28 
Journal of Agricultural Research 
Vol. XIV, No. x 
tions in the individually diseased plants which occur scattered through 
out the fields are probably produced by the alate female aphids which 
fly from blighted plants carrying the infection to healthy plants upon 
which they alight and feed. These scattered, blighted plants in turn 
may serve as centers of infection for future diseased areas. 
TablS III .—Transmission of spinach-blight from various local sources , Norfolk, Va,, 
IQX7 
Species of aphid used. 
Source of diseased material. 
Num¬ 
ber of 
plants 
inocu¬ 
lated. 
Num¬ 
ber of 
plants 
infect¬ 
ed. 
Aver¬ 
age 
length 
of incu¬ 
bation 
period. 
Num¬ 
ber of 
insects 
pESt. 
Macrosiphum solanifolii ... 
Do... 
Do....,. 
Do... 
Rhopalosiphum persicae ... 
Do.. 
Do. 
Macrosiphum solanifolii ... 
Do. 
Do. 
Rhopalosiphum persicae ... 
Do. 
Controls. 
Farm A (center of diseased 
area). 
Farm B (center of diseased 
area). 
Farm C (center of diseased 
area). 
Station farm (center of dis¬ 
eased area). 
Farm A (center erf diseased 
area}. 
Farm B (center erf diseased 
area). 
Station farm (center of dis¬ 
eased area). 
Farm A (individually dis¬ 
eased plant). 
Farm B (individually dis¬ 
eased plant). 
Station farm (individually 
diseased plant). 
Farm A (individually dis¬ 
eased plant). 
Farm B (individually dis¬ 
eased plant). 
Untreated, 20, all healthy... 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
Days . 
9 16 
10 17 
8 
9 
6 
8 
9 
8 
8 
9 
6 
7 
14.3 
16. 2 
17 
17. 7 
16 
16. 4 
14 
* 7 - 3 
19. 2 
16. 1 
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
4 
EXPERIMENTS ON THE INSECT TRANSMISSION OF SPINACH-BLIGHT, 
1916-17 
RELATIVE INFECTIVITY OF APHIDS OBTAINED FROM PLANTS IN VARIOUS 
STAGES OF THE DISEASE 
A series of experiments was begun on January 12, 1917, to determine 
the relative infectivity of the virus from plants in various stages of 
spinach-blight. As has been discussed, there are eight arbitrary but 
rather definite stages of advancement of the disease between the first 
appearance of pathogenic symptoms and the ultimate death of the plant. 
In order to make this test, it was necessary to collect typically diseased 
field plants in the various stages. A large number of lettuce-fed aphids 
were placed on the diseased plants and allowed to feed for three days 
before transferring them to healthy spinach plants. Both species of 
