Feb. io, 1923 
Genetics of Bunt Resistance in Wheat 
45i 
3. Flax resistant to wilt. (Bolley.) 
4. Cantaloup resistant to leafblight. (Blinn.) 
5. Clover resistant to anthracnose. (Bain and Essary.) 
6. Durum wheat in the Dakotas (Carleton) and Kanred wheat in 
Kansas (Roberts) resistant to rust. 
7. Tobacco resistant to rootrot. (Johnson and Milton.) 
8. Grape resistant to Phylloxera. (Viola and Ravaz.) 
A genetic analysis of the resistance of these crops after crossing them 
with susceptible races would be required to show the type of inheritance 
in each case. Such investigation would be more concise and the inter¬ 
pretation clearer if a standard method of inoculation were adopted and 
a quantitative rather than a qualitative measure of resistance were used, 
as is being done in rust resistant work with cereals ( 38 ). The analysis of 
the inheritance of rootrot in tobacco (21) should prove especially in¬ 
teresting because of the ease with which crosses are made and also on 
account of the amount of genetic work already done on tobacco. It is 
difficult to interpret much of the past work because inoculation was left 
to chance or natural agencies, which allowed part of the individuals to 
escape infection and the resulting segregates were arbitrarily classified 
as resistant or susceptible, when as a matter of fact every graduation 
from complete susceptibility to complete immunity occurred. In some 
cases (55) different stages in the life history of the organism apparently 
do not have the same infective power. 
Whether resistance of a given host against a given parasite will be 
maintained indefinitely in a given environment remains for the future to 
decide. Resistance in certain crops (46) may be only temporary. The 
resistance of the host might be apparently lost through the introduction of 
a new physiological race of the parasite to which it was susceptible. An 
example of such an occurrence is recorded by Devine and Stakman (25). 
They obtained a strain of stemrust from Oklahoma that could attack 
Kanred wheat although Kanred continued to be very resistant to the 
two common races of stemrust in Kansas. According to Evans (11) 
Bobs Rust Proof wheat was resistant to stemrust at Pretoria, South 
Africa, but was badly attacked in the Low Country of the Transvaal. 
This may have been due to the change in environment or to the presence 
of a different biologic form of the fungus encountered in the Low Country. 
Evans also reported an extraordinary case in which the pathogenic 
properties of stemrust on wheat were increased nearly five fold by passing 
one generation of its existence on the F t generation of a cross between a 
resistant and a susceptible variety (Bobs X Wol Koran). Bobs normally 
produced one-fifth as many pustules as Wol Koran, but when infected 
with spores from the F t generation it produced one-third as many. In 
this case the F 1 generation had apparently acted as a “bridge” to per¬ 
manently increase the virulency of the rust and had also decreased the 
difference between the resistant and susceptible varieties. If this 
should prove to be a general rule, breeding for resistance would not only 
be without avail but would actually be a very dangerous practice on 
account of the danger of developing and spreading super-virulent cultures 
of the parasite. It is to be regretted that Evans did not grow an F, 
generation of the host to see whether super-resistant segregates could be 
obtained. He worked with comparatively few plants in the greenhouse, 
and his results may have been due to some uncontrolled factor, for other 
