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.

Evaluation of a global spring wheat panel for stripe rust: Resistance loci validation and novel resources identification.

Stripe rust (incited by Puccinia striiformis f. sp. tritici) is airborne wheat (Triticum aestivum L.) disease with dynamic virulence evolution. Thus, anticipatory and continued screening in hotspot regions is crucial to identify new pathotypes and integrate new resistance resources to prevent potential disease epidemics. A global wheat panel consisting of 882 landraces and 912 improved accessions was evaluated in two locations in Egypt during 2016 and 2017. Five prevalent and aggressive pathotypes of stripe rust were used to inoculate the accessions during the two growing seasons and two locations under field conditions. The objectives were to evaluate the panel for stripe rust resistance at the adult plant stage, identify potentially novel QTLs associated with stripe rust resistance, and validate previously reported stripe rust QTLs under the Egyptian conditions. The results indicated that 42 landraces and 140 improved accessions were resistant to stripe rust. Moreover, 24 SNPs were associated with stripe rust resistance and were within 18 wheat functional genes. Four of these genes were involved in several plant defense mechanisms. The number of favorable alleles, based upon the associated SNPs, was significant and negatively correlated with stripe rust resistance score, i.e., as the number of resistances alleles increased the observed resistance increased. In conclusion, generating new stripe rust phenotypic information on this panel while using the publicly available molecular marker data, contributed to identifying potentially novel QTLs associated with stripe rust and validated 17 of the previously reported QTLs in one of the global hotspots for stripe rust.

Molecular markers and phenotypic characterization of adult plant resistance genes Lr 34, Lr 46, Lr 67 and Lr 68 and their association with partial resistance to leaf rust in wheat.

Thirty-nine wheat genotypes were studied to estimate their partial resistance levels to leaf rust at Behira governortae during three growing seasons, i.e. 2016/2017, 2017/2018 and 2018/2019. In these genotypes, partial resistance was characterized using final leaf rust severity (FRS %) and area under disease progress curve (AUDPC). Of the tested genotypes, only three wheat varieties; Giza 171, Misr 3 and Sohag 5 showed complete resistance. Further, 28 of the 39 genotypes had partial resistance as they revealed low and/or moderate values of FRS (%) and AUDPC (not exceeding 30% and 300, respectively). The other eight varieties were fast rusting, as they displayed the maximum values of FRS (%) and AUDPC. The four monogenic lines; Lr 34, Lr 46, Lr 67 and Lr 68 were identified in the wheat varieties using linked molecular markers; csLV34, Xgwm259, CFD71 and csGSR. Phenotypic results of the wheat varieties were confirmed by molecular marker analysis.