Durum wheat cv. Creso has been mentioned as having durable resistance to leaf rust (2--4). However, an average final disease level of 70S on the modified Cobb scale was scored on Creso across three locations in inoculated field trials in France during 2009. A mixture of two durum wheat leaf rust isolates commonly found in France was used for the inoculation, one was virulent on Lr23 and the other was avirulent on this gene, their identical avirulence/virulence formula for other genes was Lr1, 2a, 2b, 3, 3bg, 3ka, 9, 11, 13, 15, 16, 17, 19, 24, 25, 26, 27+31/Lr2c, 10, 14a, 14b, 20, 21, 33, and 44. On cv. Llareta Inia and breeding line Somateria, both of which carry the resistance gene Lr14a, the average final disease level was, respectively, 95S and 80S. Creso, Llareta Inia, and Somateria displayed average final disease levels of, respectively, 0, 10S, and 1 in field trials inoculated with race CBG/BP in 2009 at two locations in Mexico (Ciudad Obregon and El Batan). Race CBG/BP, virulent on Lr3, 10, 11, 14b, 20, 23, 27 + 31, and 33, is the most widely virulent race identified so far in Mexico where Lr14a remains effective for durum wheat. Virulence for Lr14a in durum wheat leaf rust populations was already mentioned to be present in France since 2000 (1). It has been suggested that the resistance of Creso, which has remained durable in Italy since 1975 (4), could be due to a gene close to but different from Lr14a. Alternatively, the fact that Creso's reaction was significantly lower than those of Llareta Inia or Somateria could indicate the presence of another gene, of minor effect, in addition to Lr14a. Whatever the genetic basis of the Creso resistance may be, it has been overcome by common French pathotypes and its usefulness in breeding, at a regional if not global level, has become questionable.
References: (1) H. Goyeau et al. Phytopathology 96:264, 2006. (2) S. A. Herrera-Foessel et al. Plant Dis. 92:469, 2008. (3) M. Maccaferri et al. Theor. Appl. Genet. 117:1225, 2008. (4) D. Marone et al. Mol. Breed. 24:25, 2009.
