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Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases of wheat in western Canada. Although stripe rust was an issue in southern Alberta for many years, it became important in other parts of the country after a dramatic population shift in 2000, resulting from an invasive race. Sporadic epidemics of the disease are common and cause considerable loss, due to which, an intermediate level of resistance to stripe rust was required for new varietal registrations beginning 2017. Virulence surveys are of key importance in germplasm and cultivar development as they provide breeders and pathologists the information needed to better understand host-pathogen interactions and the effectiveness of Yr genes. Virulence characterization revealed a wide range of virulence phenotypes exhibited by 33 Pst races in western Canada, although only 2-3 races were predominant. The expression of Yr genes may differ between controlled conditions and natural field conditions as previously reported. Thus, stripe rust differentials and wheat cultivars grown in western Canada are also screened at multiple locations in every year. At present, all stage resistance genes Yr1, Yr4, Yr5, Yr15, Yr76, and YrSP are effective against the predominant Pst races, whereas at the adult stage under field conditions, Yr2, Yr17, Yr28, or those carried by Yamhill are also effective. Seedling resistance genes Yr7, Yr10, Yr17, or Yr27 were the most common in Canadian wheat cultivars. Of these, only Yr17 is effective under field conditions. Adult plant resistance genes Yr18 and Yr29 are carried by many cultivars, but are not effective under high disease pressure. The effectiveness of each resistance gene may vary between the eastern and western prairies of western Canada due to differences in virulence. Regular virulence surveys using contemporary and regional cultivars facilitate the development of rust resistant cultivars.
We earlier assembled the genomes of Pst isolates collected in western Canada using assembly of Illumina paired-end sequences. Two isolates, LSW3_2012_SP2 and SWS484_SPF, were assembled with ?15,150 and ?11,700 contigs each when compared to references North American PST-78 and Chinese CYR32, respectively. In order to reduce the number of contigs and therefore obtain a longer display of contiguous genes, we used the PacBio and Illumina Mate Pair (MP) technologies to achieve that goal using the two isolates. We had to modify our current protocol for DNA isolation from Pst spores to obtain DNA fragments of ?35 Kb suitable for construction of large insert genomic sequencing libraries. Libraries of 8-10 Kb and 3.5-6 Kb were used for PacBio and Illumina MP analyses, respectively. We obtained a 26x coverage of the Pst genome with the PacBio results with an mean size of 7,400 bp and 6,500 bp for the two libraries, and a 190x coverage with the Illumina MP sequencing information. We are using the Ray assembler with the datasets and the 50x Illumina paired end sequencing information from previously independently associated isolates LSW3_2012_SP2 and SWS484_SPF. The quality of our assembly will be compared to the contigs and supercontigs available for the reference isolates PST-78 and CYR32. These results will also enable us to establish the physical relationship among isolate-specific genes. Finally, the impact of the large insert libraries on the proportion of short paired-end unassembled reads will be discussed as it was 78% and 50% for LSW3_2012_SP2 and SWS484_SPF, respectively, after assembly of 100 bp paired-end reads.