CROWN GALL 



349 



attacked. The Bacteria pass the winter in the infected regions, 

 which are sources of further infection. When growth begins in 

 the spring, a gummy substance carrying the Bacteria exudes 

 from these dead portions, and insects visiting the exudations 

 carry the disease to other trees. How would you combat Pear 

 Bhght? 



Crown Gall. — This disease is common on fruit trees, and 

 occurs on Roses, Blackberries, Alfalfa, and a number of other 

 plants. The presence of the Bac- 

 teria causes an abnormal develop- 

 ment of the infected tissues, resulting 

 in the formation of cancer-like swell- 

 ings. The disease may occur on 

 any portion of the plant but is com- 

 mon on the roots or on the stem 

 near the surface of the ground. In 

 general, nursery stock is more readily 

 affected than older trees. Plants 

 affected are much dwarfed or killed 

 (Fig. 303). 



Beans, Tomatoes, Potatoes, Sugar 

 Cane, Cotton, and most of our eco- 

 nomic plants have some form of 

 bacterial disease, but a further study 

 of the disease-producing forms must 

 be left to courses in Bacteriology 

 and Pathology. 



In summarizing, the following fea- 

 tures should be noted. Bacteria are 

 the smallest of plants, and their 

 plant body consists of a single cell 

 with protoplast poorly organized. 

 The plant body may be globular 

 or rod-shaped, either straight or curved. Some have cilia or 

 flagella and are therefore motile. They are remarkably re- 

 sistant, especially in the spore stage. With the exception of a 

 few forms, they are saprophytes or parasites. The disease- 

 producing forms are very destructive to both animals and plants. 

 They reproduce by rapid cell division and are spread, partly by 

 wind, partly by water currents, partly by their own locomotion, 



Fig. 303. — Crown Gall on 

 the Cherry tree. The cancer- 

 like swellings are due to the 

 presence of Bacteria. After 

 Bulletin 235, California Agr. 

 Exp. Sta. 



