Apr. i, 1921 
Leafroll, Net-Necrosis, and Spindling-Sprout 
65 
A close relation between the two diseases is manifested by facts given 
in Table VII and elsewhere in this paper. Net-necrosis is never present 
in lots free from leafroll, is never ifiore abundant than leafroll in lots con¬ 
taining both diseases, and is restricted to the tubers transmitting leaf- 
roll. This is in contrast with the usual independence of mosaic and 
leafroll among lots selected as healthy. Consequently there seems to be a 
closer relation between net-necrosis and leafroll than between mosaic and 
leafroll. Thesefactsare hard to explain if net-necrosis is considered to be a 
disease separatefrom leafroll, but they are easily reconciled with the theory 
that the leafroll virus may cause net-necrosis under certain conditions. 
This theory may seem to be discredited by the following facts. Leaf- 
roll hills usually produce small tubers, but net-necrosis often is found in 
large ones. Also, net-necrosis often disappears entirely or almost so 
from a stock while the leafroll remains. Both these questions will be 
considered in the discussion upon the effect of various factors upon the 
appearance of net-necrosis, where it is shown that large net-necrosis tubers 
may come from hills that contracted leafroll late in the season and that in 
some strains net-necrosis may be largely inheritable. Furthermore, the 
disappearance of net-necrosis in some varieties might well be expected 
if it is merely a leafroll symptom brought out by certain conditions. 
In this connection it may be pointed out that the indication of leafroll 
by stem-end browning of tubers was considered by Orton (ij, p. 22 ) as 
no longer being reliable. This conclusion is apparently applicable to 
stem-end browning in general. It might seem to be warranted even in 
regard to the net-necrosis type when net-necrosis is absent as a leafroll 
symptom in certain conditions, to be described in the following sections 
of this paper, and when net-necrosis modifies the usual appearance of 
leafroll plants, as described on p. 60. 
INFLUENCE OF VARIOUS FACTORS UPON THE APPEARANCE OF NET-NECROSIS 
VARIETY,* tuber weight, and opportunity for leafroll infection 
It may be noted that in the three lots (L-39, 352, and 357) of Table 
VII that are more than 50 per cent leafroll the percentage of leafroll 
tubers showing net-necrosis varies only from 65 to 71 . Apparently when 
the number and percentage of leafroll tubers are high, the conditions 
gover nin g the development of net-necrosis in such tubers may determine 
the relative amounts of the two diseases within somewhat narrow limits. 
In order to determine what factors might influence the development of 
net-necrosis, comparison was made between different parts of each of a 
number of lots of which most were divided in 1919 first according to the 
presence or absence of net-necrosis when planted and later according to 
the presence, absence, or proximity of leafroll hills in the lot or field. 
These lots were grown in 1919 either at Aroostock Farm or at the Presque 
Isle Laboratory plots. During that season all net-necrosis tubers planted 
at the latter place were in one row at the side of the plot. The tubers were 
examined for net-necrosis in the spring of 1920, and many of them were 
planted. The results of the various observations are given in Table VIII. 
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