414 SOUTHERN FIELD CROPS 



To maintain the wilt resistance in these or other varie- 

 ties, it is advisable to grow them on infected land and 

 to continue the selection each year from plants that are 

 thrifty. 



384. Cotton root-rot (Ozonium). This disease, like 

 cotton wilt, causes the sudden wilting of the plants while 

 engaged in forming fruit. However, it is confined to the 

 extreme western part of the cotton-belt, while cotton wilt 

 is a disease of the southeastern part. Cotton root-rot is 

 especially prevalent on the- stiff, lime, " black-waxy " soils 

 of Texas. It is caused by a fungus that develops 

 threads both within and upon the surface of the roots. 

 The roots of diseased plants are covered by whitish threads, 

 which later become darker. Sometimes wartlike bodies 

 appear on the surface. 



No treatment of seed or soil is effective. However, very 

 deep fall plowing and rotation of crops are helpful. In 

 such a rotation the farmer must avoid the use of other 

 plants attacked by this disease, among which are sweet 

 potatoes and alfalfa; among the plants not subject to 

 this root-rot are all the grains and grasses. 



385. Root-knot (Heteroderaradicicola) (Fig. 182). This 

 is a special kind of enlargement on the roots of many 

 plants, caused by the attacks of extremely small worms, 

 called nematodes. Cotton is attacked, but less severely 

 than are most varieties of cowpeas. Among the plants 

 not attacked are the grains and grasses and the Iron 

 variety of cowpea. The best way to combat this disease 

 consists in starving the worms, by excluding from the field 

 for two years all plants on the roots of which nematodes 

 can develop, including ordinary varieties of cowpeas, and 



