6o 
Journal of Agricultural Research 
Vol. XXI, No. T 
development and dispersal of virulent aphids. 1 The question of soil 
transmission will remain open until it is demonstrated with all aerial 
insects eliminated and even then will be solved more completely when 
the exact means, possibly either subterranean insects, root contact, soil 
water, or the soil itself, is disclosed. Wortley ( 24 ) also reports negative 
results. 
RELATION OF LEAFROLL TO NET-NECROSIS AND SPINDLING-SPROUT 
RELIABILITY of net-necrosis as an indicator of leafroll 
A number of separate lots of net-necrosis tubers have been planted 
in Maine during the past few years, beginning with 1915. The data 
concerning many of these are given in Table VI. Omission is made of 
net-necrosis lots which were known to have originated from leafroll 
hill selection and of those which were grown in the field in 1920. As 
far as is known, each of the stocks or lots considered in this table is 
unrelated to the others—that is, no two have had a common origin, 
though they may have come from the same place. The year of planting 
is given as part of the lot number. As indicated, sometimes a lot was 
divided into two or more sublots. The data for all plants grown at 
Highmoor Farm are taken from records made by Dr. W. J. Morse, and 
those for the sublots grown in 1917 at Aroostook Farm from records made 
by Mr. G. B. Ramsey. Such of these as are concerned with symptoms 
are indicated by quotation marks. The writers are responsible for data 
for all other lots. In examining this table there should be kept in mind 
both the difficulty, already discussed in regard to symptoms (p. 48-49), of 
always ascertaining net-necrosis, or tuber phloem-necrosis, without 
microscopic examination, and the possibility that plants infected with 
the leafroll virus may be so reduced in vitality that they lose the usual 
symptoms of stiffness and pronounced leaf rolling. Compare Plate 
1, A, B, for the effect of net-necrosis and leafroll in the field, Plate 2, A, 
and 4, C, for the effect of greater severity of net-necrosis in the greenhouse, 
and both the two plants of Plate 5, B, and the two pairs of plants of 
Plate 6, A, for the effect of net-necrosis upon field-grown leafroll hills. 
The difficulty regarding symptoms would be aggravated by including 
doubtful cases in a net-necrosis lot, which, when material for study of 
the disease is rather limited, may seem more desirable than excluding 
all but marked and severe cases. 
The data in Table VI indicate that there is some relation between 
net-necrosis and leafroll, inasmuch as 26 lots contained both net-necrosis 
seed and leafroll plants and came from 15 different places and com¬ 
prised about as many different varieties. The four other lots either con¬ 
tained net-necrosis seed and perhaps leafroll plants (lots 1915-W and 
1 A more recent publication by one of Quanjer’s associates (Botjbs, J. G. Oortwijn, op. cit., p. 131) 
also points out the possibility of insect transmission in the experiments in question. 
