CHAPTER XXIV 



PLANT DISEASES 



378. Disease-producing Fungi. — From previous chap- 

 ters a knowledge has been gained of the importance of bac- 

 teria, rusts, and mushrooms, and of the damage that some 

 of the members of these groups of fungi do by producing 

 diseased conditions in other plants. There are, however, 

 many fungi beside those that we have specially studied which 

 are responsible for diseases that, in the case of cultivated 

 plants, result in losses amounting to many millions of dol- 

 lars each year. Every plant of economic importance is 

 attacked by one or more parasitic fungi, and a simple enu- 

 meration of the diseases that they cause would occupy more 

 space than can be given to this chapter. So only a com- 

 paratively few diseases, which are well known and are of 

 great practical importance, will be briefly described. The 

 fungi that produce plant diseases belong to the following 

 groups : 



a. Bacteria. These were discussed in Chapter II. Some 

 of the diseases that they cause are described in §§ 379 

 and 380 below. 



b. Slime Molds. Each of these very simple plants con- 

 sists of a single cell which is sometimes quite large, 

 containing many nuclei but possessing no cell wall. 

 When it is mature, the whole cell divides into many 

 small spores. Each spore may become a free-swim- 

 ming cell which later develops into a mature plant. 

 (See § 381.) 



373 



