POWDERY SCAB OF POTATOES. 95 



disease which the writer has not had opportunity to trace from 

 the beginning, and such European autfhorities as have been 

 consulted do not state whether it results from either the warty 

 stage just described, or from the running together of the indi- 

 vidual spots of the scab or blister stage in severe attacks, or 

 from both. 



A few other characteristics of the disease as observed in 

 Maine should be noted. The most important is, perhaps, that 

 potatoes affected by powdery scab, especially in severe or mod- 

 erately severe cases, show a tendency to wither rapidly. This 

 takes place even under good storage conditions, and it becomes 

 very evident if the tubers are removed from the cellar and kept 

 at the temperature of the average living room for a few days. 

 Very frequently the skin of the potato in a circle immediately 

 surrounding the individual scab spots becomes depressed and 

 browned, taking on a co'ior very similar to that produced by 

 the common dry rot associated with the late Wight fungu'^. 

 This similarity to the late blight dry rot is greatly increased if 

 the spots of powdery scab are numerous and dose together so 

 that the discolored area of tihe skin becomes continuous. In 

 such cases the tissues dry out and shrink away very rapidly 

 beneath the discolored area, giving every appearance O'f a form 

 of dry rot. Cutting through these blackened areas of the sur- 

 face usually reveals simply a thin layer of dry, dead tissue rest- 

 ing on the apparently sound and healthy flesh of the tuber be- 

 low. Frequently a distinct stratum of dry rot has been found 

 beneath some of these discolored areas but whether or not this 

 is due simply to secondary infection by some fungus I am 

 rot yet able to say. 



Diseases Which May be Mistaken for Powdery Scab. 



Common Scab. No doubt the chief reason that powdery 

 scaib was not recognized earlier by the potato growers in tlie 

 sections where it is now known to occur is that they failed to 

 distinguish the disease from the ordinary scab which has been 

 common and widely distributed for many years. In comparing 

 the two it may be said in the first place that the lesions produced 

 by the common scab are on the average much larger. They 

 are apt to be more irregular in size and shape and never appear 

 as distinct warts or pustules containing a brownish powder. 



