Journal of Agricultural Research 
Vol. XIV, No. I 
18 
The results obtained in these early experiments indicated that the 
pathogenic agent of spinach-blight is extremely virulent and can easily 
be transmitted from one plant to another by aphids which pass from a 
diseased to a healthy plant. It is also transmissible to healthy plants by 
means of needle-prick inoculations. The incubation period, or the time 
elapsing from the inoculation to the appearance of color changes in the 
host, varied from 16 to 23 days, under the conditions present. One 
case of blight appeared on a plant to which lettuce-fed aphids had been 
transferred. Similar infections occurred about the same time in another 
set of field experiments being conducted in other cages. 
The following field experiments were begun in 1916. Holland-grown 
seed was sown in December in field cages. The seedlings had developed 
the second pair of true leaves at the time the inoculations were made. 
The diseased material used for inoculation purposes was collected from 
a typical diseased area, located on a farm about % mile distant from 
the Station property. Adult apterous individuals of Macrosiphum 
solanifolii and Rhopalosiphum persicae were transferred to the healthy 
plants. The inoculation period or the length of time the aphids remained 
on the healthy plants was 48 hours in each case. The methods employed 
in transferring the aphids and in making the inoculations are discussed 
elsewhere. The inoculations in the following experiments were made on 
January 10, 1916, and the results are given in Table II. 
Table II. Results of field experiments on the insect transmission of spinach-blight at 
Norfolk, Va., iqi6-iJ 
Method of inoculation. 
Species used. 
Num¬ 
ber of 
insects 
per 
plant. 
Num¬ 
ber of 
plants 
used. 
Num¬ 
ber of 
plants 
infect¬ 
ed. 
Per¬ 
centage 
of infec¬ 
tion. 
Aver¬ 
age 
period 
of incu¬ 
bation. 
Live virus-bearing aphids. .. 
Do. . 
Macrosiphum solan¬ 
ifolii. 
Rhopalosiphum per - 
IO 
12 
II 
91. 6 
91. 6 
Days. 
17.8 
IO 
12 
II 
17 
Juice of virus-bearing aphids 
pricked into healthy 
plants. 
Do. 
stcae . 
M. solanifolii . 
12 
T O 
6 
50 
25 
75 
A 
O T T 
R. persicae t 
M M JC 
Virus pricked into healthy 
plants. 
Pricked with sterile needle. . 
12 
12 
3 
r\ 
24. 0 
20. 6 
Controls . 
12 
IO 
IO 
9 
rv 
Nonvirus-bearing aphids 
M. solanifolii . 
IO 
IO 
U 
2 
O 
25*5 
27 
bom and reared on lettuce. 
Nonvirus-bearing aphids 
bom and reared on spin¬ 
ach. 
Oontiol plants . 
R. persicae . 
30 
Uninoculated_,., 
84 
O 
A 
O 
0 
