CHAPTER XXV 

 PLANT DISEASES 



373. How plant diseases affect crops. Farm crops are sub- 

 ject to diseases just as people are, and many diseases of plants 

 are infectious, just as are many of the diseases of animals. The 

 more generally a given crop is grown in a community, the more 

 widespread and numerous the diseases of the crop become. The 

 longer a crop is grown in a field without rotation, the greater 

 is the number of diseases affecting the crop. Diseases have 

 become so common among some of our important orchard 

 and garden crops that systematic spraying is necessary to hold 

 the diseases in check. The yearly loss in the United States 

 from potato blight alone is estimated at $36,000,000 ; from 

 grain smuts, more than $33,000,000; from grain rusts, nearly 

 $20,000,000; and from cotton wilt, no less than $10,000,000. 



374. Diseases due to parasitic plants. The most important 

 plant diseases are caused by plant parasites, and these are the 

 only ones which will be discussed at this time. Parasitic plants 

 do not take food materials directly from the soil and air but live 

 on other plants, and in so doing they often cause diseases of 

 the plants on which they live. There are parasitic flowering 

 plants, and parasitic bacteria and fungi. 



375. Parasitic flowering plants. The dodder and the mistle- 

 toe are well-known parasitic flowering plants. The dodder is a 

 yellowish, more or less twining plant which wraps itself around the 

 stems of clover, alfalfa, and other plants. By means of rootlike 



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