272 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



The recent work of Reed (1940) serves to emphasize the im- 

 portance of physiologic races in the breeding of oats resistant to 

 smuts. He differentiated 29 races of Ustilago avenae and 14 of 

 U. levis by their pathogenic behavior on strains and varieties of 

 9 species of Avena. Avena bar bat a was susceptible to all races of 

 smuts. The variety Canadian was susceptible to all physiologic 

 races of these smuts except one of each. The varieties Markton, 

 Victoria, and Navarro proved to be highly resistant to many races 

 of both loose and covered smuts. 



Literature on plant pathology contains many accounts of varie- 

 ties of crop plants that are resistant to a specific pathogen when 

 grown in one region but are susceptible when grown in another 

 region. Of course this apparent breakdown of resistance cannot 

 be attributed to one cause in every case, but the existence of dif- 

 ferent specialized races in different regions is no doubt frequently 

 the primary cause. For example, it is a common observation that 

 durum wheats in the United States are more resistant to stinking 

 smut than are vulgare wheats. The opposite situation has been 

 observed in Palestine. Abundant evidence is now at hand that 

 these conflicting observations can be explained by the existence 

 of different physiologic races of Tilletia tvitici and T. levis in these 

 two regions. It is to be expected that hybridization is less im- 

 portant in breeding crops resistant to smuts if the smuts belong 

 to the group in which the promycelium or its branches directly 

 penetrate the host tissues. Even in these species fusions between 

 different promvcelia could occur, and new races could be formed. 



From the numerous examples of interracial and interspecific 

 hybridization and of variation by sectoring that have been oh- 

 served, it is apparent, as has been indicated, that new forms are 

 continuously being produced in nature. The plant pathologist 

 must therefore first know the pathogen thoroughly, if the breed- 

 ing or selecting ot resistant host varieties is to be successful. 



LITERATURE CITED 



A \.\iodt, O. S., "Varietal trials, physiologic specialization, and breeding 

 spring wheats for resistance to Tilletia tvitici and T. levis" Can. J. Re- 

 search, 5: 501-528, 1931. 



Allison, C. C, and K. [senbeck, "Biologisches Spccialisicrung von Puccinia 

 glumarum tritici Eriksson and Henning," Phytopath. Z., 2:87-98, 1930. 



