Apr. x, 1921 
Leafroll , Net-Necrosis, and Spindling-Sprout 
57 
One such control is shown in Plate 10 with the corresponding inoculated 
plant. 
On April 15, 1920, the vines were still green but had made practically 
full growth. At this time the tubers were harvested from each of the 
inoculated plants as well as from each of the controls in order to note 
the amount of infection in the plants of the second generation. In June, 
about two months later, these tubers were planted in the field. Because 
of the very brief resting period only a few produced vines of sufficient 
size to be of value for observation. Two of the second-generation plants, 
one treated in the first generation with aphids from a leafroll plant and 
the other its corresponding untreated control, are shown in Plate 11. 
The former shows very marked leafroll, but the latter remained healthy. 
Of the controls three out of eight were sufficiently large to show that they 
were free from leafroll. The remainder had made less than 3 cm. growth 
when a killing frost ended the season; and hence, although apparently 
healthy, they were too small for notes to be significant. Of the eight plants 
whose parents were inoculated, five developed marked leafroll symptoms. 
The remainder, like the controls, were not yet large enough for notes at 
the end of the season. Even though not every plant in the second 
generation made growth adequate for observations, yet the fact that 
the remainder of the vines disclosed such marked differences, healthy 
in the case of control stock and distinctly leafroll in the case of inoculated 
stock, confirms the evidence that the leafroll of the first generation was 
transmitted by aphids. 
GREENHOUSE EXPERIMENTS WITHOUT INSECT CAGES 
During the winter of 1919-20 when cage experiments with spinach 
aphids were being conducted in the Washington greenhouse, another set 
of healthy potato vines was exposed there to aphids dispersing naturally 
from leafroll plants. The aphids were of two kinds, mixed, most of 
them being the potato aphis (Macrosiphum solanifolii Ashmead) 1 and 
the rest the spinach aphis. 1 The healthy potato plants thus exposed to 
aphid inoculation were located in greenhouse A, as is indicated in Table V. 
They were grown in 8-inch pots which were within 1.5 meters of the 
aphid-infested leafroll potato vines of the Irish Cobbler and Green 
Mountain varieties. Aphid dispersal took place freely during the last 
week in February, 1920, the healthy plants being half grown. In green¬ 
house B, as is shown in Table V, a control set of healthy plants similar to 
those in greenhouse A was set among leafroll plants. The conditions in 
greenhouse B were like those in greenhouse A with the exception that 
aphids were eliminated by weekly tobacco fumigation. 
On April 13, 1920, when the tubers were harvested, two Irish Cobbler 
plants of series No. 1 and two Green Mountain plants of series No. 2 
1 Identified by Dr. A. C. Baker, of the Bureau of Entomology, United States Department of Agri¬ 
culture. 
