Leaf rust resistance gene Lr34 associated with nonsuppression of stem rustresistance in the wheat cultivar canthatch

The common wheat cultivar Thatcher and the backcross derivative Canthatch a re moderately or fully susceptible to several races of stem rust because of a suppressor on chromosome 7DL that inhibits the expression of the relevan t resistance gene(s). The incorporation of leaf rust resistance gene Lr34 i nto 'Thatcher' is known to enhance stem rust resistance. The effect of this gene in a 'Canthatch' background and its relationship with the 7DL suppres sor were determined by replacing chromosome 7D of 'Canthatch' with 7D of 'C hinese Spring', which possesses Lr34 on the short arm. 'Canthatch' nullisom ic 7D was crossed with 'Chinese Spring', followed by a succession of backcr osses to the nullisomic recurrent parent. Homozygous resistant disomic and monosomic substitution lines were recovered that exhibited the same resista nt reaction as that of 'Thatcher possessing Lr34 and as that of 'Canthatch' nullisomic 7D, in which the 7DL suppressor is absent. The results indicate that, in 'Canthatch', Lr34 permits expression of resistance genes normally inhibited by the 7DL suppressor. Furthermore, it is likely that, in 'Thatc her' and 'Thatcher' backcross derivatives, Lr34 inactivates the 7DL suppres sor.