DISEASES OF COTTON 



239 



of the islands on the South Atlantic coast where Sea Island 

 cotton is produced. 



The first year that wilt occurs on a field it attacks only 

 a single plant or a small spot here and there. The next 

 year the spots where the plants die are larger. In a very 

 few years the fungus may become so very widespread as 

 to make it impossible to maintain a stand of cotton on 

 any part of the field (Fig. 160). 



In the stem of a cotton plant that has been attacked by 

 wilt, the woody portion is darkened or streaked with very 

 fine black lines. These black lines are the water-carrying 

 vessels that have become stopped up by the growth of the 

 fungus. Their stoppage causes the plant 

 to wilt for lack of water. A dark layer 

 occurs just under the bark (Fig. 161). 

 The germs of the disease enter the plant 

 through the roots. Cotton wilt is gener- 

 ally considered worse on land where the 

 tiny worms that produce knots on the 

 roots are present. The germs probably 

 enter more readily through the wounds 



made by these root-knot worms on the 



FIG. 161. DIAGONAL 

 roots of the plant. SECTION THROUGH 



In some of the affected areas in the COTTON STALKS 



field there may be a few stalks which do On , ri s ht > 



plant; on left, 



not contract the disease. If so, they are blackened by cotton 

 resistant and their seeds may transmit this Wllt ' 

 natural resistance. Mark them and very carefully pre- 

 serve their seeds for planting purposes. Different varie- 

 ties of cotton show marked differences in their ability to 



