412 GENETICS IN RELATION TO AGRICULTURE 



that resistance to the yellow rust, Pucdnia glumarum Eriks. & Henn., 

 in his cross between Rivet, a slightly susceptible wheat and Red King, 

 a very susceptible variety, was recessive in the F\ generation but appeared 

 in approximately one-fourth of his F% population. Tests of later genera- 

 tions proved that this character bred true. Eriksson tested Biffin's 

 work and found only slight variations in the F 2 ratio and in the intensity 

 of the resistance. However, it appears that resistance of the wheat 

 plant to other species of rust fungi may be inherited as a dominant char- 

 acter. Vavilov reports that he crossed Persian wheat, Triticum vulgar e 

 var. fuliguosum Al., which alone out of 540 varieties was immune to 

 mildew, Erisiphe graminis DC., but which was susceptible to brown rust, 

 Pucdnia tritidna Eriks., with other varieties of common bread wheats 

 and secured FI hybrids which were immune to both diseases. Thus it 

 is clear that the inheritance of rust resistance is dependent upon the 

 specific relation existing between the parasite and the host. 



The practical aspects of breeding rust-resistant cereals is greatly 

 complicated by the fact that resistance in a single variety of wheat, for 

 example, is likely to vary geographically. While this is due in part to the 

 responsiveness of the wheat plant to radical changes in environment, it 

 is probably more often due to physiological variations in the rust fungi. 

 The virility of a given parasite appears to vary not only with the host 

 but with the geographical location. A striking example of this was 

 observed by Mackie in the behavior of Kubanka, a durum wheat of 

 Russian origin. Although this wheat is remarkably rust resistant in the 

 northern Great Plains region, yet when grown on the west coast of 

 Mexico it succumbed completely to the stem rust (Pucdnia graminis 

 var. tritid) which it had resisted successfully in the Dakotas. The ex- 

 planation of this failure of a supposedly resistant wheat is found in the 

 existence of local physiological races of the species P. graminis. Thus 

 Freeman and Johnson found P. graminis var. tritid, which is supposedly 

 confined to wheat, attacking barley and rye as well. The same results 

 were obtained with oat stem rust, P. graminis var. avence, which, readily 

 attacked barley but was less virulent on wheat and rye. The stem rust of 

 barley was found to be most readily transferred to the other cereals. 

 In addition to the barberry numerous wild grasses serve as hosts of 

 the stem rusts which fact still further complicates the problem of breeding 

 for rust resistance. Starkman and Piemeisel have investigated the rusts 

 of about 35 species of grasses and have found six distinct biologic forms 

 of this species of rust, one of which came from an isolated area. Among 

 other important discoveries, they found that more than one biologic 

 form may occur on the same host in nature, sometimes even on the same 

 plant; that these biologic forms can be distinguished from each other 

 morphologically as well as parasitically; that different strains of the same 



