no 
Journal of Agricultural Research 
Vol. XXV, No. a 
imity and heavy aphid infestation together increased the spread of mosaic, 
and that with greater isolation than obtained in these fields it may be 
possible to maintain stock mosaic free if once the disease is eliminated. 
Isolation of a half mile in England has prevented infection while 
proximity increased it (jo, jo). 
INSECT CONTROL 
Aphids have proved to be effective transmitting agents in a number of 
the experiments described in this paper. Although it may not be pos¬ 
sible to control aphids sufficiently to prevent transmission in the field 
in all times and places, it at least is necessary unless there is no disease 
present. There are two distinct phases of the problem of insect control 
in relation to disease transmission. The presence of transmitting insects 
in a partly diseased field is to be expected to result in late-season infection 
of healthy hills that can not be distinguished and that therefore can not 
be eliminated in the current season. In a healthy field the mere presence 
of insects is harmless in relation to disease, but the dispersal of trans¬ 
mitting insects to either kind of field from a wholly diseased field is 
undesirable. In some situations it may be possible to prevent interhill 
transmission before removing the diseased hills and yet be impossible 
to prevent new contamination from other fields, while in other situations 
it may be possible to prevent interfield transmission and yet be impos¬ 
sible to avoid an infestation that causes interhill transmission. 
In endeavoring to follow the recommendation of entomologists regard¬ 
ing aphid control, the writers have formed the opinion that spray methods 
reported to be effective in more southern regions, at least in preventing 
directly injurious infestations, are not so useful in preventing transmis¬ 
sion in northeastern Maine. In this region high-ridge culture is practiced, 
so that many leaves lie on the surface of the soil and support aphids that 
can not be touched by the spray and that are a permanent source of 
infestation. Also, the tops in healthy fields grow so large that they form 
an expanse of foliage through which it is often impossible to walk without 
stepping over plants and in which it is difficult to drag undershot nozzles 
and to use them effectively for covering the leaves. Expensive applica¬ 
tions of nicotine solution in 1921 reduced aphids in two four-row sections 
in a seed plot containing 19 such sections. The seed plot was rogued 
of about 10 per cent of the diseased hills, partly after aphids were present, 
and these two sections had progeny with about 9 per cent diseased 
as compared with from 15 to 18 per cent for the rest of the seed plot. 
The same methods used again in 1922 were effective in killing all aphids 
except on the leaves next to the soil. The seed plot of 1922 was a 
half mile further from rose bushes than the seed plot of 1921, and the 
potato aphids arrived about two weeks later than at the site of the 1921 
plot, following roguing of all mosaic hills. The progeny in 1923 will 
indicate the value of isolation of potatoes from rose bushes (36) in reduc¬ 
ing the spread of mosaic and the value of the spraying in reducing the 
spread of the spindling-tuber disease. The latter could be rogued only 
in part of the seed plot planted by tuber units. 
In 1918, several tubers from healthy Bliss Triumph hills grown in cages 
were selected to begin a strain grown in insect-proof cages in the following 
seasons of 1919 to 1922. Every tuber from this caged stock has pro¬ 
duced healthy plants. These cages were located in the field near potatoes 
with high percentages of mosaic and leaf roll, and during the same period 
