DISEASES OF COTTON 145 



that the soil should be plowed not less than seven and 

 preferably nine inches deep if favorable results are to be 

 expected. It was also found that subsoiling was very 

 effective in decreasing the disease. 



As root-rot does not affect grasses and grains, the prev- 

 alence of the disease is greatly decreased by growing these 

 crops on the land for two or three years preceding the grow- 

 ing of cotton. The results obtained from practicing such 

 a cropping system are, however, not always uniform and 

 satisfactory. 



The application of fungicides or other chemicals or fer- 

 tilizers to the soil as a means of controlling root-rot is 

 entirely impractical. 



ROOT-KNOT (Heterodera radidcola) 



179. Occurrence. Root-knot is essentially a pest 

 characteristic of light, sandy soils. As a rule, it is not 

 serious on soils containing a large percentage of clay. 

 This disease is very often associated with cotton-wilt in its 

 occurrence. Unlike cotton-wilt, the root-knot attacks a 

 large number of plants other than cotton. Some of the 

 plants often affected by this disease are, soy bean, 

 cowpea (all varieties except Iron and Brabham and cer- 

 tain hybrids of these varieties), crimson clover, bur clover, 

 cucumber, watermelon, tomato, tobacco, peach, and pecan. 



180. Cause. This trouble is caused by microscopic 

 worms known as nematodes or eel worms which burrow 

 into the roots, thus setting up irritations which later de- 

 velop into wart-like excrescences or knots. These worms 

 vary in length from 1 / 2 o to Veo of an inch. The knots or 

 galls produced by these worms vary in size from tiny 

 enlargements on the small roots to knots an inch or more in 

 diameter on the large ones. 



