188 orton: pathology of solanum tuberosum 



The hypotheses as to the cause of leaf roll are numerous and 

 varied. It has been argued by one that leaf roll results from the 

 use of unripe tubers for seed ; by another, that it is due to the em- 

 ployment of matured tubers ; while a third believes that seed from 

 prematurely ripened plants is a cause of leaf roll. . 



Hiltner is the leading advocate of the theory that the immature 

 seedstock gives an abnormal growth. He limits this to those 

 potatoes which are prematurely ripened by drouth or other unto- 

 ward circumstances. Hiltner further holds that leaf roll may be 

 the result of an overconcentration of salts as thru excessive 

 applications of fertilizer, of unbalanced composition and applied 

 at the wrong time. 



The first appearance of leaf roll in Germany was on the variety 

 Magnum Bonum and was considered as an evidence of varietal 

 deterioration. It seems certain, however, that leaf roll is not 

 a result of "running out" of varieties thru old age, for many strains 

 originated recently are affected. Its occurrence in seedlings has 

 been observed by several workers. An interesting suggestion 

 is put forward by Hedlung that leaf roll is a pathological adapta- 

 tive mutation, and further, that since acquired characters are not ' 

 inherited the leaf roll character must be latent in normal potatoes. 



The introduction of new and more vigorous varieties affords 

 a hopeful means of ultimately controlling the situation. 



Under the name "curly-dwarf" there is to be differentiated 

 from the leaf roll a peculiar disorder known in Germany as 

 " Krauselkrankheit." This is characterized by a dwarfed develop- 

 ment of the potato plant, accompanied by a pronounced curling 

 and wrinkling of the foliage, which has been compared with Scotch 

 Kale and with Savoy cabbage. The stem and its branches, 

 the leaf petioles and even the midribs and veins of the leaves all 

 tend to be shortened in many cases to a very marked extent, par- 

 ticularly in the upper nodes of the plant, so that the foliage is 

 thickly clustered. The diminished growth of the leaf veins, in 

 proportion to the parenchyma, results in a bullate, wrinkled leaf, 

 often strongly curled downward. There seems also to be a tend- 

 ency to form more secondary branches than normal, and as these 



