256 



Diseases of Economic Plants 



The first indication of the disease is given through the leaves, 

 which droop, becoming soft and flabby as though suffering 

 from want of water, the leaves remaining green. A typical case 

 is shown in Fig. 139. Frequently the leaves on one side of the 



plant succumb earlier 

 than those on the 

 other side, and even a 

 single leaf may show 

 one-sided infection. 

 The wilted leaves dry 

 up, and eventually 

 leaves and stalk die, 

 though the stalk re- 

 mains standing with 

 its dead leaves still 

 clinging to it. 



At the stage of earl- 

 iest wilting a section 

 across the stem shows 

 a yellowish discolora- 

 tion of the woody por- 

 tion. In more ad- 

 vanced stages, or in 

 sections taken lower 

 on the stem, the wood 

 is found to be pene- 

 trated longitudinally 

 by black streaks. 

 When all the leaves 

 are wilted, the wood and bark at the base of the plant are 

 blackened and the pith decayed, leaving the stem hollow or 

 filled with a soft, rotten residue. If a badly diseased plant 

 is cut off near the ground, a dirty, yellowish, viscous exu- 

 date 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 a pionounced stage of decay. In early stages 



Fig. 140. — Tobacco plant in late stages of 

 the Granville-wilt. Original. 



