Isolate-specific QTLs for partial resistance to Puccinia hordei in barley

By using a high-density AFLP marker linkage map, six QTLs for partial resis tance to barley leaf rust (Puccinia hordei) isolate 1.2.1. have been identi fied in the RIL offspring of a cross between the partially resistant cultiv ar 'Vada' and the susceptible line L94. Three QTLs were effective at the se edling stage, and five QTLs were effective at the adult plant stage. To stu dy possible isolate specificity of the resistance, seedlings and adult plan ts of the 103 RILs from the cross L94x'Vada' were also inoculated with anot her leaf rust isolate, isolate 24. In addition to the two QTLs that were ef fective against isolate 1.2.1. at the seedling stage, an additional QTL for seedling resistance to isolate 24 was identified on the long arm of chromo some 7. Of the eight detected QTLs effective at the adult plant stage, thre e were effective in both isolates and five were effective in only one of th e two isolates. Only one QTL had a substantial effect at both the seedling and the adult plant stages. The expression of the other QTLs was developmen tal-stage specific. The isolate specificity of the QTLs supports the hypoth esis of Parlevliet and Zadoks (1977) that partial resistance may be based o n a minor-gene-for-minor-gene interaction.