96 
Journal of Agricultural Research 
Vol. XXV, No. a 
more generally distributed over the potato-growing regions than is 
shown by reports now available. 
Lack of control of these various degeneration diseases causes losses 
to seed buyers because of their appearance in plants grown from ap¬ 
parently normal and healthy tubers. This reacts on the sellers of seed. 
Control therefore will not only improve the yield of generally salable 
tubers, but will also improve the quality of the crop for seed purposes. 
GENERAL FACTORS INVOLVED IN CONTROL 
TRANSMISSION 
From the results of the experiments described in preceding parts of 
this paper, it may be concluded that by aphids and possibly other 
means there is danger of degeneration diseases spreading in the field 
from diseased to healthy hills or from diseased to healthy fields. A 
stock of Norcross (Green Mountain group) grown in a large field on 
Aroostook Farm had 55, 80, 95, 99, and 100 per cent of the hills mild 
mosaic, respectively, in the five years from 1918 to 1922. Other stocks 
of smaller size have here shown a similar steady increase of mild mosaic 
and of spindling-tuber disease, but not of leaf roll. That such frequent 
annual increase in the percentage of incidence is due to infection of 
healthy hills from the diseased ones in the same field is shown by certain 
hill selections, to be described in a later section of this paper. Also, 
the effect of different degrees of isolation of healthy stocks from diseased 
stocks will be shown in a later part of this paper. The removal of 
diseased hills serving as sources of infection, and the control of insects 
that transmit these diseases, are important phases of control to be 
given attention later. 
If any experiment should eventually indicate that transmission 
through the soil is possible, it will be necessary to test or eliminate 
various soil factors (such as subterranean insects, soil water, soil mass, 
or debris from diseased plants) in order to determine the exact method 
of transmission. At present neither soil nor root-contact transmission 
has been demonstrated. 
PERPETUATION 
The question of perpetuation in diseased weeds has been considered in 
connection with interspecific transmission. It seems to be of importance 
in some regions and not in others. 
The possibility of perpetuation in the soil exclusive of diseased tubers 
has been tested in pots, with negative results in both the first and second 
generations (40, p. 335). This experiment involved contact with some 
of the roots of plants recently removed. Field experiments also gave 
negative results with mosaic, both in the first (, 40 , p. 333-36) and second 
generations, and whether begun upon the ground of 1918 plots or of 1919 
plots. In the latter (Table XXVII, field D) dead mosaic vines were 
buried in the furrow at planting time. The same negative results 
resulted from tests with leaf roll reported by the writers (41, p. 59) by 
Quanjer (38, p. 47-42), Wortley (50), and Murphy (29, p. 32-33) and 
from tests with mosaic dwarf by Krantz and Bisby (21, p. 13-18). 
The well-known perpetuation through the tubers is of great importance. 
It requires the avoidance of fields containing viable overwintering dis¬ 
eased tubers such as were present in northeastern Maine in the spring of 
1919 (40, p. 333) and of 1921 (PI. 14, C). 
