VEGETABLE AND FIELD CROPS 303 



resulting in collapse in irregular longitudinal folds. If a 

 badly diseased plant is cut off near the ground, a dirty 

 yellowish viscous exudate issues from the cut wood. 



The root is the seat of the original infection, and any 

 plant which shows symptoms in its foliage possesses roots 

 already in pronounced stage of decay. In early stages one 

 root or more may be diseased ; in later stages all succumb ; 

 in the more advanced stages of root disease the bark is 

 black, soft, and dry, a spongy mass of fiber left by the decay 

 of the more watery parts. In the worst cases even this 

 spongy covering may drop off, leaving the wood of the root 

 bare. 



In the root as in the stem the disease manifests itself first 

 as longitudinal streaks of black in the woody cylinder close 

 to the bark. 



No corrective for a field once infected is known. After 

 infection the only recourse would be to resistant varieties, 

 if such were known, or to long rotation, and this is unsatis- 

 factory and palliative only. 



The means by which this disease spreads from field to 

 field, and methods to prevent such spreading, are suffi- 

 ciently discussed under soil diseases. 



Cercosporose (Cercospora NicotiancB Ell. & Ev.). — 

 This disease, most abundant upon the lower leaves, is 

 well illustrated in the accompanying figure. It appears 

 as brown, circular spots from the size of a pinhead to 

 a centimeter or more in diameter, thickly scattered over 

 the leaves. Older spots bear white centers bordered by a 

 darker raised line, and the centers often fall away, leav- 

 ing irregular holes. The whole leaf yellows and ripens 

 prematurely. 



