CAUSES OF PLANT DISEASES 71 



the cucurbits ; B. carotovorus, Jones, of the carrots ; 

 Pseudomonas campestris (Pammel) Smith, of the 

 cabbage, etc. (page 233) ; B. phaseoli, Smith, of the 

 beans (page 227) ; B. vascularum, of the sugar-cane ; 

 and B. pliytophthorus, Appel, of potato. 



In other cases galls or excrescences are formed on 

 various parts of the plants, and either seriously injure 

 or possibly destroy the plant, as in the case of the 

 crown gall disease of many plants. 



Bacteria are also frequently associated with other 

 organisms of diseases and decay, and materially aid 

 them in their progress. Many obscure diseases are 

 doubtless due to these minute organisms. 



There are many other plants which have this habit 

 of attaching themselves to the roots of plants, and thus 

 living partially or entirely as parasites. Some twining 

 plants assume such huge proportions as to completely 

 destroy the plants which serve them for support. 

 Seeds of some plants are sometimes caught on other 

 plants, produce roots which extend to the ground and 

 eventually attain greater vigour than the support, as in 

 the case of the palm and the ficus. 



Insects may also be the cause of diseases, either 

 directly or indirectly, and although the discussion of 

 this subject comes more strictly within the province of 

 the entomologist, there are some which, from the 

 character of the injuries which they cause, may well be 

 considered at this time. These insects are the causes 

 of abnormal plant growths known as galls or cecidia, 

 some of which are very complicated in both form and 

 structure, but very few are of any great economic 

 importance. Probably the most injurious to economic 

 plants are the mites (belonging to the genus Eriophyes 1 ). 

 The mites are not true insects, but are very minute 

 organisms, closely related to the spiders. The diseases 

 which they cause are referred to as phytoptosis, rust 

 and witches' brooms. The phytoptosis of the tomato 

 is an enlargement of the stems, which are also covered 



1 Syn. Phi/toplus. 



