THE MODERN POTATO PROBLEM 237 



plants may pass through the soil to healthy plants as 

 far as two or three yards away. More recently the 

 transmission of leaf-roll from one plant to another by 

 aphids, or plant-lice, has been demonstrated. Other 

 means of transmission are yet unknown. Healthy 

 growing plants that become infected with the virus of 

 either of these diseases respond rather slowly to the in- 

 fection, making it difficult to recognize the symptoms 

 in the first crop. Since the virus is carried in the tuber 

 these diseases are truly hereditary, and they become 

 progressively more severe with each succeeding genera- 

 tion. The impossibility of reaching the virus by seed 

 treatments coupled with the difficulty of recognizing 

 the symptoms in their incipient stages, greatly add to 

 the seriousness of the increasing prevalence of both the 

 leaf-roll and mosaic diseases. 



Of the physiological diseases of the potato, the so- 

 called spindling sprout disease is probably the most 

 important. However, to speak of a spindling sprout 

 disease is misleading, since the weak spindling sprouts 

 are merely a symptom which may have a variety of 

 causes. The true spindling sprout diseases referred to 

 here appears to be a response to unusually hot and 

 possibly dry mid-summer conditions when the tubers 

 are forming. The only reliable visible symptom of the 

 disease in the tuber is the spindliness of the sprouts and 

 the frequent appearance, in severe cases, of small tubers 

 at the base of these sprouts. Studies on this disease 

 have revealed some characteristic physiological and 

 chemical conditions of the spindling sprout tubers, but 

 it is impossible yet to say whether any one of these con- 



