170 



some of the commercial tobacco extracts through- 

 out the house. A light treatment two or three days 

 in succession at intervals of ten days is usually 

 considered the best method of control. This will 

 be much more effective in destroying the insects 

 present than one strong treatment and be much less 

 likely to injure the plants. 



The mildew is the most common disease. It may 

 be recognized by the plants affected showing areas 

 of slightly yellowish color, which later become 

 darker and finally die. In the last stages of the 

 disease the tissues of the leaves become covered 

 with a moldlike substance which is the reproductive 

 portion of the mildew. All plants showing signs 

 of the trouble should be removed and destroyed. 



Watering should be done in such a way as to pre- 

 vent wetting the leaves, and be confined entirely 

 to mornings of bright days. This trouble is much 

 less common under field conditions than it is in the 

 greenhouse. 



The lettuce rot or drop is specially destructive 

 to frame and greenhouse crops and in many 

 cases attacks the head lettuce out of doors. It 

 develops most rapidly under moist, warm condi- 

 tions, and a few days of rainy, warm weather at the 

 time when the head lettuce is beginning to head 

 may destroy the entire crop in a very short time. 

 The first indication of the trouble is a slight wilting 

 of the plant during the daytime, from which it seems 

 to recover at night. Decay quickly sets in, how- 

 ever, accompanied by offensive odors. Little can 

 be done for this trouble under field conditions ex- 

 cept to cut and destroy the diseased plants as rapidly 

 as they appear. All stable manures should be thor- 

 oughly well rotted before applying to the soil in 



