LEAFROLL, NET-NECROSIS, AND SPINDLING-SPROUT 
OF THE IRISH POTATO 1 
By E. S. Schultz, Pathologist, Cotton, Truck, and Forage Crop Disease Investigations, 
Bureau of Plant Industry, United States Department of Agriculture, and Donald 
Folsom, Associate Plant Pathologist, Maine Agricultural Experiment Station 
INTRODUCTION 
Potato ( Solatium tuberosum L.) leafroll of the type that is transmissible 
but that has not been proved to be parasitic has been differentiated from 
other types by Appel (j) 3 , Orton (15), Wortley ( 24 ), and others, and 
designated as phloem-necrosis by Quanjer (16, 17). It is considered 
by Quanjer ( 17, p. 41) to be an infectious disease caused by a virus or 
ultramicroscopic parasites. The absence of data concerning its cause 
and concerning natural and artificial means of its transmission has made 
it difficult to distinguish sharply between it and various other leafroll 
diseases of the potato and has resulted often in confusion and con¬ 
flict in the literature (3; 15, P- 3, *9, 25, 34! 'T> 2 4 )- The reviewing of 
this literature has been done previously ( 15, 17, 18) and is beyond the 
scope of this paper, which is concerned chiefly with symptoms and 
transmission. 
SYMPTOMS 
LEAFROLL 
The principal macroscopic characteristics of the type of leafroll 3 
considered in this paper include rolling, rigidity, brittleness, leathery 
texture, light green, yellowish, reddish, or purplish discoloration of the 
affected leaves; dwarfing of the vines, which usually remain erect and 
rigid; shortness of stolons; reduction in the number and size of tubers; 
and rigidity or woodiness of the tubers. Some of these characteristics 
may be absent at times. The rolling may appear upon the upper leaves 
alone or only upon the lower leaves or throughout the entire vine. It 
consists of an upward curving of the sides of each leaflet with the midrib 
at the bottom of the trough thus formed. (See PI. 1, A; 2, A.) Its 
extent depends largely upon the time when infection occurred and 
upon the age of the affected plant. Plants infected relatively late in 
their development may show rolling in the upper leaves only, while those 
1 This paper is based upon investigations carried on as a cooperative project between the Office of Cotton, 
Truck, and Forage Crop Disease Investigations of the Bureau of Plant Industry, United States Depart¬ 
ment of Agriculture, and the Department of Plant Pathology of the Maine Agricultural Experiment 
Station. Unless otherwise indicated the work was performed in northeastern Maine in the vicinity of 
Presque Isle. The order of arrangement of the authors* names is not intended to indicate that one 
cooperating institution contributed more than the other to the results. 
8 Reference is made by number (italic) to “ Literature cited,” p. 78-80. 
b Throughout this paper the term " leafroll” is employed as an abbreviation of “ apparently nonparasitic, 
transmissible leafroll” and as a synonym of “ phloem-necrosis.” 
Journal of Agricultural Research* 
Washington, D. C. 
x h 
Vol. XXI, No. 1 
Apr. 1, 1921 
Key N o. G-222 
(47) 
