VASCULAR DISCOLORATION OF IRISH POTATO TUBERS 



By H. A. Edson 



Pathologist, Office of Cotton, Truck, and Forage Crop Disease Investigations, Bureau of 

 Plant Industry, United States Department of Agriculture 



INTRODUCTION 



The exact significance of vascular discoloration in the stem-end 

 tissues of Irish potato tubers has never been fully determined. Various 

 types of both flesh and vascular necrosis are iecognized, some of which 

 are associated with the presence of Fusaria of various species or with 

 Verticillium albo-atrum. Others, however, at least in the initial stages, 

 yield no organisms when subjected to culture, nor does the microscope 

 reveal the presence of organisms. It is also recognized that a superficial 

 necrosis may develop in the stem tissues of apparently perfectly normal 

 stock. There is no such perfect natural abscission of the potato tuber 

 from the stolon as is common with fruits. Moreover, they are frequently 

 harvested before the plants are mature, and the tubers are then broken 

 off from green stolons. It has been assumed that suberization of the 

 wound thus made normally follows in two or three days, so that not 

 more than a few layers of dead cells should appear unless some aggressive 

 parasite gains entrance to the wound. A popular impression has pre- 

 vailed that any except the most superficial stem-end discoloration might 

 be taken as a trustworthy indication of the presence of Fusarium, or, at 

 least, that the stock was grown on vines affected with Fusarium or 

 Verticillium. 



Somewhat extensive preliminary observations and cultural studies, 

 made by the writer both at the time of harvest and during or at the 

 close of the rest period, on stock grown in sections where Fusarium blight 

 and wilt do not occur, as well as in sections where they are known to be 

 general, show that, while Fusarium and Verticillium undoubtedly do 

 cause vascular discoloration of potato tubers, discoloration can not be 

 accepted as proof of the presence of Fusarium or, indeed, of any other 

 organism, nor can the absence of discoloration be confidently accepted 

 as proof of the sterility of the vessels near the stolon attachment. There 

 seems to be reason to think that vascular necrosis may often arise from 

 purely physiological causes and that it need not necessarily be seriously 

 abnormal, though frequently it is. A more complete discussion of this 

 question must await the outcome of studies at present incomplete, but 

 it seems advisable to present some available data regarding the fungous 

 flora of potato stem ends. 



Journal of Agricultural Research, Vol. XX, No. 4 



Washington, D. C. Nov. 15, 1920 



vo Key No. G.-208 



(277) 



