July i, 1918 
True Nature of Spinach-Blight 
29 
aphids were used in the transfers, and inoculations were also made with 
the virus obtained from the blighted plants. The first to sixth stages, 
inclusive, were used. Plants in the seventh and eighth stages are more 
or less dried, withered, and rarely serve as food for aphids in the field. 
Ten aphids from diseased plants were transferred in each case and 
remained on the healthy plants for 48 hours. 
Table IV .—Relative injectivity of aphids obtained from plants in various stages of 
spinach-blight at Norfolk , Va., 191J 
Source of material used in making inoculations. 
Num¬ 
ber of 
plants 
inocu¬ 
lated 
by the 
trans¬ 
ference 
of 
insects. 
Num¬ 
ber of 
infec¬ 
tions. 
Aver¬ 
age 
length 
of 
period 
of in¬ 
cuba¬ 
tion. 
Num¬ 
ber of 
plants 
inocu¬ 
lated 
with 
virus. 
Num¬ 
ber of 
infec¬ 
tions 
pro¬ 
duced. 
Aver¬ 
age 
length 
period 
of in¬ 
cuba¬ 
tion. 
Plant in sixth stage of disease. 
A 
A 
Days. 
13 
14- 5 
14 
19 
22 
2 
2 
Days. 
IS 
17 
19 
21 
Plant in fifth stage of disease. 
nr 
A 
*r 
A 
2 
I 
Plant in fourth stage of disease. 
T 
A 
e T 
A 
2 
2 
Plant in third stage of disease. 
1 
A 
T 
A 
2 
I 
Plant in second stage of disease. 
*T 
A 
T 
A 
2 
2 
22. C 
Plant in first stage of disease. 
*T 
A 
2 
25- 5 
0 
2 
I 
30 
O 
Healthy plant... 
T 
A 
O 
2 
O 
# 
* 
relation between length of inoculation period and number of 
INFECTIONS PRODUCED 
From Table IV it will be seen that the ratio between the inoculations 
and infections obtained were about the same for all the stages between 
the second and sixth, inclusive, which would indicate that the virus from 
the various stages is approximately equally infectious. The most striking 
results obtained were the variation in the incubation period of the disease 
in the inoculated plants. The incubation period in the plants inoculated 
by insects transferred from the sixth-stage-diseased plants averaged 13 
days and in the plants inoculated directly with the virus by needle pricks 
it was 15 days. For plants inoculated by insects from the fifth-stage- 
diseased plant the incubation period of the disease was 14.5 days. Direct 
needle-prick inoculations with the virus from the same source resulted in 
an average incubation period of 17 days. The incubation period grad¬ 
ually increased in length with the fourth, third, and second stages until 
in the first stage it had reached a length of 25.5 days before the disease 
was produced by insects and of 30 days before the disease was produced 
by direct needle-prick inoculation with the virus. These figures indicate 
that, while the virus from the various stages may be about equally in-, 
fectious, so far as the ability to produce the disease is concerned, yet 
owing perhaps to the concentration of the infective entity in the plant 
juices, which increases as the disease advances, the virus of the more 
