198 Diseases of Economic Plants 



upon diseased refuse in any form. Compost which may be 

 contaminated with lettuce refuse bearing this disease should 

 be avoided. Those growing lettuce under glass have found 

 soil disinfection effective and practicable. 



Damping-off. See p. 19. 



Gray-mold {Botrytis cinerea Pers.). — Gray-mold is es- 

 sentially a leaf disease, beginning usually at the leaf edges, 

 and there causing wilted, flaccid patches which soon become 

 coated with a straw-colored downy fuzz, the sporiferous 

 hyphse. The whole leaf, ordinarily an outer one, may droop 

 and die. 



Weak plants may succumb entirely to such attack, the 

 disease passing slowly from the outer to the inner leaves. 

 In all eases after the death of the affected parts a charac- 

 teristic, brownish, fuzzy coating appears. This disease may 

 be distinguished from drop by the al)sence of the white 

 mycelium and sclerotia, and by the presence of the gray 

 Botrytis growth, which is not a feature of drop. The two 

 diseases, while often occurring separate, may both infest the 

 same beds or the same plants. Gray-mold may also cause 

 damping-off. 



High temperature, lack of ventilation, and lack of vigor in 

 the plant, resulting from improper care, are conditions favora- 

 ble to attack of gray-mold. Precaution with respect to 

 these details is sufficient to insure against serious loss. 



Rosette (Corticium vagum). — -A moist rot sets in, first 

 upon portions of leaves which touch the ground, and spreads 

 thence throughout the leaves, rotting away the blades but 

 leaving the midribs standing, thus distinguishing this from 

 all other diseases. Infection proceeds from leaf to leaf at 

 point of contact, often reaching the center of the head, 

 while the outer leaves are infected only, in spots. The inner 

 leaves may all ])e reduced to a slimy mass though the outer 

 leaves still remain, surrounding it as a rosette. 



Downy-mildew {Bremia lactucce Regel). — This occa- 

 sionally destructive mildew shows the character of the 

 downy mildews, ;". e., downy fuzz below, yellow spots above 



