LEGUMES FOR SEED 255 



Erw. Sm.), and the root-knot (Hctcrodcra radicicola (Greef.) 



Mull,). Both occur most commonly on sandy soils, and are 



likely to occur together. The Iron cowpea is recommended by 



the United States Bureau of Plant Industry as resistant to both 



diseases. The variety, however, is rather late in maturing, and 



not especially prolific in seed. The 



cowpea wilt does not attack other 



plants; hence growing other crops 



for two years will give temporary 



relief. The same nematode worm 



produces root-knot on other plants; 



such as, cotton, okra, peaches, and 



certain garden vegetables. Rotation 



of crops is, therefore, less effective; 



but assuming Iron cowpea not to be 



attacked by this disease, the following 



rotation of crops is recommended : 



first year, maize with Iron cowpeas 



between the rows; second year, oats 



followed by either beggar weed, vel- Healthy and diseased roots of cow- 

 pea. The diseased roots on the 



vet bean or Iron cowpeas ; third left show tufts produced by the 

 year, and, if necessary, fourth year, wilt fung j^ ter Orton) 

 cotton. 1 



318. WILT. Infection occurs by the spread of the mycelium through the 

 soil. The fungus finds entrance to the plant through the small roots, develops 

 in the water ducts of stem, small branches, and petioles of leaves. Though 

 the mycelium is white, its destructive work imparts a brown color to the 

 wood. The abundant hyphal growth in the water vessels shuts off the water 

 supply of the plant, and within a day or two after the brown color appears 

 the plant dies. 



319. ROOT-KNOT, also known as root-gall, enters the roots of cowpeas, 

 producing by irritation of plant-cells swollen, bead-like tumors on the roots. 

 It attacks both tap and lateral roots. The forms of the galls vary; the surface 

 is at first smooth, then cracked, scurfy, and more or less decayed. The eggs 

 hatch in the tissue of the old gall from which the nematodes force their way 

 into fresh parts of the same root or find their way out into the soil where 

 they hunt a new root in which to take up existence. The number of nematodes 



*U. S. Dept. Agr., Bu. PI. Ind. Bui. No. 25 (1905), p. 67. 



