highly resistant to existing races of crown 
rust in North America. 
Prevalent races of stripe (P. strii- 
formis) in both Europe and the Americas have 
changed repeatedly to render new resistant 
wheat and barley varieties susceptible. 
Thus far, all wheat varieties bred in the 
United States for seedling resistance to 
powdery mildew (Erysiphe graminis f. sp. 
tritici) have ultimately been attacked by races 
of that parasite. However, some wheats sus- 
ceptible in the seedling stage have shown 
different levels of mature-plant resistance in 
the field. Such resistances have remained at a 
useful or even high level over a period of 
years, as in the variety Knox, distributed in 
1953, which hopefully may continue to provide 
resistance against all races of the pathogen, 
The history of gains and losses in resistance 
to wheat bunt in the Pacific Northwest (16) 
closely parallels that for stem rust of wheat. 
The varieties Ridit, Albit, Rex, and Hymar, 
each resistant to certain races, have served 
to screen out and multiply races to which they 
are susceptible. Thus, when the excellent 
variety Elmar became predominant, races that 
could attack its parent Hymar were ready and 
caused devastating epidemics. Now the 
varieties Omar and Gaines have replaced 
Elmar. Thus far, they have been resistant in 
the field, but race T-18, which can attack 
Omar, is known and possibly is waiting to make 
a dramatic appearance. 
These are a few cases out of many selected 
from the area of cereal disease resistance, 
where resistance has been nullified by the 
appearance of new races. In a large measure 
these "lost'’ resistances are characterized by 
monogenic genetic control and often but not 
always by localized death of host tissue as a 
reaction to the invading pathogen. This reaction 
has been called hypersensitivity by Stakman 
(30) and the resulting killed area a "fleck." 
This visible fleck or internal necrosis com- 
monly has been associated with an ephemeral 
resistance, soon to be lost to new races. Such 
resistance has been termed racial resistance 
by Niederhauser et al. (19). 
Contrasted with these resistances, that are 
soon overcome by new pathogenic races, are 
"generalized" types of resistances, which may 
not be overcome by new races. The classical 
example of the contrasting values of these two 


121 
resistance types is found in the potato, which 
possess both types of resistance to late blight. 
The "R" genes, derived from Solanum demis- 
sum, condition hypersensitivity resistance to 
Phytophthora infestans. Such resistance alone, 
however, is of little value, as in the Kennebec 
variety introduced in 1948 and severely 
attacked by late blight in Maine in 1954, 
Niederhauser et al, (19) described the ineffec- 
tiveness of the ''R'' gene resistance in Mexico, 
where varieties with the ''R'' resistance genes 
in different combinations were killed by spe- 
cialized races of the pathogen. However, 
several Solanum demissum clones, possessing 
"partial" or ''field'' resistance, remained green 
and viable and retained their resistance in 
succeeding years although suffering a light 
attack, Likewise the European varieties Alpha, 
Gineke, and Voran also showed field resistance, 
These could be successfully grown under severe 
blight attack in the damp summer seasons at 
Toluca, Mexico, with moderate spray pro- 
grams. 
VERTICAL VERSUS HORIZONTAL 
RESISTANCE 
The persistent resistances having a gen- 
eralized effectiveness against all races of a 
pathogen have been designated by a number of 
names, none of whichis adequately meaningful. 
Those used most commonly include field 
resistance, polygenic resistance, partial re- 
sistance, generalized resistance, and tol- 
erance. The last term implying endurance of 
the pathogen rather than resistance to it is 
quite misleading. Recently van der Plank (34) 
has proposed the useful terms ''vertical re- 
sistance,'' to include those types effective 
against only certain races of a pathogen, and 
"horizontal resistance, '' whichis equally effec- 
tive against all races of a pathogen. 
Vertical resistance is typified by the "R" 
resistances to potato late blight and the 
ephemeral, hypersensitivity resistances to 
the rust fungi. It possesses the virtues of 
high-level resistance, usually simple 
inheritance, and ease and economy of 
recognition in parents and _ hybrid 
progenies in breeding programs. Its 
serious limitations are its instability and 
nullification in the presence of new races of a 
