CHAPTER XXIV 
BREEDING DISEASE-RESISTANT PLANTS 
The term, plant disease, has been restricted by some authors to those 
disorders and abnormalities caused by fungous parasites only. Other 
authors have employed the term in a more general sense, including there- 
under all abnormal conditions of structure and function which are caused 
by the different elements of the environment. We shall use the term in 
this more general sense and for the purpose of this discussion it may be 
defined as follows. Plant diseases include all the ailments and injuries 
which can be traced to specific causes or agencies as well as certain func- 
tional disorders the causes of which are obscure or difficult of analysis. 
In order to discuss profitably the breeding of disease-resistant plants it 
is necessary to consider more fully the various categories of causes. 
The Causes of Plant Diseases.—In general the diseases of plants are 
caused either by unfavorable conditions among the inanimate elements 
of the environment or by the invasions of other organisms. While every 
case of disease must be considered as the result of interrelated causal 
agencies, yet it is usually possible to discover specific agents that are 
primarily responsible for the pathological condition. It is then possible 
to determine the nature of disease resistance in particular instances with 
more or less definiteness according to the nature of the specific causes. 
The most important non-living elements of the environment affecting 
the health and vigor of cultivated plants are the soil, the water supply and 
the temperature and humidity of the atmosphere. These environmental 
factors influence plant development in so many ways that the oppor- 
tunities for maladjustment between plants and their environment are 
practically endless. Such conditions as excess of alkali or lack of suffi- 
cient moisture in the soil or the combination of excessively high temper- 
ature and low relative humidity are typical and important illustrations 
of specific environmental conditions which induce disease in plants. 
The living organisms of chief importance in causing plant diseases are 
insects, fungi and bacteria. Injurious insects may be roughly classified 
according to their ways of feeding under two heads, viz., sucking and 
biting insects. The first class includes the plant lice, phylloxerans and 
scale insects which obtain their nourishment by sucking it from the living 
plant. The second class includes all moths and butterflies whose larve 
devour living plants as well as beetles and other insects that obtain their 
400 
Digitized by Microsoft® 
