T. Johnson

Specialization, hybridization, and mutation in the cereal rusts

ConclusionThe 50 years that have elapsed since Eriksson’s discovery of physiologic specialization in the cereal rusts have seen notable advances in man’s understanding of these important pathogens. The discovery that theformae speciales or varieties, which were at first regarded as the ultimate units of specialization, were in turn made up of many specialized strains or physiologic races gave a great impetus to the study of the various rusts and, despite the fact that the existence of these physiologic races complicated the problem of developing rust resistant varieties of cereals, served also as a stimulus to greater efforts on the part of the breeders of rust resistant cereal varieties. The discovery of the existence of heterothallism in the rusts made possible studies on the crossing and selfing of the physiologic races of cereal rusts. Such studies have indicated that a great variety of physiologic races may be expected to arise by hybridization and have permitted a much more accurate evaluation than otherwise would have been possible of the significance of the alternate hosts in the production of physiologic races. These studies have also thrown considerable light on the genetics of the rust fungi and have shown that the inheritance of such rust characteristics as pathogenicity and spore color is subject to the same laws that have been found to govern inheritance in higher plants and animals.The writers are indebted to Dr. J. H. Craigie and Mr. I. L. Conners for reading and criticizing the manuscript.