CHAPTER XXXVIII 

 PLANT DISEASES AND PESTS 



256. The low forms of plant life are useful for decom- 

 posing dead plants and animals, but when they become 

 parasites on living plants and animals then they are 

 harmful. Many fungi and bacteria have become true 

 parasites and live on the higher plants cultivated by man. 

 The plant on which a parasite lives is called the host. The 

 most destructive parasitic plants are rusts, smuts, mil- 

 dews, and blight-producing fungi and bacteria. These 

 parasites attack cultivated grains, fruits, and vegetables, 

 and the damage done by them annually amounts to hun- 

 dreds of millions of dollars. 



257. Wheat Rust. Wheat rust is considered one of 

 the most destructive parasitic fungi. It passes part of 

 its life on the barberry and part on the wheat. It extracts 

 its food from the leaf of the wheat, which is soon killed, so 

 that no grain is produced. Damp, warm weather is favor- 

 able for its growth, and a crop of wheat may be killed in 

 a week. About the only remedy is to remove the bar- 

 berry. Other wheat rusts, not so destructive, live entirely 

 upon wheat. The remedy for these is not to sow wheat 

 on the same ground many years in succession. 



258. Apple Rust. The life history of the apple rust 

 or cedar rust is given in detail, in order to show how to 

 kill or control it, and also to serve as a type in under- 

 standing the nature of a double host parasite. 



