FRECUENCY OF THE IWA7257-YR78 MARKER ASSOCIATED WITH YELLOW RUST RESISTANCE IN MEXICAN BREAD WHEAT

Yr78 is an adult-plant gene that confers resistance to yellow wheat rust that was recently identified in the bread wheat line PI520108 (HORK/KALYANSONA). The gene is located on chromosome 6BS and until before this study its frequency was unknown. Within the CIMMYT gene bank wheat collection, 917 lines carrying HORK as a parent in the pedigree were identified. Approximately 12% of the samples (117 lines) were tested with the molecular marker IWA7257-Yr78, linked to the resistance gene Yr78, and with the physiological yellow rust races MEX14.191 and MEX17.83. It was determined that 76% of the lines tested positive for the IWA7257-Yr78 marker. Lines susceptible at seedling stage showed final disease severity ranging from 0 to 80% in the field, and a similar response in resistant lines at seedlings of 1MR to 60M. The remaining 24% of negative lines for the Yr78 marker, susceptible and resistant at seedling stage showed responses that ranged between 5M and 50MR, indicating the presence of other resistance genes; in addition, 64% of 621 lines from three CIMMYT spring wheat selection nurseries, analyzed molecularly, tested positive for the Yr78 marker, indicating that this gene is more common than expected in Mexican germplasm. Greenhouse and field evaluations in adult plants indicated that Yr78 induces strong chlorosis and necrosis, but does not reduce the percentage of infection; therefore, Yr78 should be used in combination with other resistance genes.