INTERVARIETAL CROSSES IN PUCCINIA GRAMINIS

Abstract

Crosses made in all possible combinations between varieties Tritici, Secalis, Avenae, Agrostidis, and Poae of Puccinia graminis Pers. have shown the existence of considerable intersterility in crosses between some of the varieties. Fertility was relatively high in crosses between vars. Tritici and Secalis, Avenae and Agrostidis, Avenae and Poae, and Agrostidis and Poae. Fertility was much lower in crosses between vars. Tritici and Avenae, Secalis and Avenae, and Secalis and Poae. In crosses between certain pairs of varieties the degree of fertility varied according to the direction of the cross. Tritici × Agrostidis, Tritici × Avenae, Tritici × Poae, Secalis × Agrostidis, and Secalis × Poae crosses succeeded much better than the reciprocal crosses. Sporidia from the F₁ teliospores of Tritici × Secalis hybrids infected barberry with the production of normal pycnia and aecia. Sporidia of Tritici × Avenae and Secalis × Avenae hybrids produced, on barberry, infections with few pycnia and these excreted little nectar; no aecia were formed but urediospores and teliospores were occasionally produced in old infections. Generally, an F₁ hybrid rust possessed a wider host range than did either of the parent varieties; but the pathogenicity of the hybrid on a given host was less than that of the parent variety that attacked that host. In hybrids of varieties that differed markedly in urediospore size the urediospores of the F₁ hybrid rusts were intermediate in size between those of the parents. The pathogenic characteristics of the F₂ (uredial) generation were studied only in Tritici × Secalis hybrids. Variation in pathogenicity was rather restricted. The races isolated were moderately pathogenic to barley and weakly pathogenic to wheat and rye.