54 
Journal of Agricultural Research 
Vol. XXV, No. a 
Verification experiments are misleading when different methods are 
used. The writers’ method was that previously reported by them 
(. 40, p. 320-26) and included severe bruising of the leaves. The method 
described by Atanasoff (4, p . 19) introduced a drop of juice into a 
split stem, resembling the capillary-tube and split-stem methods also used 
by the writers (p. 74, 83) in 1921 and discarded because of ineffectiveness. 
Even had Atanasoff’s method been similar to the writers’ and given 
negative results, it is well to make allowance for climatic factors. Nega¬ 
tive results with aphid inoculations of leaf roll by Murphy {29, p. 32) did 
not prevent later proof, and probably would not have been weighed by 
him more heavily than his negative results with mosaic and insects 
(29, p. 62) had he not been emphasizing the soil-transmission theory 
advanced by the Dutch workers but later abandoned by them. 
After tuber perpetuation, the progeny of the plants inoculated in 
1921 were used in inoculations described later in the section on “Inocu¬ 
lations performed in 1922,” and the same positive results were obtained, 
with spotting of the white corollas in addition. 
LEAF roll and net necrosis 
Leaf roll, as previously described by the writers ( 41 , 75), is character¬ 
ized chiefly by dwarfing, chlorosis, rolling, uprightness, and rigidity; 
burning also is common (PI. 6, C, 2). Phloem necrosis is a microscopic 
symptom of great value for positive identification (39 , p. 132). Leaf roll 
is less easily transmitted than any other degeneration disease considered 
in this paper (29, p. 63). The size of the tubers is reduced. Streaking 
(from phloem necrosis) in the tubers (net necrosis) previously reported 
as a symptom by the writers (41, p. 60-74) an d by Gilbert (17) and Kasai 
( 20 , p, 32 , 69) has been noted again as a symptom in plants grown in 1921 
and 1922. This tuber symptom of leaf roll was reported (41, p. 68) as 
being sometimes induced by varietal and other factors, but did not neces¬ 
sitate {41 y p . 73) long storage as suggested by Quanjer (39 , p. 132 ), and 
was not a constant symptom as he understood it. The observations 
made in 1921 on net necrosis are given in a later section (p. 64). 
Inoculations within the Green Mountain variety have been reported 
as giving positive results with stalk grafts and aphids (41 , p. 32 , 34-33 , 
57-59). Intervarietal inoculations like these, and also unsuccessful at¬ 
tempts to transmit leaf roll by leaf-mutilation inoculation, will be de¬ 
scribed later (p. 63). 
In the winter of 1921 and 1922, an experiment on the effect of healthy 
potato plants grown in contact with tuber and vines diseased with leaf 
roll (with insects excluded) was conducted at Washington, D. C. This 
test was conducted in the same manner and at the same time as the 
experiment on the effect of growing healthy potato plants in contact with 
mosaic plants. In this experiment nine healthy half tubers were planted 
in contact but not grafted with leaf-roll half tubers, while each of the 
remaining six healthy half tubers was planted 6 inches apart from the 
leaf-roll half tuber in the same 10-inch pot. Three observations on the 
vines during the course of the experiment revealed that all vines from 
leaf-roll tubers showed leaf roll, while the vines from healthy tubers 
remained free from leaf roll. Second-generation plants from the healthy 
tubers likewise were free from leaf roll, as microscopic as well as macro¬ 
scopic examinations disclosed. 
