96 M.'^JNE AGRICULTURAL EXPERIMENT STATION. I9I4. 



They never produce a browning of the surrounding skin resem- 

 bling decay, nor a pronounced shrinkage of the tubers, and 

 never form large, deep, eroded pits or cankered areas in the 

 tissues. 



While portions of the epidermis may appear turned up, sur- 

 rounding the spots of common scab, this characteristic is by no 

 means so. common or so apparent to the unaided eye as is the 

 case with powdery scab. If the affected potatoes are wet and 

 carefully w^ashed to remove superfluous dirt another distinct 

 dharacteristic can be observed. When wet, the spots produced 

 by powdery scab are invariably darker colored than those of the 

 common type. Common scab is, as a rule, nothing more than ?. 

 surface ulcer, resulting from the irritation of the growth of the 

 parasite in the superficial cells of the tissue. On account of the 

 character of the lesions thus produced the keeping qualities cf 

 the tubers are practically uninjured and the value for food pur- 

 poses is not materially lessened. It simply makes them un- 

 sightly and less easy to prepare for the table. 



Silver Scurf. This disease, which is caused by Spondyloc- 

 ladium atrovirens Harz., has been given the name of "scab" 

 and "dry scab" by some writers. Melhus* who has very re- 

 cently described the nature and distribution of this disease in 

 the United States maintains that we are not justified in applying 

 the name scab to it as we already have two or three other dif- 

 ferent types of scab. Moreover as the disease ordinarily occurs 

 on potatoes it is not a typical scab. Silver scurf is quite easily 

 detected on light skinned varieties like Irish Cobbler and Green 

 Mountain if the tubers are washed. According to the writer's 

 observations the early stages of it appear as small, faint, round- 

 ish, or irregular discolorations on the skin of the tuber, and 

 which may be easily overlooked if the latter is somewhat dirty. 

 These discolored spots are usually from one-eighth to 'one- 

 fourth inch in diameter. Later these may enlarge, run together, 

 and cover considerable of the surface area of the potato. If 

 conditions are moist the spots take on an olive color due to 

 the fruiting of the fungus. In advanced stages the discolored 

 areas change so that they present a silvery or glistening appear- 



*Melhus, I. E. Silver Scurf, A Disease of the Potato. Cir. U. S. 

 D. A. Bu. PI. Ind. 127 : 15-24. 1913. 



