240 



AGRICULTURE 



withstand the attacks of this fungus. The Dixie (Fig. 

 162), some strains of the Jackson variety, and some 

 varieties of Sea Island cotton have been made quite re- 

 sistant to cotton wilt by years of selection. 



Rotation of crops is generally the way to decrease 



the damage from 

 wilt. However, 

 the germs of wilt 

 live in the soil 

 for several years. 

 Hence, in a rota- 

 tion for land 

 where this disease 

 occurs. cotton 

 must not be grown 

 oftener than once 

 in three or four 

 years. Neither 

 should the ordi- 

 nary varieties of cowpeas be grown in such a rotation, for the 

 root-knot worms, if present, increase rapidly on the cow- 

 pea roots. This increases the number of wounds on the 

 cotton roots the next year, and hence probably the 

 number of wilt fungi entering the cotton plant. But 

 in a rotation of crops on such a field, the variety of 

 cowpeas called Iron, and also the velvet bean, may well 

 be grown, because the root-knot worms do not rapidly in- 

 crease on the roots of these plants. 



Cotton root rot. The farmers of Texas and Oklahoma 

 are not troubled with cotton wilt. Instead, their cotton 



FIG. 162. DIXIE, A WILT-RESISTANT VARIETY OF 

 COTTON 



