486 SPECIAL PLANT PATHOLOGY 



A stem section shows a browning of the vascular ring and the vessels 

 are found occupied by bacteria (Fig. i68). When the cabbage plant 

 is attacked early in the season, it is killed outright, or else it fails 

 to form the characteristic head. Infections may take place through 

 injury of the surface, but the greater part of them are through the 

 water pores, which exude drops of water, which collect during cool 



Fig. i68. — Brown-rot of turnip {Pseudomonas brassica). Cross-section from 

 middle of turnip root showing small bundle fully occupied by the bacterial organism. 

 (After Smith, E. P., Bull. 29, U. S. Bureau of Plant Industry, Jan. 17, 1903.) 



nights, and in natural infectioii slugs are responsible carriers of the 

 organism. 



Russell has found that the cauhflower is the most susceptible plant, 

 while turnips and rutabagas are not very susceptible. Edwards reports 

 that the Houser cabbage is practically immune to black-rot under field 

 conditions. The period of incubation is variable. In some cases with 

 needle punctures, the first signs of the disease appear in seven to 



