polygenic resistance cannot always be equated 
with horizontal resistance. However, the 
Chinese resistance of Knox and Vermillion has 
been fairly stable for a decade, and still gives 
protection from leaf rust in Knox, even whenit 
is surrounded by inoculum-producing sus- 
ceptible varieties. In uniform fields it may 
still give adequate protection from all current 
races. 
Shay et al. (29) question using ''monogenic" 
versus "multigenic'’ resistance to apple scab 
(Venturia inaequalis) as ''becoming academic." 
They grant that monogenic resistance is more 
highly subject to nullification by new races, 
but report that resistance breakdown may 
occur in either case. Here, however, multigenic 
resistance to apple scab is not necessarily to 
be equated with horizontal resistance. They 
could find no logical basis for selecting a 
resistance genotype with a guarantee of long 
effective life and suggest that the only safe- 
guards against loss of a variety from new 
races of V. inaequalis is the use of a many 
genes as possible from both the monogenic 
and multigenic systems. 
USE OF HORIZONTAL RESISTANCE 
Horizontal resistance, by definition as well 
as experience in some cases, is persistent or 
permanent and therefore is the preferred type 
if it is sufficiently effective. Horizontal re- 
sistance appears to result from host characters 
that limit the rate of infection reproduction 
and build-up of epidemics by the pathogenes 
rather than from host characters bringing 
about cellular, host-parasite interactions that 
are detrimental or lethal to pathogens and 
often recognized by host hypersensitivity. 
Thus, when for any reason, such as artificial 
epidemics, the pathogen becomes abundant, 
horizontal resistance appears at a dis- 
advantage, whereas a vertical resistance, 
such as hypersensitivity, may appear out- 
standing under such a test. Therefore, one 
must look further than an artificial test plot 
for the true value of horizontal resistance. 
Horizontal resistance, resulting from two or 
more specific host characters, may be ex- 
pected to be polygenically inherited, but not all 
polygenic resistance is of the horizontal type. 
123 
The simplest possible kind of horizontal 
resistance is possessed by some two-rowed 
barleys against loose smut (Ustilago nuda). To 
infect the barley embryos, the smut spores 
must first gain entrance to the young pistil of 
the flowers when they commonly open at 
pollination time. The flowers of some two-row 
barleys, such as the English Proctor variety, 
remain closed at pollination time (''close- 
flowering") and thereby exclude the smut 
spores. This character results in highly ade- 
quate resistance to all races of the smut 
fungus. A highly dependable, yet inexpensive 
method, had been devised by Pederson (20) to 
select the close-flowering types. 
In contrast to this example, horizontal re- 
sistance may result from the interaction of 
several host characters. Guzman (10) analyzed 
the nature of horizontal resistance to potato 
late blight referred to as "partial resistance." 
She found it to result from effects of the 
resistant host on the development of the 
pathogen in four ways: (1) Delayed host pene- 
tration, (2) reduced frequency of penetration, 
(3) delayed rate of host tissue invasion, and 
(4) reduced sporulation by the pathogen. These 
characters were measured effectively in the 
greenhouse to discriminate horizontally re- 
sistant from susceptible segregants in hybrid 
populations. Such tests were equally as effec- 
tive as much more costly field tests. 
The selection for leaf rust (P. recondita) 
resistance in our program of wheat breeding at 
Purdue has been largely with mature plants in 
the field under artificial epidemics of com- 
bined rust races. Under these conditions we 
have brought together mainly mature-plant 
resistances. The variety Dual, selected inthis 
manner, possesses a mature plant resistance 
superior to any of its parents (4). It is sus- 
ceptible to most races asa seedling. It appears 
likely that some elements of both horizontal 
and vertical resistance have been combined in 
this variety, which has maintained an effective 
level of field resistances during its breeding, 
selection, and commercial use from 1942 to 
the present. 
Romig (25) and Romig and Caldwell (26) 
reported that a number of leaf rust-susceptible 
but "'slow-rusting'"’ winter wheats displayed 
resistance to stomatal penetration of mature 
wheat leaf blades by the rust. The rust germ 
