332 



Diseases of Economic Plants 



reddish brown tinge. After the leaves have fallen the stem 

 dies and becomes covered with a light pink coating of the 

 spores of the wilt fungus. The spread of the disease is more 

 gradual and less conspicuous in the early part of the season, 

 but after the peas begin to set fruit they succumb rapidly, and 



a field that in July gave 

 promise of a fine crop may be 

 entirely dead before Septem- 

 ber without having matured 

 a pod. The disease usually 

 appears in spots, like the 

 VN\ 11 \ if Yf^— ^ cotton wilt, and these dis- 



^/^^^j \ //^^^ eased areas may spread until 



a whole field is involved. 



In moderate cases, or 

 where the varieties planted 

 are less subject to disease, 

 only the weaker plants are 

 killed, while the rest are 

 dwarfed and their yield re- 

 duced. Careful examination 

 of the roots shows that many 

 of the small, lateral roots are 

 dead, small tufts of roots 

 marking the points of infec- 

 tion (Fig. 174). This tufting 

 of the rootlets is similar to 

 that produced on cotton by 

 the cotton-wilt fungus. 

 In all cases the veins of the stem are brown, and the dis- 

 ease may clearly be distinguished by cutting across the stem 

 to observe whether the color of the wood is normal. This 

 discoloration, which may often be seen through the translu- 

 cent stem of the cowpea, is characteristic of this class of 

 diseases. The name "wilt" is somewhat misleading, since 

 the leaves usually drop off before there is any conspicuous 

 wilting. 



Fig. 174. — Roots of diseased cow- 

 pea at left; healthy roots on the 

 right. After Orton. 



