30 
Journal of Agricultural Research 
Vol. XIV, No. I 
advanced stages of the blight produced positive symptoms in about half 
the time that was required to produce them, when plants were inoculated 
with the virus obtained from plants in the early stages of the disease. 
It is also worthy of note that under conditions present in this experiment, 
when io adult aphids were transferred from a diseased to a healthy plant, 
positive symptoms of the disease developed in two to five days’ less time 
than when the inoculation was made with the same diseased material 
by means of the expressed virus pricked into the plants with a sterile 
needle. No infections were obtained in the transfers of lettuce-fed 
aphids to healthy spinach, and no infections were obtained when prick 
inoculations were made with the juice obtained from healthy spinach 
plants. 
In order to determine the approximate length of time which aphids 
from blighted spinach must remain on healthy plants to produce infec¬ 
tions, aphids from diseased plants were transferred to a series of healthy 
plants and allowed to remain on them for various periods of time. Since 
infections were previously obtained by allowing the insects to remain on 
plants for 48 hours, the length of the periods in the present series were 
shortened to 5 minutes, 2 hours, 14 hours, and 24 hours. As the aphids 
had been disturbed, they fed little during the 5-minute period that they 
were allowed to remain on the healthy plants. They were observed 
occasionally to plunge their beaks into the pfant tissues for an instant, 
quickly withdrawing them, moving to some other part of the plant and 
repeating the performance. The results of this experiment are given in 
Table V. 
Table V .—Results of experiments on the relation between the length of the inoculation 
period and the number of infections produced 
Length of time aphids remained 
on the plants. 
Species used or treatment. 
Num¬ 
ber of 
plants 
inocu¬ 
lated. 
Num¬ 
ber of 
plants 
in¬ 
fected. 
Aver¬ 
age 
length 
of incu¬ 
bation 
period. 
Num¬ 
ber of 
plants 
remain¬ 
ing 
healthy. 
24 hours . 
Macrosiphum solanifolii . 
14 
19 
0 
II 
Days . 
15.2 
18. 8 
2 
14 hours . 
.do . 
15 
6 
O 
A 
2 hours . 
.do . 
16.3 
24 
16.3 
i 5 
14 
2 7 - 5 
4 
2 
5 minutes. 
.do . 
7 
8 
A 
a 
A 
24 hours .. 
Rhopalosiphum persicae . 
13 
8 
12 
4 
5 
5 
2 
14 hours . 
.do . 
8 
2 hours . 
.do . 
5 
2 
5 minutes . 
.do . 
0 
O 
7 
Plants inoculated with virus 
from the original diseased 
plant . 
10 
8 
16.1 
2 
Controls . 
Untreated . 
20 
20 
Where the aphids remained on the plants for only 5 minutes less 
infections were obtained than when they fed on the plants for 2 hours or 
longer. The percentage of infections which resulted from the longer 
