DISEASES OF COTTON 143 



periments have demonstrated that these materials are 

 ineffective. As the fungus lives from year to year on the 

 organic content of the soil, the use of fungicides or steriliza- 

 tion processes are not practical. Much can be done to 

 decrease the prevalence of this disease by keeping cotton 

 off the diseased soil for a number of years. It is almost 

 impossible, however, completely to starve out cotton-wilt 

 by crop rotation, owing to the fact that the fungus will 

 live as a saprophyte on the organic matter of the soil for 

 many years even though all host plants are kept off the 

 land. 



The most effective means of avoiding injury from wilt 

 is the cultivation of wilt-resistant varieties. It has been 

 found that the commercial varieties of cotton differ greatly 

 as regards their susceptibility to wilt. Generally speaking, 

 the large-boiled varieties are more susceptible than are 

 the other groups. Beginning with some of the more or 

 less resistant small-boiled varieties as a basis, the Bureau 

 of Plant Industry has, as a result of 15 or 20 years' breed- 

 ing, developed several strains of cotton that show marked 

 power of wilt resistance. In fact, so resistant are these 

 strains that there is now little doubt as to the possibility 

 of controlling the disease in this way. The more impor- 

 tant of these resistant varieties are Dillon, Dixie, and 

 Modella. 



In the growing of these varieties much care must be ex- 

 ercised to see that no crossing from other less resistant 

 varieties is permitted and that the seed is not mixed at the 

 gin with other varieties. 



COTTON ROOT-ROT (Ozonium omnivorum) 



175. Occurrence. So far, this disease has caused 

 very little damage to cotton grown east of Texas. It is 



