Genetic progress in reducing losses to leaf rust in CIMMYT-derived Mexican spring wheat cultivars

Leaf rust, caused by Puccinia recondita Roberge ex Desmaz., is an important disease of wheat (Triticum aestivum L.) worldwide. To estimate the genetic progress in reducing grain yield losses through breeding for resistance to leaf rust, replicated trials including 15 popular CIMMYT germplasm-derived wheat cultivars released between 1966 and 1988 in northwestern Mexico were sown in that same area; normal and late planting dates were used for four and two seasons, respectively. Leaf rust epidemics were established by inoculating spreader rows planted adjacent to plots of the cultivars which were not protected hy fungicide. Average losses in grain yields of the cultivars due to leaf rust ranged between 6.6 and 62.7% and were highly correlated with final disease severity (r = 0.898, P < 0.01) and relative area under the disease progress curve (r = 0.917, P < 0,01). The losses in grain yield mere mostly due to reductions in kernel weight, kernels per square meter, and gr ain fill rate. Grain yield losses (7.7-10.4%) in slow rusting cultivars Cocoraque 75, Nacozari 76, Opata 85, and Bacanora 88 mere similar to those observed in the immune: 'Oasis 86' (6.6%) or resistant 'Ciano 79' (10.2%), The average annual progress in grain yield potential achieved through breeding averaged over the six trials vc as estimated to be 0.45% (r(2) = 0.38, P < 0.01) for fungicide protected and 2.21% (r(2) = 0.47, P < 0.01) when not protected by fungicide, We conclude that while the grain yield potential of CIMMYT-derived cultivars has increased significantly over the Sears, progress in protecting this yield potential through the incorporation of genes that confer slow rusting resistance has been more dramatic.