July 14,1923 
Degeneration Diseases of Irish Potatoes 
103 
RESULTS OF TESTS OF CONTROL MEASURES 
With the preceding data in mind, tests of various control measures can 
be made and interpreted more intelligently. Seed treatment seems use¬ 
less, since the plant juice containing the contagium can not well be 
poisoned. Heat does not injure the virus before injuring the tuber 
(#, 19). The only apparent solution is the selection of noninfected seed 
by one or more of the methods which will be discussed. 
TUBER SELECTION 
The selection of hills and strains is in the broad sense a type of tuber 
selection, but the term “tuber selection'* is used here as meaning the 
selection of tubers in the bin without knowledge as to the health of the 
parent plant. The elimination of tubers with net necrosis of a certain 
type will reduce the amount of severe leaf roll in the progeny (41), 
Discarding tubers with the spindling-tuber symptoms (produced by 
plants in the second year of infection) will reduce the amount of third- 
year infection of the spindling-tuber and unmottled curly-dwarf diseases. 
(First-year infection in field occurs late and does not show in vines or 
tubers.) The use of only the largest potatoes will reduce the amount 
of third-year infection of mosaic and leaf roll (41, p. 76-77). How¬ 
ever, there will probably always be tubers of good size and shape that 
were produced by plants infected the previous year or before and that 
are perpetuating disease, unless the field producing the crop was free 
from infection. 
With the spindling-tuber disease it is possible to eliminate a large 
percentage of the spindle-shaped or “run long" tubers and so reduce the 
percentage of this disease. However, tuber selection alone, even with 
the spindling-tuber disease, does not necessarily result in eliminating 
this malady, since many of the normal-shaped tubers will be infected, 
due to late season transmission by aphids in the field. Also, symptoms 
are not always conspicuous (PI. 9, C). 
Selection of normal-shaped tubers from a Green Mountain and an 
Irish Cobbler lot from 1917 to 1922 did not produce stock free from spind¬ 
ling tuber. In fact, the Irish Cobbler lot showed over 90 per cent spind¬ 
ling tuber in 1922, or an increase of about 90 per cent in six seasons. 
Although the conditions for field infection in this lot, being grown near 
diseased stock in experimental plots, were very favorable, nevertheless 
this indicates that tuber selection alone does not insure freedom from this 
malady. 
With the spindling-tuber disease then, as with other insect-borne 
diseases of the potato, the futility of attempting control by means of 
tuber selection alone is very evident from the results of insect 
transmission. 
HILL SELECTION 
In 1918 hills were selected as healthy, both next to mosaic hills and 
also with varying numbers of healthy hills between them and the nearest 
mosaic hill in the same row, and those next to diseased hills produced 
more mosaic progeny than the others ( 40 , p. 334 ). In 1919 Green 
Mountain hills that apparently were healthy were dug in two places at 
progressively later dates in the season. One set was secured at the 
Presque Isle laboratory plots and consisted of 50 hills each grown in 
