RESISTANCE TO PUCCINIA-RECONDITA F SP TRITICI IN 50 MEXICAN BREAD WHEAT CULTIVARS

Bread wheat (Triticum aestivum L.) cultivars derived from CIMMYT germplasm are currently grown on a large area worldwide. Since limited information is available on genes for resistance to leaf rust in these wheats, the two main objectives of the study were to Postulate the known Lr genes conferring low seeding reactions to 18 pathotypes of Puccinia recondita Roberge ex Desmaz. f. sp. tritici in 50 Mexican cultivars released between 1960 and 1988; and to evaluate the presence of additional adult-plant resistance. Thirteen known genes were postulated to be present either singly or in combination: Lr1 (in 12 cultivars), Lr3 (1), Lr3bg (2) Lr10 (12), Lr13 (25) Lr14a (5), Lr16 (5), Lr17 (8), Lr19 (1), Lr23 (3), Lr26 (9), and Lr27 + Lr31 (7). Additional temperature-sensitive seedling resistance occurred in 25 cultivars. Area under the disease progress curve for these 25 cultivars indicated variable levels of adult plant resistance. Thirteen other cultivars with seedling susceptibility to one or more of the predominant field pathotypes were also partially resistant in the field. High levels of adult plant resistance occurred in some cultivars, even in the absence of known genes with major effects. 'Penjamo 62' and 'Siete Cerros 66' possessed the gene Ne2 for hybrid necrosis but lacked Lr13, indicating that recombination can occur between the two loci.