POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 39 



OCCURRENCE AND DISTRIBUTION. 



In Europe curly-dwarf is apparently not sufficiently common to 

 have any economic significance. The literature on this subject must, 

 however, be interpreted with an understanding of the confusion of 

 terms among the older authors, who often used the word ''Krausel- 

 krankheit" as a collective term for curly-dwarf, leaf-roll, bacterial 

 ring disease, and still others (Appel and Schlumberger, 1911; Frank, 

 1897; Kiihn, 1859). Evidence is also found in the old English 

 literature that a varietal deterioration called ^'curl" was frequent 

 even in the nineteenth century (Dickson, 1814; Shirreff, 1814; 

 Townley, 1847; Foster, 1905). It is impossible to know whether 

 this trouble was leaf -roll or curly-dwarf; but the thought suggests 

 itseK that there have been periods, or cycles, of decline in potato 

 varieties, followed by the rejuvenation due to introduction of new 

 sorts. It may be that such a period of decline is now beginning, as 

 manifested by the appearance of leaf-roll and similar troubles in the 

 principal potato coruitries during recent years. 



In the United States it is probable that curly-dwarf plays a large 

 r61e in the deterioration of potatoes. It is commonly met with in 

 New England and New York fields, though not always recognized, as 

 the larger plants overshadow and conceal the weaklings. The writer 

 has sought this type of deterioration in potato fields in many States 

 from Maine to California, and has foimd it to be not infrequent in 

 occurrence, but that its presence in the average field is limited to 

 scattered plants, usually less than 2 per cent. Field-to-field inspec- 

 tion in important potato districts has, however, resulted in the dis- 

 covery of some fields where a larger percentage, even half or more of 

 the plants, showed curly-dwarf. Some of these fields showed weak- 

 ness in other ways, through failures to germinate, blackleg, mosaic 

 disease, and general lack of vigor. 



In another instance a strain of potatoes was being grown by a pro- 

 gressive, careful farmer, who had adopted the hill-selection method 

 to increase vigor and productiveness, yet a considerable proportion 

 went down with curly-dwarf in 1913, three years after the selection 

 ceased. The 1912 crop was normal in appearance but was subjected 

 to severe drought. This is of interest in connection with the belief 

 that prevails in some quarters that dry years induce this type of 

 trouble. The same grower shipped a portion of his 1912 crop to a 

 southern State for the early spring planting, and ranch curly-dwarf 

 appeared in the fields of the purchaser. 



It seems evident that this is a physiological disorder, resulting in 

 a permanent deterioration of the stock. It may develop at any time 

 as a result of conditions not yet fully understood, and the vigor of the 

 affected strain apparently can not be restored. 



