394 



DISEASES OF ECONOMIC PLANTS 



the Southern States from North CaroHna to Florida, and 

 west to Texas, and is yearly noted in new localities. 



Owing to the all-important position the cowpea occupies 

 in southern agriculture as a nitrogen crop, where the best 



rotation demands its 

 frequent recurrence 

 upon the same soil, the 

 disease is of pecuhar 

 moment. 



No remedy is known 

 except the use of the 

 iron cowpea, the re- 

 sistance of which was 

 noted by Mr. T. S. 

 Williams of South Car- 

 olina in 1900, and fur- 

 ther tested by W. A. 

 Orton of the United 

 States Department of 

 Agriculture. 



Leaf spot {Amerosporium ceconomicum Ell. & Tracy). 

 — This disease is readily distinguished from cercosporose 

 by the fact that the spots are circular, are of shiny white- 

 ness, and are studded all over with httle black pycnidia, 

 smaller than an ordinary pinhole. Fig. 170. The disease is 

 often abundant, but is usually not so serious as cercos- 

 porose. 



Powdery v:uldew (Erysiphe polygoni DC). — The white 

 spots of this mildew are strictly circular when young, but 

 as they enlarge they coalesce and cover the whole upper 

 surface of the leaves with a white powder. 



Fig. 171. — Cowpea leaflet ST)ttoi wHli poW' 

 dery mildew. Ori.-^inal. 



