QTL Mapping of Adult Plant Resistance to Stripe Rust in a Doubled Haploid Wheat Population.

Stripe rust caused by Puccinia striiformis Westend. f. sp. tritici. is a major bread wheat disease worldwide with yield losses of up to 100% under severe disease pressure. The deployment of resistant cultivars with adult plant resistance to the disease provides a long-term solution to stripe rust of wheat. An advanced line from the International Winter Wheat Improvement Program (IWWIP) 130675 (Avd/Vee#1//1-27-6275/Cf 1770/3/MV171-C-17466) showed a high level of adult plant resistance to stripe rust in the field. To identify the adult plant resistance genes in this elite line, a mapping population of 190 doubled haploid (DH) lines was developed from a cross between line 130675 and the universal stripe rust-susceptible variety Avocet S. The DH population was evaluated at precision wheat stripe rust phenotyping platform, in Izmir during 2019, 2020, and 2021 cropping seasons under artificial inoculations. Composite interval mapping (CIM) identified two stable QTLs QYr.rcrrc-3B.1, and QYr.rcrrc-3B.2, which were detected in multiple years. In addition to these two QTLs, five more QTLs, QYr.rcrrc-1B, QYr.rcrrc-2A, QYr.rcrrc-3A, QYr.rcrrc-5A, and QYr.rcrrc-7D, were identified, which were specific to the cropping year (environment). All QTLs were derived from the resistant parent, except QYr.rcrrc-3A. The significant QTLs explained 3.4-20.6% of the phenotypic variance. SNP markers flanking the QTL regions can be amenable to marker-assisted selection. The best DH lines with high yield, end-use quality, and stripe rust resistance can be used for further selection for improved germplasm. SNP markers flanking the QTL regions can aid in identifying such lines.

First Report of Ug99 Race TTKTT of Wheat Stem Rust (Puccinia graminis f. sp. tritici) in Iraq.

A wheat rust survey was conducted in Iraq in 2019 and collected 27 stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Erikks. & E. Henn.) samples. Seven samples were viable, and they were tested for races of P. graminis f. sp. tritici at the Regional Cereal Rust Research Center (RCRRC) in Izmir, Turkey under strict quarantine procedures. Two 0.5 cm segments of each infected stem sheath were incubated in a petri dish at 20°C for three hours for re-hydration of urediniospores, which were multiplied on 10-day old seedlings of susceptible cultivar Morocco grown in a spore free growth chamber at 18°C and 16 hours light. Inoculated seedlings underwent a dew period at 18°C for 16 hours dark and 8 hours fluorescent light and 95% relative humidity. Three days after moving the pots to a growth chamber with eight hours dark at 18°C and 16 hours light (300 µmol m-2s-1), each pot was covered using a cellophane bag. Bulk urediniospores of each collection were collected 14 days post-inoculation from a cellophane bag using a mini cyclone spore collector connected to a gelatin capsule. One ml of 3M Novec™ oil was added to each capsule, and spores were inoculated onto 20 North American stem rust differential lines using the standard procedures (Jin et al. 2008). Pre-inoculation, inoculation, incubation, and post-inoculation conditions were the same as above. Seedling infection types (ITs) were recorded 14 days post-inoculation using 0 to 4 scale (Stakman et al. 1962). Race designation followed the five- letter code nomenclature described by Jin et al. (2008). Out of the seven samples, four were typed as TKKTF, two as TKTTF, and one collected from an advanced breeding bread wheat line "Shahoo 2" (Inqalab 91*2/Tukuru) in a trial site at Halabja governorate showed mixed ITs of 11+ and 3+ for lines carrying Sr11, Sr24, Sr36, and Sr31. Three single pustule (SP) isolates were developed from the IT of 3+ pustules collected from the Sr31 tester line, and one SP isolate was developed from the IT 11+ pustule collected from the Sr11 source. After spore multiplication, the SP-derived isolates were inoculated on the 20 North American differential lines. To confirm virulence/avirulence on Sr24, Sr31, and Sr36, cultivars Siouxland (PI 483469, Sr24+Sr31) and Sisson (PI 617053, Sr36+Sr31) were also inoculated. All seedling assays were repeated three times. The three SP isolates virulent on Sr31 were designated as race TTKTT, and the SP isolate avirulent on Sr11 was designated as TKTTF. Seedling ITs of 3+ and 0; were recorded for Siouxland and Sisson against TTKTT, respectively, and both cultivars showed IT of 1+ against TKTTF. Race TKTTF was similar to TKKTF except for additional virulence on Sr36, and TTKTT differed from the other two races being virulent on Sr24 and Sr31. DNA analysis of three TTKTT isolates from Kenya and the TTKTT isolate from Iraq using a diagnostic qPCR assay developed by the USDA-ARS Cereals Disease Laboratory (Ug99 RG stage 1, Szabo unpublished) confirmed that all tested isolates belonged to the Ug99 lineage. Race TTKTT was first reported from Kenya in 2014 (Patpour et al. 2016), and in 2018 from Ethiopia (Hei et al. 2020). We report the first detection of TTKTT in Iraq and the Middle East region. This represents only the third instance of a member of the Ug99 race group outside of Africa since first detection of race TTKSK in Yemen in 2006, and Iran in 2007 (Nazari et al. 2009). The continued spread of stem rust races with complex virulence and the increasing frequency and early onset of stem rust infections in the Middle East is a cause for concern. Continuous support for rust surveillance and race typing in this region remains crucial. References: Hei, N. B., et al. 2020. Plant Dis. 104:982. Jin, Y., et al. 2008. Plant Dis. 92:923-926. Nazari, K., et al. 2009. Plant Dis. 93:317. Patpour, M., et al. 2016. Plant Dis. 100:522. Stakman, E. C., et al. 1962. Identification of physiological races of Puccinia graminis var. tritici. U. S. Dep. Agric. ARS E-617.

Genome-wide association study of resistance to PstS2 and Warrior races of Puccinia striiformis f. sp. tritici (stripe rust) in bread wheat landraces.

Stripe or yellow rust, caused by Puccinia striiformis Westend. f. sp. tritici is a major threat to bread wheat production worldwide. The breakdown in resistance of certain major genes and newly emerging aggressive races of stripe rusts pose serious concerns in all main wheat growing areas of the world. To identify new sources of resistance and associated QTL for effective utilization in future breeding programs an association mapping (AM) panel comprising of 600 bread wheat landraces collected from eight different countries conserved at ICARDA gene bank were evaluated for seedling and adult plant resistance against the PstS2 and Warrior races of stripe rust at the Regional Cereal Rust Research Center (RCRRC), Izmir, Turkey during 2016, 2018 and 2019. A set of 25,169 informative SNP markers covering the whole genome were used to examine the population structure, linkage disequilibrium and marker-trait associations in the AM panel. The genome-wide association study (GWAS) was carried out using a Mixed Linear Model (MLM). We identified 47 SNP markers across 19 chromosomes with significant SNP-trait associations for both seedling stage and adult plant resistance. The threshold of significance for all SNP-trait associations was determined by the false discovery rate (q) ≤ 0.05. Three genomic regions (QYr.1D_APR, QYr.3A_seedling and QYr.7D_seedling) identified in this study do not correspond to previously reported Yr genes or QTL, suggesting new genomic regions for stripe rust resistance.