344 



Diseases of Economic Plants 



more slowly, the intervals between the different stages are 

 more prolonged. 



In light sand the disease may progress very rapidly and 

 may consist of two stages only, the sudden wilted condition 

 followed by speedy death. The final diagnostic symptom of 

 the disease, however, as in the case of many other wilts, is 

 the darkening of the affected veins, which change from the 

 normal white to light brown or black. Though in most 

 cases the plant dies, occasionally one revives and seems to 



Fig. 183. — A variety of cotton resistant to I lie wilt; note the 

 complete failure of the other varieties. After Orton. 



outgrow the disease. In such plants relapses may follow, 

 showing somewhat different symptoms, among them decay 

 of the boll, and a different sequence of color changes. 



Rotation of crops should be practiced, the diseased plants 

 should be pulled and burned to check the spread of the 

 disease in the soil, and in general the recommendations 

 given under soil diseases should be followed. Cotton follow- 

 ing cowpeas infected with root-knot, is highly susceptible. 

 Therefore, only cowpeas resistant to nematodes should be 

 used in cotton rotation (Fig. 177). The ultimate solution of 

 the question must be in the employment of resistant varieties. 



