GENETICS OF ADULT-PLANT RESISTANCE TO STRIPE RUST IN 10 SPRING BREAD WHEATS

Nine Mexican spring bread wheat (Triticum aestivum L.) cultivars deriv ed from CIMMYT germplasm and the U.S. spring wheat cultivar Wheaten we re susceptible to the Mexican Puccinia striiformis pathotype 14E14 in seedling growth stage, but displayed different levels of adult plant r esistances to the same pathotype when tested in the field. One hundred and eighteen random F, plant derived F; and F, lines from the crosses of these ten adult plant resistant wheats and susceptible cultivar Ju pateco 73S were evaluated in the field. The moderate adult plant resis tance of Penjamo 62, Lerma Rojo 64, Nacozari 76, Tesia 79, and Wheaten was under monogenic genetic control and was attributed to the adult p lant stripe rust resistance gene Yr18. The moderate resistances of Cle opatra 74, Zaragoza 75, and Apache 81 were also monogenic, but gene Yr 18 was absent. Pavon 76 carried two partially effective additive genes ; and the adult plant resistance of Tonichi 81 was based on additive i nteraction involving Yr18 and two additional partially effective genes . Tonichi 81 does not carry any seedling resistance gene, however, the adult plant resistance is highly effective worldwide. This resistance , designated as the Yr18 complex, is of a durable nature. The partial adult plant resistance of Pavon 76 has also remained durable in Mexico and other countries where it is grown.