tubes that failed to penetrate frequently de- 
veloped deformed appressoria. Field tests 
indicated that leaf rust epidemics spread more 
slowly in plots of these varieties than in 
normally rusting varieties. These slow-rusting 
varieties appear to have at least one important 
host character for resistance that alone or 
collectively with other types may bring about 
an effective field resistance of the horizontal 
type. 
Hayden (11) found Marquis wheat to become 
15 to 20 times as severely infected with stem 
rust in the field as did the field-resistant 
variety Sentry when exposed to equal inocula- 
tion. This resistance of Sentry, in the hands of 
future breeders, may prove to be a valuable 
element of horizontal resistance. The Marquis 
wheat variety dominated the spring wheat 
region, where it was grown on nearly 12 million 
acres from 1914 to about 1930. During the 
1920's, my observations in South Dakota were 
that it was not severely attacked by leaf rust. 
However the variety Kota, introduced in the 
1920's, suffered the most serious leaf rust 
attack I have ever seen. Prior to those epi- 
demics, leaf rust in that area was not con- 
sidered to be a serious wheat disease and 
caused little concern. Marquis, however, is 
susceptible upon inoculation in the seedling 
stage. This record suggests that, when grown 
on vast and continuous areas, it may have 
possessed enough horizontal leaf rust re- 
sistance at maturity to prevent the development 
of serious epidemics. The varieties Kota, 
Ceres, and Thatcher, which were released to 
replace Marquis, were seriously attacked by 
leaf rust. Records in the U.S. (interstate) 
Uniform Spring Wheat Rust Nurseries show 
Marquis to have been consistently less severely 
attacked than Kota, Ceres, and Thatcher during 
the 1930's, even when surrounded by rust 
breeding-susceptible varieties. A study of the 
nature of leaf rust resistance in Marquis might 
well provide valuable guidance to resistance 
breeding. 
The persistence of the stem rust resistance 
of Selkirk spring wheat mentioned earlier is 
highly encouraging. This variety has not been 
severely attacked in the field for the past 11 
years despite the fact that race 15B3 and two 
biotypes of race 29 were found attacking it as 
far back as 1952-54. Such stem rust races 
124 
must have occurred repeatedly since 1952, but 
have not produced damaging epidemics. Selkirk 
possesses the Sr6 gene for vertical stem rust 
resistance and some or all of the resistance of 
its parent Redman. Redman and other 
derivatives of Hope and H-44, while susceptible 
to race 15B as seedlings, have not shown 
extremely high mature-plant susceptibility as 
have certain durum varieties. This field re- 
sistance was reported by Peturson (21) and 
Stakman and Christensen (31). It has been 
repeatedly observed on Hope derivatives in 
our nurseries inoculated with different clones 
of race 15B. It may be possible that Selkirk 
combines a seedling vertical resistance, com- 
bined with a mature-plant horizontal re- 
sistance, that suppresses the multiplication of 
the new races that can attack it in the seedling 
stage. If found to be true, this would be indeed 
fortunate and could point the way toward 
breeding far more stable stem rust resistance. 
Mention has been made of the successful use 
of a field resistance to control the northern 
leaf blight of corn. This resistance is ex-= 
pressed in restricted growth of the parasite 
in the host and smaller host lesions (14), and 
is polygenic in inheritance. These charac- 
teristics and the lack of precise host special- 
ization of the pathogen on the resistant corn 
inbreds (23) suggest strongly that this re- 
sistance may be of the stable horizontal type 
that may not be overcome by new races. 
One cannot at this time be so hopefulfor the 
monogenic resistance to this disease recently 
described in corn inbred G.E,. 440 and Lady- 
finger popcorn (12). The monogenic inheri- 
tence, and comparatively high resistance con- 
ferred are suggestive of the vertical type of 
resistance. However, this resistance has been 
enthusiastically received as offering an easier 
way to breed resistant corn, and is being 
widely used. 
A serious problem is posed concerning the 
method of using this resistance, which in some 
ways appears superior to those now in use. 
The problem is emphasized here because it is 
typical of many to be faced in the future. In 
view of the past uncertain and, in some cases, 
sad experiences in the use of monogenic 
vertical resistance to the rusts, smuts, potato 
late blight, and the powdery mildews, it would 
appear unwise to risk such an experience with 
