First Detection of Virulence to Genes Lr9 and Lr25 Conferring Resistance to Leaf Rust of Wheat Caused by Puccinia triticina in Mexico.

Leaf rust resistance gene Lr9 was transferred to wheat from Triticum umbellulatum (1). In 1971, 4 years after cultivars with this gene were released in the United States, leaf rust races with virulence to Lr9 were detected. In Mexico, cultivars with Lr9 have not been released and leaf rust races with Lr9 virulence have not been detected. Resistance gene Lr25 was transferred to wheat from Rosen rye (1) and has been effective worldwide, but this gene has not been used commercially because of possible negative effects on yield (1). Rust samples collected at different locations in Nuevo Leon State of Mexico during the 2004-2005 and 2005-2006 wheat crop seasons were analyzed on seedlings of wheat leaf rust differentials in the greenhouse from single pustule isolates. The race nomenclature used is described in Singh (2). Among the races identified from this area, seven, MLJ/SP, TNM/JP, TLB/JP, TNM/KP, TNR/JP, TNB/JN, and TMB/JP, were virulent to both of these genes. Virulence to Lr9 and Lr25 were confirmed by repeating tests using Lr9 differentials 'RL6010' and 'McNair 701' and Transec and three other Lr25 wheat lines developed at CIMMYT, which carry Lr25. To our knowledge, this is the first confirmed report of virulence to Lr9 in Mexico and virulence to Lr25 in the world. These isolates are conserved at CIMMYT for future reference. References: (1) R. A. McIntosh et al. Wheat Rusts: An Atlas of Resistance Genes. CSIRO Publishing, Melbourne, Australia, 1995. (2) R. P. Singh. Plant Dis. 75:790, 1991.