226 VEGETABLE FORCING 



ventilation. The fungus first causes the wilting of the 

 outside leaves of the plant, and finally the rotting of the 

 stem at the surface of the ground. Head lettuce is most 

 susceptible to attack, but the disease often appears on 

 Grand Rapids and other loose-heading sorts, and may 

 cause heavy losses in houses which are not properly 

 sterilized. This fungus is both parasitic and saprophytic, 

 living over by means of vegetative bodies called 

 sclerotinia. Since this fungus may live over in the soil 

 or in the old plant remains, it is necessary to apply some 

 treatment which will kill the fungus. Formaldehyde or 

 steam sterilization is usually effective. Thorough ven- 

 tilation, careful watering and the maintenance of proper 

 temperatures are also important factors in the control of 

 this disease. There are other forms of rots, but this is 

 the most important. 



Lettuce mildew (Brcmia Lactuccc Reg.) appears some- 

 times in houses where there is excessive moisture, and 

 when there is little sunshine, but more particularly on 

 frame lettuce in the fall. It is first seen on the upper 

 surfaces of the outer leaves, as yellow spots, making the 

 leaves paler in color and finally causing them to wilt. If 

 proper sanitary conditions are maintained, mildew is not 

 likely to cause serious losses. 



Gray mold (Botrytis vulgaris) often accompanies let- 

 tuce drop. It is entirely saprophytic and does not spread 

 so rapidly as lettuce drop. The edges of the leaves wilt 

 and the leaves soon droop and die, their surfaces becom- 

 ing covered with gray mycelium. Thorough sterilization 

 with steam or formalin is effective as a preventive 

 measure. 



Dwarf, stunted heads or tufts of leaves, generally 

 called rosettes, sometimes appear in beds of lettuce. 

 They are most commonly caused by the fungus Rhizoc- 

 tonia, which feeds on the roots of lettuce plants and thus 

 interferes with their proper nutrition. As the old roots 



