10 A STUDY IN CEREAL RUSTS 



P. graminis hordei (stem rust of barley) on barley, wheat, and 

 rye. 



P. graminis secalis (stem rust of rye) on rye and barley. 



P. graminis avenae (stem rust of oats) on oats. 



If these forms are merely adaptations it ought to be possible to 

 change their parasitic tendencies by restricting or changing their en- 

 vironment. It ought to be possible to break down the biologic forms 

 under conditions abnormal for host or parasite. Various methods 

 have been tried in attempting to break down the specialization of para- 

 sitism of different fungi. Salmon's work along this line on the Ery- 

 siphaceae has already been mentioned. Ray (1903) states that by 

 subjecting maize to ether vapor and then inoculating it with spores of 

 Ustilago zeae, the resulting infection was much more virulent than 

 that on plants not so treated. 



Comparatively little work of this nature has been done with Puc- 

 cinia graminis. The fact that physiological races of rusts behave dif- 

 ferently under different conditions has been known for some time. 

 Much of the information was, however, gathered from incidental 

 observations. In breeding wheats for the purpose of obtaining rust- 

 resistant forms it would be very helpful to be able to correlate certain 

 characters with rust resistance. For this reason this phase of the 

 question, from both practical and scientific points of view, is of much 

 importance. The same is true of physiological races. It is important 

 to know if they can be broken down by means of a high degree of soil- 

 fertilization, if they become generalized by growing on the alternate 

 host, and if they adapt themselves readily to new hosts. The present 

 investigation was therefore undertaken with the object of determining 

 the possibility of developing and breaking down physiological races 

 and of obtaining definite information concerning the factors influencing 

 varying resistance in immune or semi-immune varieties of wheat. 



EXPERIMENTAL 



METHODS 



The rusts used in making inoculations were obtained originally 

 from their respective hosts in the fields at University Farm, St. Paul, 

 Minnesota. They were then artificially transferred to plants growing 

 in the greenhouse. Transfers were made to new plants about once 

 every three weeks until the rust had been confined to its own host for 

 at least twelve successive transfer generations. In nearly all the exper- 

 iments with biologic forms the rust had been confined to its own host 

 for at least twenty generations, thus giving assurance that it was the 

 particular biologic form desired. 



The seeds of the host plants were planted in rich loam soil in four- 



