Natural selection has been an important 
factor in the evolution of all economic species, 
both before and after domestication. Major 
among the selective pressures that were 
associated with survival in the wild were 
ravages by pests. Characters associated with 
survival are not necessarily those that makea 
domesticated species useful to man; neverthe- 
less, many characters that contribute to sur- 
vival in wild species also are associated with 
quality and quantity of productivity of these 
same species when domesticated. Such is the 
case with genetic resistance to pests. 
Pest control by genetic means is undoubtedly 
one of the oldest forms of biological control 
that man has pursued intensively in his attempts 
to tilt the ''balance of nature"! in his favor. As 
a consequence of the famine in Ireland in the 
middle of the 19thcentury, varieties of potatoes 
resistant to late blight were developed prior to 
1900. Resistance to insects was recorded as 
early as 1860, with the discovery that American 
rootstocks of grape were resistant to 
phylloxera. One of the earliest recorded in- 
stances of development of varieties resistant 
to insects, however, occurred with the breeding 
of wheat varieties resistant to the Hessian fly 
in the early 1920's. Barley varieties resistant 
to the cereal cyst nematode were developed in 
Europe also in the early 1920's, 
Although differences in animal resistance to 
pests, such as resistance of zebu cattle to the 
Texas fever tick, have been reported, the cost 
of exposing large numbers of animals has pre- 
cluded the possibility of active breeding pro- 
grams to exploit such resistance, 
Breeding of genetic resistance in domestic 
species is one of the most economical methods 
of combating damage and destruction by pests. 
In most cases, this type of control is not 
permanent. The ability of parasitic organisms 
to adapt themselves through natural selection 
to previously resistant hosts necessitates 
constant alertness for new attacks and requires 
continuing research programs to maintain 
resistance. 
ADVANCES AND CHALLENGES IN THE CONTROL OF PLANT 
DISEASES THROUGH BREEDING 
Ralph M. Caldwell, Department of Botany and Plant Pathology, 
Purdue University, Lafayette, Ind. 
The development of crop varieties that resist, 
tolerate, or escape the attack of pathogenic 
organisms or viruses has been a prominent 
phase of scientific agricultural development. 
A multitude of such accomplishments, old and 
new, have been built into the structure of 
modern agricultural production. Many of these 
successes are time tested and solid and en- 
courage effort to extend such gains to all dis- 
eases that threaten the plants we value or 
must produce. Some advances have been of 
ephemeral value and require costly repetition 
of effort, because we have not possessed the 
requisite knowledge or wisdom to do a more 
permanent, better job. It will be possible to 
review only a few of the problems and 
accomplishments that mark the path of progress 
toward plant disease control through breeding. 
My field of first-hand knowledge and experi- 
ence is that of disease resistance of the field 
al7/ 
crops. For convenience and security, and not 
because I consider it unique or requiring 
special attention, I will draw heavily on this 
area to develop certain ideas on disease re- 
sistance. 
TIME-TESTED VICTORIES OVER 
PLANT DISEASES 
The breeding of disease-resistant crop plants 
has been going on for a long time. Even in 
prehistoric times, one really smashing success 
was achieved. This was the development of 
maize essentially resistant to its leaf rust 
disease caused by Puccinia sorghi. This dis- 
ease has been well under control in field corn 
during the entire history of its culture in the 
Corn Belt of the United States. There is no 
record of sweeping epidemics of this rust, only 
light infections that have been common and 
