REED: PHYTOPATHOLOGY 165 



1840's and ever since efforts have been continued to secure resistant varieties. 

 In a few cases promising results were secured, especially when a new breed- 

 ing stock was obtained from Mexico or Peru. Since 1918, Reddick in the United 

 States, Salaman in England, IVIiiller and Schick in Germany, and workers in 

 Russia, have developed blight-resistant potato breeding programs. However, 

 the discovery of specialized races of the pathogen in 1933 by Miiller and by 

 Miss O'Conner and Peterson (1933) have made the program more difficult. 

 Another example of the difficulties in the successful development of resistant 

 varieties is found in breeding oats for smut resistance. The variety Victoria was 

 imported from Uruguay by the United States Department of Agriculture in 

 1927. After its introduction it proved to be resistant to all races of loose and 

 covered smut known at that time. It was crossed with other varieties and by 

 1940 many valuable selections had been obtained which combined smut resis- 

 tance with other desirable qualities. The discovery of a new race of smut in 

 1941, which attacks Victoria and most of the selections derived from its crosses, 

 necessitates a new breeding program. 



The genetics of disease resistance has been investigated by many workers. 

 Biffen (1904) early published data on the yellow rust of wheat, Puccinia 

 glumarmn, which indicated that the inheritance of resistance followed the Men- 

 delian laws. The rusts have been suitable for such studies, since the results from 

 an experiment may be secured in seven to ten days. However, environal factors 

 must be carefully considered. Many hybrids have been studied by Hayes et al. 

 ( 1920) , Harrington and Aamodt ( 1923) , Clark and Ausemus ( 1928) , Goulden 

 et al. (1928), McFadden (1930), as well as other investigators. Sometimes 

 the results have indicated a simple relation, while in others the genetic situation 

 is quite complex. 



The smuts of cereals have been favorable subjects for the study of the inheri- 

 tance of disease resistance. One of the difficulties, however, is the long period 

 of time required for securing the data, and another is the great importance of 

 the control of environal factors at the time of infection. Gaines (1923) obtained 

 a complicated situation in his studies of the genetics of bunt resistance. Briggs 

 (1926 and after) secured quite clear-cut results which usually indicated mono- 

 hybrid ratios. He reported, however, the occurence of several factors for resis- 

 tance found in different varieties. Crosses between resistant and susceptible 

 varieties of oats have been studied with reference to their resistance to loose 

 and covered smuts, beginning with Wakabayashi (1921), Gaines (1925), and 

 Reed (1925). Many different hybrids have been studied by workers, and the 

 results sometimes indicate clearly a single factor difference, while in other 

 crosses two, three, or even more factors are required to explain the data. Mains 

 (1934) studied the resistance to powdery mildew of wheat hybrids and Briggs 

 (1935 and later) carried out a series of experiments with different hybrids of 



