142 Diseases of Truck Crops 



more serious on greenhouse lettuce than on that 

 grown in the open. In the field it usually attacks fall 

 lettuce. 



Symptoms. Affected leaves lose their natural 

 green color and turn yellow. A careful examination 

 will disclose a delicate downy web on the imder side 

 of the foliage which will have a wilted appearance. 

 The downy web consists of the conidiophores of the 

 ftmgus. These appear singly and are much branched. 

 The conidia germinate by means of a germ tube. 

 Downy nuldew attacks not only lettuce, but also chic- 

 ory and numerous other Compositas. 



Control. Spraying lettuce is not recommended. 

 Clean culture and careful regulation of the soil mois- 

 ture will help to control this trouble. Downy 

 mildew is seldom found on well drained lands. In 

 greenhouses, the disease may be checked by the 

 sudden lowering of temperature for a day or two. 

 Soil sterilization with formaldehyde (see p. 53) will 

 also be effective. 



Gray Mold 



Caused by Sclerotinia Fuckeliana De Bary 



Gray mold attacks grapes in Europe but in the 

 United States it is commonly niet with in the lettuce 

 fields, especially on plants which are fully developed 

 and somewhat overgrown. 



Symptoms. The disease is manifested by soft, 

 watersoaked spots on the foliage causing a wilting. 

 The spots soon become coated '""*^_ ^^f" f""t of a 



