Apr. i, 1921 
Leaf roll, Net-Necrosis, and Spindling-Sprout 
53 
leafroll. At this time the five ungrafted hills from the same tuber units 
were still free from leafroll. In the fall of 1919 the tubers from these 
grafted and ungrafted hills were reserved for planting in 1920. Observa¬ 
tions on the second-generation plants in 1920 showed that all the tubers 
from the leafroll grafted vines produced leafroll plants, while those 
from four of the five ungrafted hills gave rise to healthy vines. The 
infection of one control hill was probably due to inoculation by aphids 
in 1919. These results confirm similar ones secured by Quanjer (17) 
with grafts. 
In view of results already obtained with aphids and to be described 
later, grafts that were made in the field in 1920 were covered with insect 
cages. Three Irish Cobbler tubers were taken from a lot which was 
rogued in 1919 and which contained less than 1 per cent of leafroll in 
1920. Each was cut into 3 pieces, and each tuber unit was planted 
under a cloth-covered insect cage. Scions from leafroll plants in the 
open field were grafted on July 15 upon 4 stalks in each cage, 2 in the 
second and 2 in the third hill of each tuber unit. All the 12 scions but 
1 lived until August 25, when the hills were dug. In spite of precau¬ 
tions, one or more individuals of the potato aphid (Macrosiphum solani- 
folii Ashm.) 1 were introduced into each of the first and second cages, 
with the result that by August 20 such aphids were numerous. None 
were found in the third cage at any time. The results were the same for 
all 3 tuber units, and it seems probable that the aphids had not been 
abundant long enough to contribute to them. On August 25 each of the 
6 hills with grafts, except 1 in the second cage, showed leafroll in the top 
of a branch coming from a stock just below the graft. One such branch 
and a control are shown in Plate 6, B, C. 
TRANSMISSION BY APHIDS 
INDICATIONS OP NATURAL TRANSMISSION FROM FIELD OBSERVATIONS 
During 1917 each healthy control lot was planted in the same row with, 
or in an adjoining row to, leafroll stock of the same strain and variety! 
Leafroll appeared in 1918 in the progeny of hills that during the pre¬ 
vious season apparently were free from leafroll. The healthy hill- 
selected lots were planted in 1918 and 1919 in a section of the plot 
more apart than in 1917 from the leafroll lots of the same variety, being 
from 2 to 12 meters distant. Twenty-five per cent of the 438 hills of 
control lots, representing Green Mountain, Irish Cobbler, Early Rose, 
and Burbank varieties, showed leafroll in 1918. Only 13 per cent of 
the 240 control hills, representing the same varieties, were leafroll in 
1919- In 1920 the percentage of leafroll hills, from a total of 336 con¬ 
trol hills of the same varieties as in 1919, was 12. It is assumed that, as 
1 Identified by Dr. Edith M. Patch, Entomologist of the Maine Agricultural Experiment Station. 
