78 THE EXISTS OF GRAINS IN THE UNITED STATES. 



desirable varieties with some variety of known resistance, and select- 

 ing the resistant plants. 



In breeding for resistance to disease, the disease must be present 

 every year. Rust occurs in epidemical proportions only at irregular 

 intervals, and, therefore, in order to breed for resistance to rust 

 special breeding plats must be employed, where the disease can be 

 produced yearly by conditions particularly favoring its propagation. 

 Wherever efficient breeding of rust-resistant cereals is to be done, 

 such a breeding plat is absolutely necessary. 



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