58 
Journal of Agricultural Research 
Vol. XXI, No. I 
showed leafroll. The tubers of these four plants as well as those of the 
remainder in both greenhouses, were reserved for study of the second- 
generation plants. The appearance of this second-generation stock in 
the experimental field plots is indicated in Table V. 
Table V .—Transmission of leafroll by aphids dispersing naturally in the greenhouse 
First-generation plants. 
Second - generation 
plants. 
Se¬ 
ries 
No. 
Variety. 
Time of 
planting. 
Location of experi¬ 
ment. 
Distance from leafroll 
plants. 
Total 
num¬ 
ber. 
Num¬ 
ber 
leaf- 
roll. 
Per¬ 
cent 
age 
leaf. 
roll. 
1 
Irish Cobbler... 
Nov. 28,1919 
Dec. 3*1919 
Jan. 12,1920 
.do. 
Greenhouse A; leaf- 
roll plants and 
aphids present. 
.do. 
0.5 to 1 meter. 
16 
56 
49 
63 
33 
s 
Green Moun- 
0.5 to 1 meter. 
7 
tain. 
.do. 
.do. 
Next to leafroll plants 
T.5 meters. 
8 
4 
.do. 
18 
5 
Irish Cobbler... 
Jan. 29,1920 
Dec. 18,1919 
Greenhouse B; leaf- 
roll plants but no 
aphids present. 
.do. 
0.5 to 1.5 meters. 
6 
Green Moun- 
1.5 to 3 meters. 
M 
tain. 
From Table V it is evident that leafroll developed in a high percentage 
of the progeny of the healthy potato plants subjected to infestation by 
aphids from leafroll potato vines. On the other hand, it is equally 
apparent that all the healthy plants not exposed to aphid infestation 
remained entirely free from leafroll in both the first and second genera¬ 
tions. Furthermore, it is clear that a higher percentage of leafroll devel¬ 
oped as the plants of the first generation were closer to leafroll plants. 
It will also be noted that contact of healthy vines with leafroll foliage 
was neither sufficient nor essential for transmission of this disease, and 
that there was no contact of roots. The dispersal of aphids from leafroll 
foliage to healthy foliage was alone effective in transmitting leafroll. 
In the spring of 1920, 120 mostly healthy tubers were split into halves 
and were put into a warm place and caused to sprout. One-hundred of 
these were used for other experiments, and four of the 100 produced leaf- 
roll plants from both halves, as a result of infection of the stock in 1919 
or before. The remaining 20 furnished 40 halves. A half from each 
of the 20 was kept in an individual insect cage for a week together with a 
leaflet infested, when introduced, with spinach aphids and secured from 
a diseased potato plant grown in the Maine Agricultural Experiment 
Station greenhouse at Orono, Me. The aphids varied in number from 
about 30 to about 100, and at least some of them dispersed to the sprouts, 
which were from 2 to 5 mm. in length, before being killed by tobacco 
fumigation. The other half of each tuber was untreated except for 
similar fumigation. The half tubers were planted without further cutting. 
Ten halves were fed upon by aphids from mosaic potato foliage; of these, 
