POTATO IMPROVEMENT 231 



toes has been traced to certain plant diseases (Stewart, 1916; 

 Orton, 1914) which have been variously named as leaf roll, 

 mosaic, and curly dwarf. Quanjer (1920) has presented evidence 

 to show that these three diseases may be stages of the same 

 disease, which is transmissible from plant to plant. Similar 

 results have been obtained at University Farm. 1 The disease 

 is called " mosaic dwarf" by Krantz and Bisby in unpublished 

 investigations. That rejuvenation of a variety is possible 

 through its introduction and growth under a more favorable 

 environment is illustrated by studies which have been carried on 



FIG. 57. Progeny of single tubers as grown at University Farm, 1918. 

 Some tubers give vigorous progeny, others produce only small, weak, degenerate 

 plants. (Courtesy of Krantz.) 



cooperatively between the Division of Horticulture of the Min- 

 nesota Central Station and the sub-stations. Yields for Min- 

 nesota No. 2 at University Farm for 1914, 1915 and 1916, 

 respectively, were 196, 169 and 22 bu. This shows the rapid 

 reduction in yield which is obtained by the continued use of 

 tubers saved at University Farm. Tubers of Minnesota No. 

 2 saved from the 1916 University Farm plot gave a yield cf 

 170 bu. at Duluth in 1917. Tubers from this Duluth plot yielded 

 300 bu. at Grand Rapids in 1918. Whether a badly diseased 



1 Data on running out and on field experiments in Minnesota were 

 furnished by F. A. KRANTZ of the Division of Horticulture, Minnesota 

 Experiment Station. 



