orton: pathology of solanum tuberosum 187 



In general the yield is very much reduced. The diseased hills 

 have numerous tubers very much smaller than normal so that 

 the yield is only about half that of a healthy field. If one uses 

 these potatoes again for seed, the greater part fail to develop, 

 and an uneven stand is the result. The stronger tubers succeed 

 in growing, but the stem remains weak, the leaves are from the 

 beginning considerably rolled and more or less colored. Few or 

 no tubers are found in such hills, so that a complete crop failure 

 results. Stem end browning of tubers is no longer considered a 

 reliable evidence of leaf roll. 



The true leaf roll is inheritable. The tubers from diseased 

 plants produce diseased progeny as a general rule. This affords 

 a means of distinguishing from genuine leaf roll those temporary 

 conditions which give rise to a similar appearance of the plants. 

 It is now quite generally admitted that the presence of fungous 

 mycelium is not a character of the leaf roll. The leaf roll diseased 

 plants in America have been free from fungous infection. 



The leaf roll disease of potatoes first came into the public eye 

 in Europe in 1905 in Westphalia. In 1907 a more general out- 

 break occurred in Germany and much alarm was expressed. Its 

 occurrence is certain in Germany, Austria-Hungary, Switzerland, 

 the Netherlands, Denmark, Norway and Sweden, as well as in the 

 United States. 



Two developments of leaf roll in this country have been studied. 

 One in a collection of seedlings grown by the Bureau of Plant 

 Industry, the other a destructive outbreak in eastern Colorado 

 and western Nebraska during 1911 and 1912, which was the cause 

 of immense losses, the shipments from one district falling from an 

 expected 7000 cars to 200 cars. 



The seedling potatoes showed every degree pf variation in 

 plant characters, and in addition many showed distinct evidence 

 of the diseased condition herein described as leaf roll. It is note- 

 worthy that in neither field was there any trace of Fusarium wilt, 

 nor of Verticillium wilt, ''black leg" or "mosaic," altho the latter 

 three were common in adjoining fields. The evidence indicates 

 that leaf roll and curly leaf are manifestations of physiological 

 weakness and associated with decline or loss of vigor of the strain. 



