186 BACTERIAL DISEASES OF PLANTS 



pigment especially when Witte's peptone is used. On agar plates 

 the young surface colonies are rather watery and will often flow 

 if the plates are tilted to a vertical or semi-vertical position (Fig. 

 133). On gelatin plates the colonies are small and not charac- 

 teristic. On steamed potato cylinders the bacterial slime is 

 white at first, becoming gradually dark brown or black. It does 

 not grow readily on raw potato (contrast with No. VII). Some 

 of its most striking characters are: irregularly roundish, small, 



FIG. 129. Cross-section of a mature Florida potato attacked by Bacterium 

 solanacearum showing brown stain and gray ooze in the vascular region. A 

 natural infection by way of the rhizome. 3? 



opalescent surface colonies on agar-poured plates, sensitiveness 

 to dry air, bipolar staining, aerobism, liberation of brown stain 

 in tissues of its hosts, dark stain on steamed potato and agar, slow 

 clearing of milk without precipitation of the casein, bluing of 

 cream-free litmus milk, non-liquefaction of gelatin (at least 

 during the first few weeks), reduction of nitrates and failure 

 to grow in Cohn's solution. 



It loses virulence rather quickly, dies out early on various 



eased vanilla, which was inoculated from tomato, which was inoculated from dis- 

 eased Georgia Ricinus. Sub-cultures from single colonies on agar-poured plates 

 were used in each case. Six plants (of which these were 1 and 2) were inoculated 

 and all became diseased and badly dwarfed. See Fig. 131. Bacteria were abundant 

 in the xylem vessels of the stem and also in the root 2 inches below the crown (12 

 inches below the needle pricks) in both plants. There was a brown stain in the 

 vascular system. Time, 10 days. Plants inoculated July 12, 1918. Photographed 

 July 22. \-2 natural size. 



