REPORT OF THE SOCIETY 45 



PHYSIOLOGICAL DISEASES OF POTATOES 



Dr. M. F. Barrus, Cornell University 



Potatoes are so high priced this spring that a discussion of any phase of 

 culture looking toward greater production has an interest not only to scien- 

 tific men and to farmers but to the general public as well. A large amount of 

 investigational work on potatoes with special reference to disease is being 

 conducted by various agricultural institutions and as a result new ideas are 

 being formulated and new aspects of the problem continually opened up. 

 The scope of investigations is indeed so large that the writer is content to 

 present only a small part, and that as the result of observation and study 

 rather than active participation in research of this kind. 



The high prices that potatoes have commanded this winter have in- 

 duced many farmers to sell until they are short of seed or have reduced the 

 quality of that which they have reserved. The principal reduction in 

 quality has been in size of seed, for the larger tubers being more marketable 

 will have been sorted out and sold. It is doubtless unnecessary to point out 

 to you, as should be done generally to farmers, that diseases transmitted by 

 the tuber are often responsible for a large number of the small tubers. When 

 one uses for planting tubers large and small as they run, the danger of 

 increasing the percentage of these diseases is not great, but when marketable 

 ones are sorted out and the remainder is used for seed the chances of securing 

 a large proportion of tubers that came from diseased plants is very great. 

 Such affected tubers are sure to give a disappointing yield. The matter 

 would not be so serious were it possible to detect readily these tubers from 

 healthy ones, but the very nature of the diseases, as will be seen, makes this 

 impossible except in a few cases. 



Of the more serious diseases affecting the plant so as greatly to reduce 

 the yield are the so-called "physiological troubles" or constitutional diseases, 

 leaf roll, curly dwarf, and mosaic. No attempt will be made to describe 

 them in detail in this paper, but their names serve to do this to some extent. 

 In each case the foliage lacks the deep green colour and normal spread of 

 leaves that healthy plants have. The cause of these diseases is to a large 

 extent a matter of speculation among scientists. They seem to be much 

 related especially in certain cases and may be varied manifestations of the 

 same disorder. That affected plants give reduced yields is evident from 

 the average smaller size of the tubers they produce. Such tubers show no 

 indication of disease and are apparently in no way different from healthy 



