Accomplishments in wheat rust research in South Africa

Wheat Stem Rust Surveillance Reveals Two New Races of Puccinia graminis f. sp. tritici in South Africa During 2016 to 2020

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is an important disease of wheat in South Africa (SA) and is primarily controlled using resistant cultivars. Understanding virulence diversity of Pgt is essential for successful breeding of resistant cultivars. Samples of infected wheat stems were collected across the major wheat-growing regions of SA from 2016 to 2020 to determine the pathogenic variability of Pgt isolates. Seven races were identified from 517 isolates pathotyped. The most frequently found races were 2SA104 (BPGSC + Sr9h,27,Kw) (35% frequency) and 2SA88 (TTKSF + Sr8b) (33%). Race 2SA42 (PTKSK + Sr8b), which was found in 2017, and 2SA5 (BFGSF + Sr9h), identified in 2017, are new races. The Ug99 variant race 2SA42 is similar in its virulence to 2SA107 (PTKST + Sr8b) except for avirulence to Sr24 and virulence to Sr8155B1. Race 2SA5 is closely related in its virulence to existing races that commonly infect triticale. Certain races showed limited geographical distribution. Races 2SA5, 2SA105, and 2SA108 were found only in the Western Cape, whereas 2SA107 and 2SA42 were detected only in the Free State province. The new and existing races were compared using microsatellite (SSR) marker analysis and their virulence on commercial cultivars was also determined. Seedling response of 113 wheat entries against the new races, using 2SA88, 2SA88+9h, 2SA106, and 2SA107 as controls, revealed 2SA107 as the most virulent (67 entries susceptible), followed by 2SA42 (64), 2SA106 (60), 2SA88+9h (59), 2SA88 (25), and 2SA5 (17). Thus, 2SA5 may not pose a significant threat to local wheat production. SSR genotyping revealed that 2SA5 is genetically distinct from all other SA Pgt races.

High-resolution mapping of SrTm4, a recessive resistance gene to wheat stem rust

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The diploid wheat recessive stem rust resistance gene SrTm4 was fine-mapped to a 754-kb region on chromosome arm 2AmL and potential candidate genes were identified. Race Ug99 of Puccinia graminis f. sp. tritici (Pgt), the causal agent of wheat stem (or black) rust is one of the most serious threats to global wheat production. The identification, mapping, and deployment of effective stem rust resistance (Sr) genes are critical to reduce this threat. In this study, we generated SrTm4 monogenic lines and found that this gene confers resistance to North American and Chinese Pgt races. Using a large mapping population (9522 gametes), we mapped SrTm4 within a 0.06 cM interval flanked by marker loci CS4211 and 130K1519, which corresponds to a 1.0-Mb region in the Chinese Spring reference genome v2.1. A physical map of the SrTm4 region was constructed with 11 overlapping BACs from the resistant Triticum monococcum PI 306540. Comparison of the 754-kb physical map with the genomic sequence of Chinese Spring and a discontinuous BAC sequence of DV92 revealed a 593-kb chromosomal inversion in PI 306540. Within the candidate region, we identified an L-type lectin-domain containing receptor kinase (LLK1), which was disrupted by the proximal inversion breakpoint, as a potential candidate gene. Two diagnostic dominant markers were developed to detect the inversion breakpoints. In a survey of T. monococcum accessions, we identified 10 domesticated T. monococcum subsp. monococcum genotypes, mainly from the Balkans, carrying the inversion and showing similar mesothetic resistant infection types against Pgt races. The high-density map and tightly linked molecular markers developed in this study are useful tools to accelerate the deployment of SrTm4-mediated resistance in wheat breeding programs.