Virulence to Yr10 and Yr24 in Mexican yellow rust fungal population and implications for CIMMYT durum and bread wheat germplasm

Stripe rust, caused by Puccinia striiformis tritici (Pst), continues its evolution towards virulence to race-specific resistance genes. Identification of Mexican Pst isolates MEX16-03 and MEX16.04 that changed infection types of Yr10 testers from 1 to 9 and for Yr24 (=Yr26) testers from 3 to 9 indicated that a mutation for virulence to these resistance genes has occurred in a predominant race detected in 2014 and maintained at CIMMYT as MEX14.191 and at INIFAP as CMEX14.25. Isolate MEX14.191 was responsible for the susceptibility of popular varieties Nana F2007 and Luminaria F2014 grown in central Mexican highlands. Isolate 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 using the Avocet near-isolines and other known testers. Virulence to Yr10 and Yr24 (=Yr26) were also confirmed by testing seedlings of cultivars Moro (Yr10), Chuanmai 42, and Neimai 836 (Yr24). Seedling tests carried on 200 bread wheat, 550 durum, and 460 synthetic hexaploid wheats with their respective durum parents from CIMMYT collection indicated that MEX16.03 and MEX16.04 do not represent a major threat because a majority of the lines remained resistant to these isolates. However, it is worth mentioning that durum cultivars, such as Khofa, Desert King, Anatoly, Movas, and Llareta INIA, and 10 primary synthetic hexaploid or synthetic-derived bread wheats that were resistant to MEX14.191 became susceptible to MEX16.03 and MEX16.04. Our results indicate that resistance gene Yr10 was absent and Yr24 occurred in low frequency in CIMMYT bread wheat germplasm. A majority of CIMMYT durum wheat possibly carried Yr24 in combination with other effective gene(s).