Suppressors of genes for stem rust (Puccinia graminis f. sp. tritici Eriks. and E. Henn.) resistance in wheat (Triticum aestivum L. and T. turgidum L.) have occasionally been reported. In my last three published papers, I have reported the occurrenceof supressors in Thatcher spring wheat. Triumph 64 winter wheat and Medea durum wheat. To obtain a better idea of the frequency of suppressors, seven durum wheat cultivars susceptible to race TMHT (15B-1) were intercrossed in all combinations but one. The Fl plants were all susceptible to TMHT. Eleven of the 20 crosses segregated for resistance in the F2 and resistance was confirmed in the F3 generation. The same seven durum wheat cultivars were crossed with a susceptible common wheat and all seven crosses segregated for resistance in the F2 generation. A similar test was run with seven susceptible common wheat cultivars. Again, all of the Fl plants from the 21 crosses were susceptible to race TMHT. Five of the six crosses involving Gabo plus one other cross segregated for resistance in the F2 generation. The results indicate that suppressors of genes for stem rust resistance are much more common than expected. This raises questions about their selective advantage in the evolution of wheat.
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