Apr. i, 1921 
59 
Leafroil, Net-Necrosis , and Spindling-Sprout 
3 produced mosaic plants as did the corresponding untreated halves, 2 
produced healthy plants as did the corresponding untreated halves, 5 
produced mosaic plants with the corresponding plants healthy, and none 
whether treated or not produced a leafroll plant. Ten other halves were 
fed upon by aphids from leafroll potato foliage; of these, 5 produced 
leafroll plants while the corresponding halves and the other five pairs of 
halves produced plants with no leafroll. Thus aphids from leafroll plants 
inoculated the sprouts of 50 per cent of the half tubers fed upon, while in 
the control set aphids from mosaic plants inoculated 71 per cent of the 
half tubers not already diseased. The method used in this experiment 
is probably not duplicated by natural conditions except for the remote 
possibility of inoculation at the rhizome scar by aphids reaching and 
adhering to the tubers at digging time. However, it served to confirm 
in one generation of plants that which could be proved only by growing 
the second generation in certain other experiments described in this 
paper—that is, the ability of aphids to transmit leafroll about as readily 
as they can transmit mosaic. 
TEST OF SOIL TRANSMISSION 
In a test regarding the soil harboring of mosaic (20, Table VIII), 
19 rows of mostly healthy Green Mountain potatoes were planted in 1919 
across the location of 14 plots of the preceding year, when 1 plot was of 
Irish Cobbler potatoes all leafroll and another was of miscellaneous 
varieties partly leafroll. No leafroll was noted in any of this stock in 
1919, or in 1920 except 4 hills in the part of the stock grown on 
the ground previously planted with miscellaneous varieties. Unless 
leafroll differs from mosaic by being nontransferable between varieties 
(20 p. 324), these negative results indicate that the danger of leafroll 
overwintering in the soil is small and is probably due to ungathered 
diseased tubers which may grow and produce other such tubers and also 
produce sources for infection by aphids. 
Similar negative results in regard to overwintering have been secured 
by Quanjer (17). His explanation of the positive results of proximity 
on the basis of contamination passing through the soil is, in the opinion 
of the writers, hardly more plausible than one on the basis of transmission 
by dispersing aphids. His experiments, as far as can be learned by the 
writers, were not performed in such a way as to preclude the possibility 
of inoculation by aphids, and the experience of the writers has shown 
that with special precautions it is very difficult to control these insects 
completely and that ordinarily they are abundant. Judging from his 
figures (18 ), apparent soil transmission was correlated in the first gener¬ 
ation with other differences than root contact alone—namely, less 
complete isolation of the plants by means of pots, more vigorous growth, 
and a difference in location. All these factors may have influenced the 
