224 



Diseases of Economic Plants 



scab tendency. The benefit of treating seed to be used on 

 infested soil varies largely with local conditions. 



Wart'-''^' ^*'^' ^^^ {Synchytrium eridohiotiaim (Schil.) Perc, also 

 known as Chrysophlyctis) . — This disease invaded America 

 (Newfoundland) about 1909, and recently was (Sept., 1918) 

 found in several counties of Pennsylvania; still more recently 

 in Virginia and West Virginia. It has long been known in 



Fig. 123. — Potato infested with powdery- 

 scab. After Melhus. 



England and Europe. Badly diseased potatoes are entirely 

 replaced by a white, warty mass; in milder cases, only a few 

 eyes are affected with either brown or black spots, or with 

 dark warts of size varying from that of a pinhead, upward. 

 Soil once infested must not be used for potatoes, neither 

 may diseased seed be used. Great care should be exercised 

 to prevent wider distribution of this dangerous disease in 

 America. 



Powdery-scab -"*' (Spongospora subferranea (Wallr.) Johns.). 

 — Known in Europe since 1841, this disease was first re- 

 ported in the United States in 1918, prol)ably having been 



