36 DISEASES OF FIELD & GARDEN CROPS. [CH.VI. 



at the time of digging, were apparently quite sound. In 

 bad cases small discoloured spots first appear on the bark 

 of the tuber ; these increase in size and become confluent, 

 till at length the entire skin of the potato is discoloured. 

 The cuticle then bursts in many places, and the olive-green 

 spores are liberated. Like many other plant diseases, 

 potato smut is in some seasons rare, at other times it is 

 very common and destructive. No potatoes showing 

 traces of smut should be planted as seed. 



