivar Triumph 64 and differentiation of SrTmp, SrCad, and Sr42

Resistance to Ug99 stem rust in Triumph 64 was conferred by SrTmp on chromosome arm 6DS and was mapped to the same position as SrCad and Sr42 , however, the three genes show functional differences. Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is an important disease of wheat that can be controlled by effective stem rust resistance (Sr) genes. The emergence of virulent Pgt races in Africa, namely Ug99 and its variants, has stimulated the search for new Sr genes and genetic characterization of known sources of resistance. Triumph 64 is a winter wheat cultivar that carries gene SrTmp, which confers resistance to Ug99. The goals of this study were to genetically map SrTmp and examine its relationship with other Sr genes occupying a similar chromosome location. A doubled haploid (DH) population from the cross LMPG-6S/Triumph 64 was inoculated with Ug99 at the seedling stage. A single gene conditioning resistance to Ug99 segregated in the population. Genetic mapping with SSR markers placed SrTmp on chromosome arm 6DS in a region similar to SrCad and Sr42. SNP markers developed for SrCad were used to further map SrTmp and were also added to a genetic map of Sr42 using a DH population (LMPG-6S/Norin 40). Three SNP markers that co-segregated with SrTmp also co-segregated with SrCad and Sr42. The SNP markers showed no difference in the map locations of SrTmp, SrCad, and Sr42. Multi-race testing with DH lines from the Triumph 64 and Norin 40 populations and a recombinant inbred-line population from the cross LMPG-6S/AC Cadillac showed that SrTmp, SrCad, and Sr42 confer different spectra of resistance. Markers closely linked to SrTmp are suitable for marker-assisted breeding and germplasm development.

Genetic mapping of SrCad and SNP marker development for marker-assisted selection of Ug99 stem rust resistance in wheat

New SNP markers that can be used for marker-assisted selection and map-based cloning saturate the chromosome region carrying SrCad , a wheat gene that confers resistance to Ug99 stem rust. Wheat stem rust, caused by Puccinia graminis f. sp. tritici, is a devastating disease of wheat worldwide. Development of cultivars with effective resistance has been the primary means to control this disease, but the appearance of new virulent strains such as Ug99 has rendered most wheat varieties vulnerable. The stem rust resistance gene SrCad located on chromosome arm 6DS has provided excellent resistance to various strains of Ug99 in field nurseries conducted in Njoro, Kenya since 2005. Three genetic populations were used to identify SNP markers closely linked to the SrCad locus. Of 220 SNP markers evaluated, 27 were found to be located within a 2 cM region surrounding SrCad. The diagnostic potential of these SNPs was evaluated in a diverse set of 50 wheat lines that were primarily of Canadian origin with known presence or absence of SrCad. Three SNP markers tightly linked proximally to SrCad and one SNP that co-segregated with SrCad were completely predictive of the presence or absence of SrCad. These markers also differentiated SrCad from Sr42 and SrTmp which are also located in the same region of chromosome arm 6DS. These markers should be useful in marker-assisted breeding to develop new wheat varieties containing SrCad-based resistance to Ug99 stem rust.

Genetics and mapping of seedling resistance to Ug99 stem rust in Canadian wheat cultivars 'Peace' and 'AC Cadillac'

Stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.) has re-emerged as a threat to wheat production with the evolution of new pathogen races, namely TTKSK (Ug99) and its variants, in Africa. Deployment of resistant wheat cultivars has provided long-term control of stem rust. Identification of new resistance genes will contribute to future cultivars with broad resistance to stem rust. The related Canadian cultivars Peace and AC Cadillac show resistance to Ug99 at the seedling stage and in the field. The purpose of this study was to elucidate the inheritance and genetically map resistance to Ug99 in these two cultivars. Two populations were produced, an F(2:3) population from LMPG/AC Cadillac and a doubled haploid (DH) population from RL6071/Peace. Both populations showed segregation at the seedling stage for a single stem rust resistance (Sr) gene, temporarily named SrCad. SrCad was mapped to chromosome 6DS in both populations with microsatellite markers and a marker (FSD_RSA) that is tightly linked to the common bunt resistance gene Bt10. FSD_RSA was the closest marker to SrCad (≈ 1.6 cM). Evaluation of the RL6071/Peace DH population and a second DH population, AC Karma/87E03-S2B1, in Kenya showed that the combination of SrCad and leaf rust resistance gene Lr34 provided a high level of resistance to Ug99-type races in the field, whereas in the absence of Lr34 SrCad conferred moderate resistance. A survey confirmed that SrCad is the basis for all of the seedling resistance to Ug99 in Canadian wheat cultivars. While further study is needed to determine the relationship between SrCad and other Sr genes on chromosome 6DS, SrCad represents a valuable genetic resource for producing stem rust resistant wheat cultivars.

Letter Code System of Nomenclature for Puccinia graminis f. sp. avenae

Current systems that describe the virulence phenotype in Puccinia graminis f. sp. avenae lack a systematic approach for the naming of races or to provide easily made comparisons of virulence among races. A new nomenclature system that simply and systematically characterizes virulence in P. graminis f. sp. avenae is described. The new system has the distinct advantage of providing easily seen relationships among races in contrast to previous nomenclature systems. This allows for easier interpretation of virulence relationships in the oat stem rust population and provides a large amount of virulence information with a minimum of written characters. This system uses single-gene differential lines with the resistance genes Pg1, Pg2, Pg3, Pg4, Pg6, Pg8, Pg9, Pg10, Pg12, Pg13, Pg15, and Pg16, grouped into three subsets of four lines in sequential Pg gene order. By grouping in sequential gene number order, the relationship of the new system to the "standard" system is easily seen. Each race is designated by a three-letter code, based on the seedling reaction (low or high) on 12 differential lines. The letter code nomenclature system is open ended and can be updated easily as new differential genes are identified. This system simply and precisely describes the virulence phenotypes of isolates of P. graminis f. sp. avenae, and allows for easily made comparisons of virulence of isolates collected over time and across geographical locations worldwide.

Evaluation of Avena spp. Accessions for Resistance to Oat Stem Rust

Oat stem rust, caused by Puccinia graminis f. sp. avenae, can cause significant yield losses in the eastern prairie region of western Canada. Currently, the predominant race of P. graminis f. sp. avenae in this region is NA67. Few genes confer resistance to NA67, and none are present in any oat cultivars registered for production in Canada. To detect lines exhibiting resistance to race NA67, we evaluated 9,978 accessions from 22 Avena spp. in field nurseries from 2001 to 2004. In all, 35 accessions were highly resistant and 12 were moderately resistant, comprised mostly of the species A. strigosa. Seventy-one accessions had an intermediate response, comprised mostly of A. abyssinica, A. barbata, A. sterilis, and A. vaviloviana. All other accessions (9,860 = 98.8%) were susceptible to race NA67. Some highly resistant accessions were found to have been incorrectly classified previously as hexaploid species A. sativa or A. sterilis, and were confirmed by chromosome counts to be diploid or tetraploid. The most promising source of novel stem rust resistance is from the diploid species A. strigosa. Transfer of resistance from diploid and tetraploid species to A. sativa is very difficult, but the lines identified in this study should prove useful as new sources of resistance to oat stem rust.

Diversity and Sources of Multiple Disease Resistance in Hordeum spontaneum

Hordeum spontaneum, the progenitor of cultivated barley, is known to be a rich source of disease resistance genes. The objective of this study was to assess the diversity of H. spontaneum accessions from Israel and Jordan for their reaction to six fungal pathogens of importance to cultivated barley in the United States and Canada. Overall, a high level of macro-scale (across collection sites) and micro-scale (within a collection site) diversity for disease reaction was found in the 116 accessions of H. spontaneum evaluated at the seedling stage. Additionally, genetic heterozygosity for resistance loci was common in H. spontaneum. The frequency of resistance in accessions from Jordan and Israel was high for Septoria speckled leaf blotch (77 and 98%, respectively), leaf rust (70 and 90%), net blotch (72 and 68%), and powdery mildew (58 and 70%); intermediate for spot blotch (53 and 46%); and low for stem rust (2 and 26%). The level of disease resistance in H. spontaneum was not strongly correlated with any of the weather variables (temperature, precipitation, and humidity) monitored near the collection sites. However, in general, resistance was more often found in germ plasm from mesic (e.g., Mediterranean coast) than in xeric (e.g., Negev Desert) areas. Two H. spontaneum accessions (Shechem 12-32 and Damon 11-11) were resistant to all six pathogens and may be useful parents in programs breeding barley for multiple disease resistance. The high level of diversity and heterozygosity for disease reaction found in this study indicates that H. spontaneum is an extraordinarily rich and largely untapped source of unique disease resistance alleles for cultivated barley improvement.

Fusarium Species Pathogenic to Barley and Their Associated Mycotoxins

Epidemics of Fusarium head blight (FHB) occurred on barley in Minnesota, North Dakota, and South Dakota from 1993 to 1998. The Red River Valley region was most severely impacted by the disease based on assessments of FHB severity in grain samples harvested from commercial fields. Fusarium graminearum was the primary pathogen causing these FHB epidemics. It comprised from 62 to 64% of all Fusarium species isolated from infected kernels from 1994 to 1996. Fusarium poae (range of isolation 13 to 20%),F. sporotrichioides (10 to 17%), and F. avenaceum (6 to 10%) also were isolated from barley kernels and were likely involved in causing some FHB infection, but to a very limited extent. All four Fusarium species were pathogenic on barley in inoculation tests conducted in both the greenhouse and the field. Mycotoxin screens were performed on barley spikes inoculated with the respective species in the greenhouse. Spikes infected with F. graminearum contained deoxynivalenol and 15-acetyldeoxyni-valenol; those infected with F. sporotrichioides contained T-2 toxin, HT-2 toxin, and T-2 tetraol; and those infected with F. poae contained nivalenol. Some isolates of F. poae also produced 15-acetoxyscirpenol and scirpentriol. Although F. graminearum and DON are recognized as the primary FHB pathogen and mycotoxin, respectively, in barley, the possible presence of other Fusarium species and mycotoxins should not be overlooked.