Diseases op the Potato. 309 



itself is to be strengthened, and the removal of as many injurious 

 influences as posible is desired. 



There appears to exist another trouble which is generally con- 

 founded with the early blight; in fact it is very difficult to distin- 

 guish the two without the aid of laboratory methods. This dis- 

 ease, which is entirely of a physiological nature, has been thor- 

 oughly discussed by Sturgis.* It has been found only to a lim- 

 ited extent in New York, for almost invariably cultures made 

 from affected leaves freely developed the early blight fungus. 

 According to Sturgis the same conditions which produce the early 

 blight also bring about the death of certain parts of the leaf tissue 

 in a manner almost identical with that of the macrosporium. He 

 says: "Both classes of injury appeared in connection with the 

 marks of the flea-beetle. Both were exhibited as brown spots and 

 blotches marked with concentric rings; but in specimens charac- 

 terized by the presence of the fungus, the spots were more sharply 

 defined and darker in color. This difference was sufficiently 

 marked to enable a close observer to distinguish either one in the 

 field after a little practice, but a comparison between the two pro- 

 duced the impression that both classes of injury might have been 

 caused by the same agency, and that the slight difference in 

 appearance might be due to the fact that in the one case a fungus 

 had ocupied the injured tissue, and in the other had not done so." 



It seems, therefore, that the causes which lead to the appear- 

 ance of these two diseases are the same, and the same lines of 

 treatment are consequently indicated. 



3. Potato scab (Oospora scabies, Thaxter). — This disease is one 

 which is well known to potato growers. Figure 99 represents 

 affected tubers. The uneven, warty growths upon the surface 

 of the potatoes are composed of material produced by the tubers 

 in consequence of the irritation of parasitic organisms which live 

 upon the substance of the potatoes. Under favorable circum- 

 stances these injuries are very extensive, for the entire surface of 

 the tuber may be affected, and although the affected parts do not 

 always penetrate very deeply, the blemished appearance of the 

 tubers and the actual loss of material may become very serious. 



*Aim. Rcpt, Conu. Agric. Exp. Sta. 1894, 127-134. 



