76 
Journal of Agricultural Research 
Vol. XXI, No. I 
variation to have a marked influence upon the spread of mosaic (19, 
P- 33i)- 
Reduction of the spread of leafroll by shade, in comparison with the 
spread in drouth conditions, has been reported by Wortley (24, p. 519). 
It seems possible that aphids might be more restless and inclined to dis¬ 
perse from plant to plant in drouth conditions and that their dispersal 
might be reduced in shade conditions by a fungous disease. Such a dis¬ 
ease has been noted in Maine by the writers as being more prevalent in 
insect cages than in the open field during sunny weather and as practically 
eradicating an abundant infestation within a few days of cloudy weather. 
It may be suggested that the normal weather conditions of northeastern 
Maine specially favor any fungous disease of aphids, even as they favor 
the growth of the potato plant and the development of its important 
malady, the late blight (Phytophthora infestans de By). 
CONTROL OF LEAFROLL 
Roguing tests were begun in 1919 in order to ascertain whether or 
not it was possible to free infected potato stock from leafroll plants. 
Two stocks, lots 20 and 21 of Table XIII, have been grown upon the 
experimental plots in northeastern Maine since 1916. They were at 
first a single healthy stock which did not yet contain any leafroll plants 
in 1918 when it was divided and located in two rows next to a 100 per 
cent leafroll row. It was exposed to heavy aphid infestation in 1918, 
and in 1919 it developed leafroll in 5 per cent of the plants from one row 
and in 11 per cent of those from the other. In 1919 a part (sublot 1) 
of each of these i-row lots was rogued, while another part (sublot 2) was 
not rogued. That season these lots were again located, as indicated, near 
or next to wholly leafroll stock, and numerous aphids, though not so many 
as in 1918, appeared during the latter part of the season. At harvesting 
time only tubers of approximately 4 ounces and more were selected. 
Lot 22 as noted was taken from a commercial field having 14 per cent 
leafroll infection. This field is located fully half a mile from the experi¬ 
mental plots on which lots 20 and 21 were grown. It happened that 
aphids were very scarce there, only a few individuals to a plant being 
noted late in the season. Part of this stock was rogued and designated 
as sublot 1, while sublot 2 of this same stock was not rogued. The 
selection of tubers was the same as for lots 20 and 21. Observations on 
these lots are reported in Table XIII. 
In view of the varying conditions obtaining in connection with the 
different lots described in Table XIII, it will be impossible to consider 
that a single factor caused the differences in leafroll percentages in 1920. 
Apparently selecting tubers in 1919 reduced the leafroll percentage in 
the unrogued sublots of lots 1919-21 and 1919-22, while such an effect 
was prevented in lot 1919-20 by. greater proximity to a leafroll row. In 
sublot 1 of lot 1919-20 roguing prevented an increase in leafroll per- 
