Vegetable and Field Crops 199 



(see grape). It is frequently reported as causing much loss to 

 lettuce under glass, especially 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 green- 

 house, especially as to moisture, ventilation, and tempera- 

 ture, usually suffice to hold it in check. Evaporated sulfur 

 also aids in suppressing the mildew. 



Bacterial rots -■^'' (Pseudmnonas sps.). — Several different 

 rots due to bacteria occur upon this host. In some instances 

 the head rots at the center, or at the top. In other cases 

 leaves may become spotted with large brown or black areas, 

 or shriveled, or give way to a soft rot, or the tips of the heart 

 leaves may die. In other instances, small translucent leaf 

 spots occur; extending and coalescing, they occupy large 

 areas, the affected tissues becoming diy and papery. 



Various species of Pseudomonas, associated with these rots 

 have been considered causal agents, but predisposing condi- 

 tions, as extreme cold, or other unfavorable conditions of 

 temperature, ventilation, or nutriment seem to be necessary 

 to enable these bacteria to gain entrance into the plant: 



Leaf-perforation, anthracnose -^^ (Marssonina panat- 

 toniana (Berl.) Magn.). — The most conspicuous character 

 is perforation resulting from the fall of the tissue killed by 

 the attacking fungus. These perforations are from 1-2 mm., 

 or more, in diameter, with irregular borders. Upon the 

 midrib and petiole depressed spots result. With small, 

 young leaves dwarfing and distortion is produced, especially 

 at the apex. The disease appears to be limited to green- 

 houses and to be spread largely by watering. 



It was first described by Selby in Ohio in 1896 and has 

 since been noted in several other states, ranging from the 

 Atlantic to the Pacific, arid in Europe. Diseased lettuce 

 should be l)urned. Watering by overhead systems leads to a 

 minimum of infection. 



Blight (Septoria laducce Pass, and Septoria condmilis E. & 

 M.). — Upon the old, outer head-leaves there often appear 



