THE EFFECTS OF GENOTYPE AND TEMPERATURE ON THE RESISTANCE TO PUCCINIA GRAMINIS TRITICI CONTROLLED BY THE GENE Sr6 IN TRITICUM AESTIVUM

A line of wheat (Triticum aestivum L.). Kenya 58/10* Marquis (K58Mq10), homozygous for gene Sr6 for resistance to stem rust (Puccinia graminis tritici Eriks. and Henn.), was crossed with four rust susceptible cultivars or lines. The F1 seedlings were tested with rust races 56 and 15B-1, twice under greenhouse conditions and three times in growth chambers, and F2 seedlings were tested twice in the greenhouse. In conditions that resulted in maximum resistance, Sr6 was dominant with both race 56 and race 15B-1 in two crosses, dominant with race 56 and recessive with race 15B-1 in one cross, and recessive with both races in the fourth cross. Thus, the dominance of Sr6 depended on the susceptible parent with which K58Mq10 had been crossed. The data indicated that the dominance of Sr6 was not controlled either by a single independent modifier locus or by the general genetic background of the susceptible parents. Although the possibility of a linked modifier has not been eliminated, it appears that the results may be due to the effects of different alleles for susceptibility at the Sr6 locus. The degree of resistance conditioned by Sr6 is determined by a complex interaction between genotype and environment.