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

Abstract Twenty‐one bread‐wheat entries were selected after careful screening for complete or near‐complete resistance to yellow rust (Puccinia striiformis), 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 F2 populations were advanced to F7 under selection for complete resistance to the three rusts and for good agronomic types. In 1992 the 21 parents and 140 selected F7 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 F7 lines, all still resistant in F6, 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 patho‐systems when the selection is for complete or near‐complete resistance.