142 FIELD CROPS FOR THE COTTON-BELT 



spores by means of which the disease is propagated. Any 

 agency that will transfer these spores or the infected soil 

 from one part of the field to another will serve to spread the 

 disease. Chief among these agencies are cultivating tools, 

 wind, drainage water, and the feet of men or of work 

 animals. 



The fungi that produce the wilts of cowpeas, tomatoes, 

 watermelons, tobacco, and okra are thought to be closely 

 related to the cotton-wilt fungus. There is no proof, 

 however, that these diseases are communicable to cotton. 



173. Symptoms. — The first appearance of this dis- 

 ease is indicated by the yellowing of the leaves at their 

 margins and between the veins. Later the leaves wilt 

 and fall from the plants. The characteristic tendency 

 of cotton plants to wilt when infected with this disease is 

 due to the growth of the fungus in the water-carrying 

 vessels of the roots and stems, such a growth cutting off 

 the water supply to the upper portions of the plant. Us- 

 ually the badly affected plants are completely killed while 

 others may lose only a portion of their leaves, but the 

 plants thereafter possess a stunted appearance. An ex- 

 amination of the tap-root or lower part of the main-stem 

 of a cotton plant affected with wilt will reveal a brownish 

 color of the wood in the region of the water-ducts. This 

 darkened color is the result of the closely woven hyphse 

 of the fungus growing in the water-carrying vessels. 



Cotton-wilt usually makes its appearance at first in 

 small restricted areas throughout the cotton field, which 

 gradually become larger until the entire field is affected, 

 provided cotton is grown on the same land year after year. 



174. Remedies. — Although barnyard manure and 

 various fertilizing materials have been suggested as a 

 means of controlling wilt, both farm experience and ex- 



