412 GENETICS IN RELATION TO AGRICULTURE 
that resistance to the yellow rust, Puccinia glumarum Eriks. & Henn., © 
in his cross between Rivet, a slightly susceptible wheat and Red King, 
avery susceptible variety, was recessive in the F; generation but appeared 
in approximately one-fourth of his Ff, population. Tests of later genera- 
tions proved that this character bred true. Eriksson tested Biffin’s 
work and found only slight variations in the 7, 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, Tritzcum vulgare 
var. fuliguosum Al., which alone out of 540 varieties was immune to 
mildew, Hristphe graminis DC., but which was susceptible to brown rust, 
Puccinia triticina Eriks., with other varieties of common bread wheats 
and secured Fy 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 (Puccinia graminis 
var. tritict) 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. tritici, which is supposedly 
confined to wheat, attacking barley and rye as well. The same results 
were obtained with oat stem rust, P. graminis var. avene, 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 
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