Potato Breeding 73 



of the wild forms. The stress of uncongenial environment 

 and hirge production of tubers causes most varieties to lose 

 this abnormal (from the standpoint of the plant) produc- 

 ing power and to decline unless selection is constantly- 

 practiced. Disease may be partially responsible for this 

 degeneration. 



Running out of varieties is often caused by repeated 

 planting of culls and poorer specimens while the best are 

 sent to market. 



Newman ^ says, that in vSweden there is a potato known 

 as the Dala, which is said to have been introduced about 

 150 years ago and is still one of their best sorts. The 

 opinion held at Svalof is that there is no period of old age 

 in a variety of potatoes ; that where suitable sorts are 

 used, and where suitable tubers of these sorts are used for 

 seed purposes each year, the planting of the variety may 

 be maintained indefinitely under all favorable conditions 

 of soil and climate. The main considerations are mainte- 

 nance of vigor and control of disease, and this implies 

 careful selection of seed tubers, careful cultivation and 

 spraying and rotation of crops. Special emphasis is 

 laid upon the latter point. 



Orton sa}^s : " The same system of seed selection and 

 treatment and crop rotation that will free the potato 

 fields of wilt, leaf-roll, and curly-dwarf will at the same 

 time not only bring under control the blackleg and some 

 other diseases but will insure the maintenance of the 

 strains cultivated in their most vigorous and productive 

 condition and be free from objectionable mixtures with 

 other varieties." 



The following statement from Fruwirth is representa- 



• " Plant Breeding in Scandinavia." Canadian Seed Growers Associa- 

 tion, Ottawa (1912). 



