64 
Journal of Agricultural Research 
Vol. XXI. No. i 
POSSIBILITY OF "a COMMON CAUSE OF LEAFROLL AND NET-N ECROSIa 
In the preceding section of this paper data have been submitted show¬ 
ing that in Maine net-necrosis has been consistently accompanied by 
leafroll. It may be that net-necrosis, a tuber phloem-necrosis, is caused 
by the same virus 1 as is leafroll or phloem-necrosis. This hypothesis 
will be tested further by detailed microscopic studies now under way 
and by various inoculation experiments begun in 1919, but it explains 
the available facts and explains them at least as well as does the assump¬ 
tion that the two diseases are caused by different viruses. 
Experiments indicate that net-necrosis is apparently nonparasitic in 
nature and transmissible, like mosaic and leafroll. Numerous attempts 
by the writers and by others in the same laboratories to find one kind of 
organism associated consistently with the disease have resulted only in 
failure. The usual origin of discoloration at the rhizome scar indicates 
that the source of infection comes from the parent plant. The. rare 
cases already noted as found by the writers in which the phloem discol¬ 
oration was restricted to the eye-end portion suggest that the disease is 
carried with the juice. Transmissibility by aphids was demonstrated in 
the field experiment with insect cages, described on page 54. The 
leafroll plants from which the insects were taken had been produced by 
net-necrosis tubers. At planting time in 1920, severe net-necrosis was 
found in one of the eight tubers from the plant given the third treatment 
described in Table III, while none was found in the 584 control tubers. 
The fact that this single tuber showed net-necrosis is explained easily by 
the theory that one virus causes leafroll and, in the proper conditions, 
net-necrosis as well. 
Correlation between the amount of net-necrosis and leafroll in a num¬ 
ber of stocks grown in 1920 has been partly indicated on page 60. This 
correlation is shown further by data upon the 1,000 tubers in the 10 lots 
that supplied the majority of the net-necrosis tubers, given in Table VII. 
Table VII .—Leafroll and net-necrosis in 1920 in lots healthy in 1919 
Total 
Tubers producing leaf- 
roll plants. 
Tubers 
showing 
net- 
Percentage 
of leafroll 
tubers 
Lot No. 
number 
of tubers. 
Number. 
Percentage. 
necrosis 
at planting 
time. 
showing 
net- 
necrosis. 
L- ■5T . 
227 
146 
15 
O 
7 
2 
13 
d l . 
0 
O 
•Lr3 4. 
139 
120 
2 
1 
I 
50 
L-18 . 
15 
13 
9 
60 
T .-70 . .... 
102 
87 
85 
62 
71 
oy . 
III 
55 
50 
36 
65 
nCA .. 
50 
10 
20 
5 
50 
0 
0 
0 
355. 
J J 
38 
40 
22 
58 
i5 
68 
2 c 8 . 
6 
i5 
5 
83 
35°. 
1 
1 See p. 74, with footnote, for the writers’ use of the term “ virus.” 
