102 



Salad Crops 



may spread until the destruction of the entire plant results. A 

 characteristic gray fungous growth from which the name of 

 the disease is derived occurs on the rotted tissues. The para- 

 site is most destructive in greenhouses. Control: Care in ven- 

 tilating and watering will do much toward preventing the de- 

 velopment of this rot. The prompt removal of all debris is 

 desirable. 



Anthracnose {Marssonina panattoniana) . — Leaf lesions 

 appear first as somewhat circular water-soaked spots which 

 later become brown. In the later stages, the affected tissues 

 die and drop out, giving the leaf a shot-holed appearance. On 

 the midrib the brownish spots are sunken and elongated. 

 Control: Prompt removal of affected plants, together with 

 rotation of crops in the field, is desirable. Sanitation in the 

 greenhouse is important. Slightly higher temperature than is 

 usual, together with careful ventilation, will check the disease. 



Mildew {Bremia lactuccr). — Yellow areas are evident on the 

 upper side of affected leaves and a w^hite mildew is present 

 on the under surface of such spots. Mildew is primarily a 

 greenhouse disease, although it may occur in the field in cool 

 weather. Control: Care in ventilating and watering will pre- 

 vent the development of this disease. A slight increase in tem- 

 perature may tend to check its development. 



Tip-burn. — A blackening of the leaf margins, frequently 

 evident only on the inner leaves, is characteristic of tip-burn. 

 Apparently this disease is not due to a casual organism but 

 to unfavorable environmental conditions. Control: Careful 

 watering and ventilating in the greenhouse W'ill aid in pre- 

 venting this trouble. There is some indication that an excess 

 of nitrate and excessive applications of fertilizers in midsum- 

 mer may increase the development of tip-burn. 



Cabbage looper (Autographa 'brassicce) and Celery looper 

 (Autographa falcigera). — Both of these common looping cater- 

 pillars sometimes attack lettuce. As an arsenical cannot be 

 used, hand-picking is the only available measure. 



Plant-lice (several species). — Lettuce both in the green- 



