CHAPTER XV 



PLANT DISEASES 



Interference with plant growth. The influence of a 

 fertile, carefully prepared and cultivated soil and of high- 

 yielding plants in crop production has been considered some- 

 what in detail. But even when these important and necessary 

 provisions for producing crops are made, production will not 

 necessarily come up to its possibilities, owing to the inter- 

 ference of harmful agencies that are always operating to a 

 certain extent. 



Such agencies are plant diseases, weeds and insects. While 

 the losses occasioned by them cannot be prevented entirely, 

 they may be controlled in some measure. Such control must 

 be directed by an understanding of the nature of the injuries 

 and of the agent concerned. In order to present a basis for 

 such an understanding, a chapter will be devoted to the 

 discussion of some of the most important facts relating to 

 each of these agencies. 



What is meant by plant diseases. Our association with 

 plants is largely with those that bear flowers and produce 

 seeds. But there are many thousand kinds of other plants, 

 sometimes referred to as lower plants, which we do not easily 

 notice. Of the lower plants, those of one group, known as 

 fungi, are distinguished by the absence of chlorophyll, or 

 leaf green, and consequently are unable to make carbo- 

 hydrates, such as sugar and starch. For this reason the 



161 



