Lack of durability of resistance to cereal rusts in wheat when selection is for complete resistance

Twenty-one bread-wheat entries were selected after careful screening for complete or near-complete resistance to yellow rust (Puccinia striformis), stem rust (P. graminis), and leaf rust (P. recondita). In 1987, the 21 entries were intercrossed in a near-half diallel scheme. The resulting 190 F-2 populations were advanced to F-7 under selection for complete resistance to the three lusts and for good agronomic types. In 1992 the 21 parents and 140 selected F-7 lines were assessed for their resistance to the three rusts. Of the 21 parents, 12 showed a breakdown of yellow rust resistance, five a breakdown of stem rust resistance and two a breakdown of leaf rust resistance. In addition, several of the 140 selected F-7 lines, all still resistant in F-6, had become susceptible to one or more of the rusts. It appears that a progression towards more complex races, especially of yellow rust, is inevitable for the wheat-cereal rust pathosystems when the selection is for complete or near-complete resistance.