July x. 1918 
True Nature of Spinach-Blight 
43 
Table; XI .—Transmission of the infectious entity of spinach-blight by virus-bearing aphids 
to their offspring —Continued. 
/ 
JT 
Species used. 
Generation. 
Num¬ 
ber of 
insects 
per 
plant. 
Num¬ 
ber of 
plants 
inocu¬ 
lated. 
Number of 
plants 
infected. 
Aver¬ 
age 
length 
erf incu¬ 
bation 
period. 
Outdoor series: 
M. so lanifo Hi (Auburn, 
Ala., strain). 
Bo. 
Original. 
2 
IO 
IO 
Days. 
21.3 
First... 
2 
IO 
Healthy. 
2 
Bo. 
Second. 
2 
IO 
35 
38 
Bo. 
Third. 
2 
IO 
2 
Bo. 
Fourth. 
2 
IO 
Greenhouse series: 
R. persicae (Baton Rouge, 
La., strain). 
Bo... 
Original. 
5 
5 
c 
20 
19 
2 
TO 
First. 
20 
28 
Bo. 
Second. 
20 
Healthy. 
1 
Bo. 
Third. 
0 
5 
20 
3 i 
Bo. 
Fourth. 
20 
Healthy. 
8 
Outdoor series: 
R. persicae (Baton Rouge, 
La., strain). 
Bo. 
Original. 
5 
5 
5 
5 
e 
IO 
24 
First. 
IO 
3 
1 
Bo. 
Second. 
TO 
*! 
3<5 
■2 c 
Bo. 
Third. 
TO 
2 
Bo. 
Fourth. 
IO 
O J 
j 
The original virus-bearing Norfolk strain of Macrosiphum sokinifolii 
gave high percentages of infection both on the insectary and field plants. 
One hundred per cent of these plants became infected in the field cages; 
this was unusual, considering that the aphids had fed on lettuce for 
several days between the time they were taken from diseased and the time 
they were placed on the healthy spinach plants. Eight infections out of 
25 inoculations, or 32 per cent, occurred on those inoculated in the 
insectary with the first-generation offspring bom and reared on lettuce. 
This was the highest percentage of infection obtained in any of the 
experiments with the lettuce-fed offspring of virus-bearing aphids. 
In the field the aphids from the same lot produced 2 infections from 10 
inoculations. These from the second generation gave 30 per cent of 
infection in the field and 8 per cent in the insectary experiments. The 
third generation produced 8 per cent of infection in the insectary and 19 
per cent in the field, while the fourth generation produced 4 per cent of 
infection in the insectary and none in the field. There was a gradual 
increase in the length of the average incubation period of the disease, 
from 18.1 days with the original virus-bearing aphids to 37 days with 
the fourth-generation offspring. 
The results of the transfers of the Norfolk strain of Rhopalosiphum 
persicae were similar to those of local Macrosiphum solanifolii. With 
R . persicae , however, smaller percentages of infection were obtained 
