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 (Bremia Lactuce 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 
