COTTON DISEASES 413 



382. Treatment of wilt by means of rotation of crops. 



In spite of this long life of the germ of cotton wilt, the 

 only effective treatment of the soil consists in starving 

 the germs. This is done to a considerable extent by keep- 

 ing cotton out of the field for three years ; a longer banish- 

 ment of cotton still more nearly gets rid of the disease. 

 Meantime the field may be used for corn, oats, grasses, 

 the Iron variety of cowpeas, and certain other plants. 



It has been found that cotton wilt is most prevalent 

 on soils which contain, not only the germs of the wilt 

 fungus, but also the minute worms that cause root-knot 

 (see paragraph 385) on the roots of cotton and of numerous 

 other plants. It is thought that the wounding of the roots 

 of cotton by these tiny nematode worms more readily per- 

 mits the entrance of the germs of cotton wilt. Hence, 

 in a field where both troubles occur, no plants should be 

 grown on which nematode worms thrive and multiply. 



383. Use of resistant varieties. Not every cotton 

 plant in a diseased spot dies. The plants that live and 

 thrive are resistant, and the seed saved from them produce 

 plants, the majority of which are resistant. Thus, by 

 selecting for several generations healthy plants and grow- 

 ing them each year on diseased spots, a variety of wilt- 

 resistant cotton may be bred up. This can probably be 

 done with many varieties. However, present varieties 

 differ greatly in the degree to which they resist cotton 

 wilt. The varieties Dixie and Dillon have been thus 

 bred up by the United States Department of Agriculture, 

 until they are able to produce profitable crops in fields 

 that have been ruined for most other varieties by the pres- 

 ence of this disease. 



