VEGETABLE AND FIELD CROPS 245 



spreads thence throughout the leaves, rotting away the 

 blades but leaving the midribs standing/ thus distinguish- 

 ing 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 be reduced to a slimy 

 mass though the outer leaves still remain, surrounding it 

 as a rosette. 



Downy mildew (Bremia Laducce Kegel). — This occa- 

 sionally destructive mildew shows the character of the 

 downy mildews, i.e., downy fuzz below, yellow spots 

 above (see grape). It has been reported from Massa- 

 chusetts as causing much loss to lettuce under glass, es- 

 pecially upon the older leaves. It is also noted upon old 

 leaves on seed plants of both cultivated and wild lettuce. 



Close attention to proper cultural conditions in the 

 greenhouse, especially as to moisture, ventilation, and 

 temperature, usually suffice to hold it in check. Evap- 

 orated sulphur also aids in suppressing the mildew. 



Soft rot {Bacteria). — Several different diseases due to 

 bacteria occur upon this host. Notable among them is a 

 soft rot of the head and a hollowing of the stems. Peculiar 

 translucent watery spots upon the leaf are also caused by 

 bacteria. 



These diseases have not yet been fully studied nor treat- 

 ments suggested. 



Leaf perforation, anthracnose (Marsonia pei'forans Ell, & 

 Ev.). — The most conspicuous character is the perforation 

 resulting from the fall of the tissue killed by the attacking 



1 stone, Geo. E., and Smith, R. E., Mass, Agr. Exp. Sta. Bui. 69, 

 September, 1900. 



