STUDIES ON VARIATION IN PATHOGENICITY IN LEAF RUST OF WHEAT, PUCCINIA TRITICINA ERIKSS.

In experiments designed to clarify the causes of the pathogenic changes that have recently been encountered in leaf rust of wheat, two factors were given consideration, (1) the possibility that nuclear exchanges might occur between the mycelia of different races in the uredial stage, and (2) the possible role of Thalictrum species in originating new rust strains. Infection studies with mixtures of races 9 and 15 and mixtures of races 5 and 9 did not produce any evidence that these races could interchange nuclei and thus initiate new pathogenic strains of leaf rust.In greenhouse infection tests with native Thalictrum spp., aecia were produced on T. dasycarpum, pycnia only on T. dioicum and T. polygamum, while no infection of T. venulosum took place. In similar tests with the introduced species T. glaucum and T. dipterocarpum, abundant production of aecia occurred on both species. In out-of-doors inoculation tests with T. dasycarpum and T. venulosum, no infection of these two species took place, whereas, under the same conditions, heavy aecial production occurred on the introduced species T. glaucum.Selfing studies in which Thalictrum glaucum was infected with known physiologic races have indicated that some races of leaf rust are homozygous, and others heterozygous for pathogenic characters. A culture of race 5 appeared to be homozygous, whereas a culture of race 3 was heterozygous, giving rise to races 3, 15, 32, 68, and three undescribed races. A culture of race 76 was heterozygous for both pathogenicity and urediospore color. Aeciospores of this race produced uredia of two different shades of yellow in addition to uredia of normal color. Aeciospores derived from teliospores collected in the field also gave rise to uredial strains of yellow spore color. Most of the yellow rust strains were decidedly low in pathogenic vigor.