First Detection of Virulence in Puccinia striiformis f. sp. tritici to Wheat Resistance Genes Yr10 and Yr24 (=Yr26) in Mexico
Stripe rust, caused by Puccinia striiformis f. sp. tritici, was observed on bread wheat (Triticum aestivum L.) lines previously considered to be resistant. Specifically, near isogenic lines (NIL) carrying the resistance genes Yr10, Yr24, and Yr26 at the CIMMYT-Toluca research station were observed to be infected with stripe rust in 2015. Several leaves exhibiting stripe rust uredinia and urediniospores were collected from field plots containing the Yr24 (MEX16.04) and Yr26 (MEX16.03) NILs. More than 20 pustules were isolated from the diseased leaves and inoculated onto 12-day-old wheat seedlings of cultivars Morocco and Avocet Yr24. Inoculated plants were incubated in a dew chamber with 100% relative humidity for 20 h in the dark, maintained between 7 and 10°C. After incubation, plants were moved to a greenhouse where each pot was isolated in its own compartment. Greenhouse temperature was maintained at 15 to 18°C, supplemented with 10,000 lx of fluorescent light for 8 h per day. Urediniospores representing each of 24 isolates were collected 12 days after inoculation and maintained at 4°C. Virulence spectra of each isolate were determined by inoculating 30 differential lines that contained known Yr resistance genes. Cultivars Moro (Yr10), Chuanmai 42, and Neimai 836 (Yr24) were also included. Infection types were recorded approximately 2 weeks postinoculation using a 0 to 9 scale (McNeal et al. 1971). A change in the infection types from 1 to 9 for wheat lines containing Yr10 and from 3 to 9 for Yr24 and Yr26 indicated that a mutation for virulence to Yr10 and Yr24 (= Yr26) occurred in a recently identified isolate accessioned and maintained at CIMMYT as MEX14.141. MEX14.141, among others, had caused a severe yellow rust epidemic during 2014 on Mexican cultivars Nana F2007 and Luminaria F2014 (Solis et al. 2016). One of the 20 isolates mentioned, designated as MEX16.04, has the following avirulence/virulence formula: Yr1, 5, 15, Sp/Yr2, 3, 6, 7, 8, 9, 10, (17), 24, 26, 27, 28, 31, 32. MEX16.04 is avirulent onto the most susceptible barley cultivar Kaputar, which allowed us to classify the isolate as P. striiformis f. sp. tritici instead of P. striiformis f. sp. hordei. MEX16.04 belongs to the aggressive race group first identified in North America in 2000 and which became predominant in subsequent years (Milus et al. 2009). Seedling tests using the above mentioned methodology on 12-day-old seedlings of 167 bread wheat, 508 durum, and 460 member set composed of synthetic hexaploid wheats with their respective durum parents from CIMMYT indicated that MEX16.04 does not represent a major threat since a majority of the lines remained resistant to this isolate. Moreover, I is not known to be present in CIMMYT bread wheat germplasm and I occurs in low frequency; however, I is present in combination with other effective gene(s) in durum wheat. Purified urediniospores of MEX16.03 and MEX16.04 isolates are stored in the CIMMYT and INIFAP yellow rust collections.