Lettuce Diseases 147 



Bacterial Blight 



Caused by Pseudomonas viridilividum Br. 



Symptoms. The disease seems to attack only the 

 outer leaves of a head. The affected foliage is first 

 covered with numerous watersoaked spots which en- 

 large, fuse together, and involve the entire area of 

 the affected leaves. The latter either soften or dry 

 up, opening up the way for the entrance of other 

 decay organisms, which may now attack the other- 

 wise sound head. 



The Organism is rod-shaped, occurring singly, in 

 pairs, or in chains, and it moves about by means of 

 polar flagella. On agar, the young colonies are round 

 with entire smooth margins; they are translucent, 

 cream white in reflected light, but bluish in trans- 

 mitted light. The older colonies are not always uni- 

 form in color, but may take on yellowish bands or 

 become mottled. The organism does not form gas 

 and it liquefies gelatine slowly. It is not especially 

 sensitive to sunlight. 



Control. Since the disease may be introduced 

 with infected soil, soil sterilization is recommended. 



The SotTTH Carolina Disease 



Caused by Pseudomonas vitans Br. 



Symptoms. The disease may attack the stems or 

 leaves. At first, diseased plants become pale and 

 lose their normal green. Later the head wilts and 

 rots (fig. 26, a.). The rot may be confined to the 



