First report of expansion of virulence in Puccinia striiformis f. sp. tritici to wheat resistance genes Yr10 and Yr24 in the Middle East.

Over the last decade, field assessments of the yellow rust differential lines for resistance genes Yr10 and Yr24 and race analysis in the Middle East have demonstrated efficient yellow rust control by Yr10 and Yr24 (=Yr26). Yellow rust samples collected during 2018-21 in Central West Asia & North and sub-Saharan Africa underwent race analysis at the Regional Cereal Rust Research Center in Izmir, Türkiye. The infected leaf segments were subjected to rehydration at 20°C for three hours. Subsequently, the leaf segments were rubbed on the first leaves of seedlings of susceptible cultivar Morocco. Inoculated seedlings were placed at 10°C in dark conditions with 95% humidity for 24 hrs, then moved to a growth chamber with a 16-hr light (220 µmolm-2s-1) cycle at 15°C and an eight-hour dark period at 12°C. Urediniospores were collected 15 days post-inoculation. A set of yellow rust differential lines including Morocco, Avocet 'S', Avocet 'R', Yr1/6* Avocet 'S', Kalyansona (Yr2), Vilmorin 23 (Yr3), Hybrid 46 (Yr4), Yr6/6* Avocet 'S', Yr7/6* Avocet 'S', Yr8/6* Avocet 'S', Yr9/6* Avocet 'S', Yr10/6* Avocet 'S', Moro (Yr10+), Yr17/6*Avocet 'S', Yr24/6* Avocet 'S', TP1295 (Yr25), Yr27/6* Avocet 'S', YrSp/6* Avocet 'S', Spalding Prolific (YrSP), Strubes Dickkopf (YrSD), Tres/6*Avocet'S', Cham 1, and Ambition was used in race analysis. A mixture of 2 mL Soltrol® and 0.5 mg fresh urediniospores was used to inoculate 10-day-old seedlings of the 23 differential varieties. Pre-inoculation, incubation, and post-inoculation conditions were the same as above. Seedling infection types (ITs) were recorded 15 days post-inoculation on a scale of 0 to 9 (McNeal et al. 1971), where ITs 0 to 6 are classified as low infection types (LITs= avirulent) and ITs 7 to 9 categorized as high infection types (HITs= virulent). HITs of 7 to 9 were observed for the first time on Yr10/6* Avocet 'S', Yr24/6* Avocet 'S', as well as on Moro (Yr10+) for 25 sample of the total 50 isolates from Lebanon and Türkiye in 2018. During the race analysis in 2019 to 2021, virulence for Yr10 and Yr24 was identified among tested samples from Egypt, Lebanon, Jordan, Syria, and Türkiye, indicating the expansion of virulence for Yr10 and Yr24 into new regions. HITs were observed for the durum wheat cultivar Cham 1 and wheat cultivar Ambition in all races. Virulence for YrA, Yr2, Yr6, Yr7, Yr8, Yr17, and 32 was common within the Yr10 and Yr24 virulent races, and virulence for YrSp and Yr27 were observed in low frequency. Molecular genotyping of 209 isolates, including the Yr10 virulent races, was performed using 19 microsatellite markers (Ali et al. 2017; Rodriguez-Algaba et al. 2017) and aligned with the Puccinia striiformis nomenclature system of the Global Rust Reference Center (GRRC). The results showed that 66 isolates were identical to the genotyping lineage "ME2018" identified in Egypt in 2018 by GRRC. This genetic lineage has now been designated as PstS17 (Hovmøller et al. 2023). The durum wheat cultivars have always been resistant to yellow rust in the Middle East. Seedling tests of 50 durum advanced lines from CIMMYT's International Durum Wheat Yield Nursery showed LITs in 45 accessions (90%) against an avirulent race for Yr10 and Yr24 (PstS2), but only 12% remained resistant while tested with a PstS17 (virulent for Yr10 and Yr24). This observation provides compelling evidence of the Yr10 and/or Yr24 presence within tested durum wheat germplasm. Continued monitoring of virulence and resistance of wheat germplasm to yellow rust is critical for successful breeding for rust resistance.

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.