LETTUCE 287 



Diseases. Both forced and field lettuce are subject to two seri- 

 ous diseases mildew and blight. These diseases are the result 

 of an attack by a low form of plant life known as a fungus, which 

 is a parasitic organism ; that is, an organism which lives upon 

 and derives its sustenance from other growing tissue. 



Mildew. This is undoubtedly the most troublesome disease that 

 the grower of lettuce under glass has to contend with. Plants 

 affected with this show slightly discolored areas, areas of a less 

 intense green color or of a slightly yellow cast, which later be- 

 come either dry and dark or soft and flabby, according to the con- 

 ditions in the house. In the advanced stage of the disease the tissue 

 of the plant becomes covered with a growth which, to the casual 

 observer, looks like ordinary mold and is the result of the develop- 

 ment of the fruiting or reproductive portions of the mildew. All 

 plants which show even the early stages of the disease should be 

 removed and destroyed. 



The best way to avoid mildew is to exercise special care in water- 

 ing the plants. Water should be applied in such a manner as to 

 prevent its coming in contact with the leaves, and then only on 

 mornings of bright days ; for cloudy, close, rather warm weather is 

 particularly conducive to the development of both rot and mildew. 

 Thorough ventilation and a careful regulation of the heat so that 

 a night temperature of from 40 to 50 F. and a day temperature 

 of from 60 to 65 F. can be maintained are the best safeguards 

 against the mildew. If the disease makes its appearance in spite 

 of these precautions, the heating pipes should be coated with sul- 

 phur, and, if possible, as a precautionary measure, such a coating 

 should be kept on the pipes during the entire forcing season. In 

 addition to this, if the outbreak is severe and threatening, sul- 

 phur should be distilled over a sand bath heated by a low flame, 

 as described on page 251. 



Rot. Lettuce rot, or drop, is the result of a parasitic organism, 

 similar in many respects to the one causing the mildew but differ- 

 ent in its effect upon the growing plant. While the rot sometimes 

 causes considerable damage to lettuce under glass, as a rule it is 

 much less troublesome under such conditions than when the crop 

 is grown in the open or in frames where muslins are used for 

 protective purposes. If the weather is extremely wet, or if it is 



