orton: pathology of solanum tuberosum 



185 



There is another group of apparently physiological or inheri- 

 table potato diseases, of which ''leaf roll" and "curly dwarf" are 

 most important, which I bring into this discussion because they 

 are probably deterioration phenomena connected in some way not 

 yet fully understood with the effect of unfavorable environmental 

 conditions. 



The great losses that have been caused by these diseases both 

 in Europe and America have caused much alarm and led to much 

 investigation and discussion. The suggestion that our potato 



■ No of Years CountBd 



Fig. 3. Average monthly rainfall for four potato centers in the United States. 

 Data from U. S. Weather Bureau. The heavy upper border of the shaded area 

 shows the mean rainfall for the number of years indicated at the base of the 

 column. The figures within the squares in the shaded area show the number of 

 years within the period of record when the rainfall has been below the amount 

 indicated. The corresponding figures in the white squares show the number of 

 years during the period of record when the rainfall has exceeded the amount 

 indicated. 



varieties may suffer rapid deterioration thruout extensive dis- 

 tricts and without discoverable cause is naturally provocative of 

 uneasiness. While the nature and cause of ''leaf roll" and related 

 troubles is still largely a matter of speculation, the haze is parti- 

 ally cleared by the separation of this complex into at least five 

 different troubles, three of which are attributable to known para- 

 sites. 



As pointed out by the writer in December, 1911, in a paper 

 before the American Phytopathological Society, the following dis- 

 eases have been more or less confused by various observers: 



