250 Diseases of Economic Plants 



fectual. Losses range anywhere from 10 to 95 per cent of 

 the crop, and the total annual loss is at least three-quarters 

 of a million dollars. Healthy potatoes must be used for 

 seed; the hotbed should occupy uninfested soil, and rotation 

 should be practiced. The suggestions given under soil dis- 

 eases apply here. 



Minor diseases 



Dry-rot {Diaporthe batalatis H. & F., Photnopsis) . — The 

 affected part, often the upper end of the potato, becomes 

 dry, wrinkled, and covered with numerous pycnidia, the 

 flesh within changing to an almost powdery condition. 

 Affected potatoes should be destroyed. Java dry-rot ''"'' 

 (Diplodia iiibericola (E. & E.) Taub.). ^ The potatoes show 

 dark, shriveled patches with scattered pycnidia. The inner 

 tissue is brittle and black. Scurf ^'^-^ (Monilochcetes infuscans 

 E. & H.). — A brown or rusty coat often forms over the 

 whole potato. It causes no decay, but does result in shrink- 

 ing, loss of volume, and vmsightliness, which reduces the 

 money value of the crop. Blue-mold rot (Penicillium expan- 

 sum). — ^ The diseased parts, which may involve the whole 

 potato, are of almost chalky color and consistency, dry, and 

 inoffensive in character. 



White-rust {Albugo ipomoece-pandurana; (Schw.) Swingle). 

 — White, glistening sori, 1 nun. or so in diameter, appear 

 upon the under sides of the leaves and upon the stems. 

 These rupture and set loose a mass of powdery, snow-white 

 spores. The tissue surrounding the sorus is pale, or yellow, as 

 seen from the opposite side of the leaf. Later, brown patches 

 result. 



Leaf-blight {Phijllosticta batatas Cke.). — Spots of brown, 

 dead tissue from 1-10 nun. in diameter, round or angular, 

 are frecjuent upon the leaf. Thej" are very definitely bordered 

 by a dark Ijand, and bear numerous pycnidia, visible to the 

 naked eye. In many instances these spots become so nu- 

 merous as to cause the whole leaf to yellow and fall, and thus 

 reduce the crop. 



