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 larvae 

 devour living plants as well as beetles and other insects that obtain their 



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