44 U. S. DEPARTMENT OF AGRICULTURE. 



Silver scurf is a skin disease which is purely superficial, 

 and does not penetrate below the cork layer. The infected 

 areas are marked by their silvery appearance, which is espe- 

 cially pronounced if the tubers are washed. In late stages 

 of the disease, the affected skin becomes wrinkled and 

 sloughs off. 



Generally, affected areas occur near the stem end, but 

 they may be found over the entire surface. Diseased areas 

 vary from one-fourth inch to one inch or more in diameter. 



In storage, especially under moist conditions, a sooty layer 

 composed of the spores of the fungus may be found on the 

 diseased areas. 



Infected stock is fit for table use. 



Ref. (43); (57). 



POTATO: WART. 



Cause: A fungus (Chrysophlyctis endobiotica). 



In its early stages, wart is characterized by small, warty 

 outgrowths which usually occur at the eyes of the tuber. 

 These warts later become very large, and in severe cases, 

 cover or involve the entire tuber. They are rough and black 

 in color. 



This disease is serious in certain parts of Europe, and has 

 recently made its appearance in the United States (Penn- 

 sylvania). It is a very destructive field disease, and may be 

 carried with diseased seed tubers. Once having gained a 

 foothold, the fungus persists for long periods in the soil. 



Infection takes place in the field while the tubers are 

 growing. The disease does not progress in storage, and does 

 not spread to healthy tubers. Diseased stock is very subject 

 to secondary rots, and in this way it is a menace to healthy 

 stock. Slightly affected tubers may be used for table 

 purposes. 



Precaution against the use of diseased tubers for seed 

 purposes is of prime importance in the prevention of this 

 disease. With this end in view a National quarantine has 

 been established. 



Ref. (35); (63). 



POTATO: COMMON SCAB. 



Cause: A fungus (Actinomyces scabies). 



Scab is characterized by rough corky elevations, or by 

 pits. If tubers are attacked early, the pits are deep ; if late, 

 they may be shallow and superficial. In the early stages, 

 affected tubers are marked with minute, reddish or brown- 

 ish surface lesions. Cork formation usually occurs under- 

 neath these lesions. Often the lesions coalesce and in severe 

 cases the entire tuber may be covered by a rough incrusta- 

 tion. Scabby potatoes are little if any more disposed to 

 decay than clean potatoes. 



