78 
Journal of Agricultural Research 
Vol.XXI, No. i 
removed from the seed plot the less chance there will be for infection 
from such plants. This is more particularly the case in those sections of 
the country where aphids become very numerous relatively late in the 
development of the vines. Furthermore, isolation of the seed plot from 
diseased plants in adjoining plots or fields also will contribute to the 
effectiveness of roguing. 
SUMMARY 
(1) Potato leafroll of the apparently nonparasitic transmissible type, 
also called phloem-necrosis, is probably as widely distributed over the 
earth as are potatoes. Its exact geographical distribution is difficult to 
ascertain, however, because of the present impossibility of easily identi¬ 
fying it by the isolation of a pathogene or other cause and because of 
the resulting tendency to apply the name to unrelated diseases with 
somewhat similar symptoms. The same may be said of net-necrosis 
and spindling-sprout. 
(2) The chief and perhaps the only manner of transmission from season 
to season in the soil or elsewhere is by means of tubers from diseased 
plants. 
(3) Leafroll usually reduces the yield considerably, and, inasmuch as it 
is inheritable, it decreases the value of the tubers for seed. It may also 
lessen the value of the tubers for other purposes. 
(4) Leafroll is transmissible from one plant to another by means of 
grafting either tubers or stalks and by means of aphids. 
(5) Net-necrosis is apparently a leafroll symptom, being a discolora¬ 
tion which results from tuber phloem-necrosis and which appears more 
often as conditions of variety, recency of infection, and weight of tuber 
are more favorable. It develops in the dormant tubers without relation 
to differences in the storage temperature. When it occurs as a symptom 
of leafroll, the effects of the latter are still more detrimental, one being 
a decided spindliness of the sprouts. Transmission and control of leaf- 
roll are thus concerned indirectly with net-necrosis and spindling-sprout. 
(6) Leafroll and mosaic are somewhat similar types of diseases. 
(7) Inter-regional differences in the spread of leafroll may depend 
upon differences in climate and in the abundance of aphids. 
(8) Roguing has proved to be much more effective in eliminating leaf- 
roll than it has been for mosaic, at least in northeastern Maine. 
LITERATURE CITED 
(1) Allard, H. A. 
1915. EFFECT OF DILUTION UPON THE INFECTIVITY OF THE VIRUS OF THE MOSAIC 
disease of tobacco. In Jour. Agr. Research, v. 3, no. 4, p. 295-299 
(2) - 
1916. SOME PROPERTIES OF THE VIRUS OF THE MOSAIC DISEASE OF TOBACCO. 
In Jour. Agr. Research, v. 6, no. 17, p. 649-674, pi. 91. Literature 
cited, p. 673-674. 
(3) Appel, Otto. 
1915. LEAF ROLL DISEASES OF THE POTATO. In Phytopathology, v. 5, p. 139-148. 
