6 



THE CONTROL OF COTTON AVILT AND EOOT-KNOT. 



Fig. 8.— Female of the nema- 

 tode galhvorm (Ilcterodera 

 radkicola) magnifled 85 di- 

 ameters: a, Mouth; 6, spher- 

 ical sucking bulb; c, c, ova- 

 ries, as seen through the 

 body wall; (f. anus: e, small 

 whitespots showing approx- 

 imately the natural size cf 

 these worms. They are 

 usually white. It is gener- 

 ally not difficult to isolate 

 them in water by breakmg 

 open the galls containing 

 them. (After X. A. Cobb.) 



very much subject to root-knot (fig. 7), and experience lias shown 

 tliat when a farmer has observed that his cotton wilts worse after a 

 cowpea crop the cause is almost always root-knot. 



For a complete discussion of 

 root-knot, its cause and control, 

 see Bulletin 217 of the Bureau 

 of Plant Industry, entitled 

 "Root-Knot and Its Control." 



IMPORTANT FACTS CONCERN- 

 ING WILT AND ROOT-KNOT. 



Cause. — Wilt is caused by a 

 parasitic fungus, a minute 

 threadlike organism, which en- 

 ters the roots from the soil and 

 plugs the water-carrvnng vessels 

 of the root and stem. 



Root-laiot is caused by minute 

 eelworms, or nematodes, which 

 bore into the roots. (See figs. 8 

 and 9.) The irritation caused by 

 these worms results in enlarged 

 growths of the root at the point 

 of attack. 



Plants attacked. — The cotton- 

 wilt fungus attacks only cotton 

 and okra. There are very simi- 

 lar wilt diseases of tlie cowpea, watermelon, tomato, and 

 other crops, but they are not communicable to cotton. 

 The root-knot nematode attacks a very large number 

 of plants and spreads freely from one to another. A 

 few ])lants are immune; see list on page 7. 



Duration of infection. — The wilt fungus lives many 

 years in the soil. Rotation of crops affords only slight 

 relief. 



The root-knot nematode may be starved out by two 

 to four year rotations of crops on which it can not live. 

 Fertilizers and fungicides ineffective. — Neither cotton 

 wilt nor root-knot can be prevented by the use of kainit, 

 salt, phosphate, or other fertilizers; nor by sulphur, 

 lime, or other fungicides applied to the soil. Stable 

 manure in some cases affords a degree of temporary 

 rehef from wilt. 



B elation to soil. — Both wilt and root-knot are mainly confined to 

 sandy or light loam soils. Clay soils are but seldom infected. 



[Cir. 92] 



Fig. 9.— The adult 

 male of Heierodera 

 radkicola. or gall- 

 worm: I, Worm 

 in profile view; II, 

 head of the same, 

 more highly mag- 

 nified; ///, middle 

 region of the worm 

 showing blind 

 ends of the sexual 

 organs; 71', pos- 

 terior extremity. 

 The drawings 

 were prepared 

 from stained spec- 

 imens, examined 

 in carbolic-acid so- 

 lution. 



a, Lips; b, 

 oesophageal tube; 

 c, median bulb; d, 

 excretory pore; e, 

 spear;/", intestine; 

 g, blindendsoftes- 

 ticles; Ji. testicles; 

 i, spicula; /, rudi- 

 mentary bursa; fc, 

 anus. (After N. 

 A.Cobb.) 



