Suppression of stem rust resistance in the hexaploid wheat cv. Canthatch by chromosome 7DL

The nullisomic for chromosome 7D (2n = 40) of the hexaploid wheat cv. Canthatch (Triticum aestivum L., 2n = 42) was found to be resistant in both the seedling and adult-plant stages to several races of stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. and E. Henn) to which normal disomic Canthatch (2n = 42) was susceptible. Since ditelosomic 7DL (2n = 40 + tt) reacted in the same manner as Canthatch, it appears that a gene(s) is present on the long arm of chromosome 7D that suppresses resistance determined by other genes, probably on chromosomes of the A and B genomes. Evidence indicated that the suppressor was derived from the ancestral, diploid progenitor of common wheat, Aegilops squarrosa L., rather than by mutation at the hexaploid level. The results provide a genetic explanation for apparent failures of some attempts to transfer stem rust and leaf rust resistance to hexaploid wheat from diploid and tetraploid relatives.