July 14, 1923 
Degeneration Diseases of Irish Potatoes 
63 
As reported previously (41, p. 54-55), in 1919 aphids were transferred 
from a plant with both mild mosaic and leaf roll to three healthy hills 
in an insect cage, and a higher percentage of the progeny were leaf roll 
(86 per cent) than were mosaic (57 per cent). As is shown in Table II, 
aphids from hills both mild mosaic and spindling-tuber, with contact 
with those hills, resulted in mosaic infection of two hills, spindling tuber 
infection of two hills, and in no infection of a fifth hill. As described 
on page 51, aphids from a “mottled curly-dwarf 77 (or spindling tuber+ 
leaf-rolling mosaic) hill infected a hill in an insect cage partly with 
curly dwarf and partly with the spindling-tuber disease alone (PI. 3, 
B, 1, 2). 
CONCLUSIONS REGARDING TRANSMISSION AND DIAGNOSIS IN THE GREEN 
MOUNTAIN VARIETY 
Within one variety in a given time and place, it has been possible to 
distinguish diagnostically several degeneration diseases of potatoes, 
occurring both singly and in combinations. These several degeneration 
diseases can be studied to advantage by means of inoculations with 
grafts, aphids, and the leaf-mutilation method. In this way further 
distinctions can be made between the more similar diseases such as the 
types of mosaic, and combinations of diseases can be divided. The 
same methods have been used in Minnesota in this variety in “mosaic- 
dwarf 77 inoculation by grafting, leaf mutilation, root contact, and expo¬ 
sure to insect dispersal from diseased plants, with similar results, positive 
except with root contact (, 21 , p. 13-22). 
TRANSMISSION AND DIAGNOSIS WITHIN VARIETIES OTHER THAN 
THE GREEN MOUNTAIN 
It is thought that the several diseases described in the preceding pages 
in Green Mountains are present also in other varieties. Assuming that 
there may be varietal modification of symptoms, as will be shown later 
in this paper to be possible, intervarietal inoculation is necessary for 
conclusive proof that a symptom complex in one variety is caused by 
the same virus as a symptom complex in another variety. However, 
it is thought that leafroll and the spindling-tuber disease can often be 
correctly diagnosed outside of Green Mountains without intervarietal 
inoculations. 
In 1920, as is described partly in a previous paper (41, p. 53), leaf-roll 
inoculations were made on Irish Cobblers inside of insect cages by means 
of grafting, leaf mutilation, aphids, and contact. The progeny were 
observed in 1921, with the data given in Table X. It is clear that 
grafting and aphids w T ere the only effective means employed, and that 
aphids were not as effective as with mosaic in the same season (Table 
I). Corresponding leaf-mutilation inoculations in the open field in 
the two hills of each of 12 four-hill tuber units, gave negative results in 
both generations. Kasai (20, p. 66) describes reduction of leaf-roll 
contamination by the use of field insect cages in Japan. 
Leaf-mutilation inoculations were made in the Orono greenhouse 
during the winter of 1921-22 in Irish Cobblers, with rugose mosaic, 
with leaf roll, and with a combination of the two. The four hills inocu¬ 
lated with rugose mosaic showed leaf dropping in from 25 to 50 days. 
Their 12 progeny were mosaic, with leaf dropping. The four controls 
