orton: pathology of solanum tuberosum 189 



remain short and with curly leaves, the compactness of the plants 

 is more striking. 



The color of the fohage in curly-dwarf is typically normal 

 green. The tuber yield is greatly curtailed. Severe cases have 

 no tubers. In others, a few small potatoes are formed. The 

 hereditary nature of the trouble is attested by the German authori- 

 ties and has been observed by the writer. 



In the United States curly-dwarf plays a larger role in the 

 deterioration of our potatoes than in Europe. It must be re- 

 garded as a physiological disorder, which crops out in previously 

 healthy stocks, under conditions not yet known to us. Once 

 developed, it is apparently not possible to restore the vigor of the 

 affected hills. 



Examination of a variety or seedling collection shows that 

 there are all grades of the condition above described from pro- 

 nounced types of curlj-dwarf to those approaching normal vigor. 

 It will furthermore be apparent that this is a difference inherent 

 in the varieties or strain under observation. 



Both leaf roll and Qurly dwarf develop suddenly from hitherto 

 health}^ stocks and both are transmitted by planting tubers from 

 diseased plants. That whole districts should be affected as in 

 Westphalia in 1907 and in Colorado in 1911 indicates a physiologi- 

 cal deterioration due to environmental relations, unless a parasite 

 should be demonstrated, which has not yet been done. 



The climatic charts presented show that there is a great defi- 

 ciency of moisture in Germany and Colorado in comparison with 

 Chili. Is it possible that under these conditions varieties of 

 potatoes may lose their vigor and undergo physiological changes 

 comparable with those already noted for southern grown seed? 



That the leaf roll disease is being brought under control in 

 Germany by the use of healthy seed potatoes from outside the 

 affected districts supports these hypotheses and lends still more 

 strength to the argument for potato breeding for a higher degree 

 of climatic adaptation. 



Altho nothing like the present outbreak of leaf roll has occurred 

 during the last forty years, an examination of old literature 

 shows that about 1770 and in subsequent years there were epi- 



