of Biffen of England showed that it was possible to breed by 

 crossing a strain of wheat resistant to Yellow Rust {Puccin- 

 ia glumarum), but all attempts to breed a strain resistant to 

 Black Stem Rust {P. grmninis) have met with disappoint- 

 ment. Recent researches go to show that this latter rust 

 exists in America as six or more distinct physiological races 

 or biologic forms and strains differing in their virulence, and 

 that the nature of rust-resistance is not yet thoroughly 

 understood. A variety of wheat resistant in one region to 

 rust is sometimes susceptible in ano.her region. 



The inheritance of rust-resistance depends, therefore, 

 upon the specific relation between host and fungus. Tl is 

 likely, therefore, that by selection, rather than breeding, 

 resistant strains of wheat will be found for limited areas, 

 which are capable of resisting the local biological strains of 

 rust. (Read E. C. Stakman's Article on Resistance to and 

 Biologic Forms of Black Stem Rust, in Journ. Agric. 

 Research, 1915 and 1917). 



In general, the resistance of plants to other fungous 

 diseases is specific, depending upon the strain of variety and 

 upon the physiological differences between strains of the 

 fungus. Orton has developed a watermelon {Cilrullus 

 vulgaris) that is resistant to the wilt disease {Fusarium 

 niveum), and a strain of cotton resistant to the cotton wilt 

 (F vasinjectum). Hansen has produced hybrid peal's that 

 are largely resistant to pear blight {Bacilhis amylovorus). 



Prof. R. H. Biffen's experiments on the breeding of a 

 strain of wheat that was at the same time hard, high- 

 yielding and resistant to rust {Puccinia glumarum) are in- 

 teresting and suggestive. 



First he determined that hardness was dominant to 

 softness by crossing Red Fife with Rough Chaff, an English 

 wheat. In the Fo generation he was able to obtain a form 

 that combined hardness with the high-yielding quality. 



Next, Biffen crossed Red Rivet an English wheat, very 

 resistant to rust, with Michigan Bronze, a strain of Ameri- 

 can wheat very susceptible to rust. The Fi generation 

 showed that susceptibility was dominant to immunity, 

 but in the F2 generation one fourth of the plants were 

 found to be immune. 



Similar results were obtained when he crossed Ameri- 

 can and Michigan Bronze. 



Biffen also crossed Squarehead's Master (rust suscept- 

 ible) with a Ghurka wheat (rust resistant) and obtained a 



160 



