Family Compositae 141 



outer leaves of a head. The afifected foliage is first 

 covered with numerous watersoaked spots which 

 enlarge, 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 or 

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

 of polar flageUa. On agar, the young colonies are 

 rotmd with entire smooth margins; they are trans- 

 lucent cream white in reflected light but bluish in 

 transmitted light. The older colonies are not always 

 uniform 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. It is not as yet known whether Bordeaux 

 mixture wiU control the disease. The disease shotild 

 not be allowed to gain any headway in the field. 

 Diseased plants should not be fed to cattle nor al- 

 lowed to find a place in the mantire pile, but should 

 be destroyed by fire. In severely affected fields other 

 crops should be grown and the land be given a rest 

 from lettuce for at least three years. 



Downy Mildew 



Caused by Bremia lactucce Reg. 



Downy mildew is a disease which is more trouble- 

 some in Europe than in the United States, and it is 



