8EED AND PLANT DISTRIBUTION. 67 



DISEASES OF THE COWPEA. 



The Iron cow pea is most noteworthy for its resistance to thecowpea 

 wilt disease and root-knot. A full description of these diseases, with 

 accounts of experiments with remedies, etc., is published in Bulletin 

 No. 17. Bureau of Plant Industry, Tinted States Department of Agri- 

 culture, and will be sent free on request. A brief account is given 

 here to enable the farmer to recognize these troubles. 



THE COWPEA WILT. 



The wilt of the cowpea is common only on light or sandy soils, and 

 occurs principally on land where cowpeas have been grown for several 

 years. It appears about August in spots of varying size, which 

 spread gradually over the field. The plants in these areas turn yel- 

 low, lose their leaves, and die. The stems have a reddish-brown 

 tinge, and, when broken, the inside will also be found discolored. 

 Later, these stems become covered with the light-pink spores of the 

 fungus which causes the disease. This fungus enters the roots from 

 the soil and, growing upward, tills the water-carrying vessels of the 

 stem with its threads, thus shutting off the water supply and causing 

 the death of the plant. 



Remedies. — The cultivation of the Iron pea is the best means of 

 relief, as it will grow where all other kinds fail. Rotation of crops 

 for two years will give temporary relief, or since the disease does not 

 attack any other crop than the cowpea. velvet beans or other legumes 

 may be substituted. 



ROOT-KNOT. 



Root-knot, like the wilt, is most injurious on sandy soil, and the 

 two diseases are often found occurring together. It is caused by a 

 minute nematode, or eel worm, which enters the roots and produces 

 large, irregular swellings or galls. These very injurious enlargements 

 should not be confused with the bacterial tubercles found on all healthy 

 cowpea roots. The latter are small and regular in form and greatly 

 benefit the plant by enabling it to draw nitrogen from the air. The 

 accompanying figures illustrate this distinction. A few bacterial 

 tubercles appear on the healthy roots in PI. Ill, tig. 2, while the roots 

 in fio - . 3 are deformed bv root-knot. 



Root-knot is also produced on several other plants by the same 

 nematode that attacks cowpeas. Cotton, okra, peaches, and most 

 garden vegetables are greatly injured by it. This is the most serious 

 feature of the disease, since the cultivation of the ordinary varieties 

 of cowpea on nematode-infected land so greats increases the number 

 of the parasites in the soil that succeeding cotton or other crops are 

 much injured. 



