Lettuce Diseases 149 



This disease does not cause a rot of the leaves. The 

 trouble is confined mainly to the edges of the foliage, 

 marring the appearance and the market value of the 

 product. The varieties most afltected seem to be 

 the Black-seeded Simpson, the Improved Hansen, 

 and the Big Boston. Of the varieties less suscep- 

 tible may be mentioned the Early Curled Simpson, 

 and Vaughan's All Season. The variety Grand 

 Rapids seems to be immune. 



The Organism. Pseudomonas marginale is a 

 short rod, rounded at both ends and motile by means 

 of polar fiagella. It forms capsules but no endo- 

 spores, liquefies gelatin quickly and produces no gas. 



Control. Infected material should be destroyed* 

 by fire. In watering, splashing should be avoided. 

 Soil sterilization with steam or formaldehyde (see 

 pp. 32-43) is recommended. 



Downy Mildew 



Caused by Bremia lactuce Reg. 



Symptoms. Affected leaves lose their natural 

 green color and turn yellow. A careful examination 

 will disclose a delicate downy web on the under side 

 of the foliage which will have a wilted appearance. 

 The downy web consists of the conidiophores of 

 the fungus. These appear singly and are much 

 branched. The conidia germinate by means of a 

 germ tube. Downy mildew is a disease which is 

 more troublesome in Europe than in the United 

 States, and it is more serious on greenhouse lettuce 



