Jan. 20, 1919 New Biologic Forms of Puccinia Graminis 105 



tion on the barberry may be worth investigating. Of course mutation 

 and adaptation must be considered also in any attempt to explain how 

 so many forms originated. 



The fact that the same variety of wheat may be immune in one 

 locality and susceptible in another is clearly explained. Formerly 

 recourse was taken to the theory that the environmental conditions 

 changed the physiologic processes and materials of wheat varieties so 

 fundamentally that the resistance of the plants broke down. The real 

 explanation of this phenomenon however is the fact that there are 

 many biologic forms of the rust fungus. This has actually been demon- 

 strated in field experiments. There is also preliminary evidence to 

 show that the same thing may be true of Pvccinia triticina on wheat. 



Methods for breeding for rust resistance must be changed funda- 

 mentally — if indeed it is worth while to do such work at all until more 

 is known about the specialization of the rust fungus. The breeder must 

 know and work with those forms of rust which occur in the region for 

 which his new variety is intended; and even then breeding must be 

 very largely a regional or even a local problem. For instance, in the 

 breeding plots at the Minnesota Agricultural Experiment Station cer- 

 tain varieties were practically immune to stemrust, but rust forms 

 have been found within 50 miles of the plots which can attack these 

 varieties so heavily as to make them worthless for rust resistance. 



The discovery of so man)'' forms of stemrust on wheat complicates 

 the rust problem seriously. Extensive experiments are under way to 

 determine the number, characteristics, and distribution of biologic 

 forms as well as their constancy and probable origin. 



