July 14, 1923 
Degeneration Diseases of Irish Potatoes 
65 
intervarietal transmission and varietal modification of 
SYMPTOMS 
Observations by the writers, made largely of plants grown or fur¬ 
nished by the Office of Horticultural and Pomological Investigations,^ 
United States Department of Agriculture, have disclosed a great varia¬ 
tion in apparently diseased plants in the many commercial and seedling 
varieties available. Many of the observations were made on an un¬ 
sprayed plot, maintained for the study of resistance to late blight, 
Phytophthora infestans deBy., where mottling was not masked as much 
as in frequently sprayed fields. Here, both in 1920 and in 1921, there 
were such differences between the lots and between the hills of a given 
lot, as well as between the symptoms observed on one date and another 
in the same hills, that it seemed probable that there existed varietal 
susceptibility and varietal modification of symptoms. The question of 
the existence of such modification has been considered in experiments 
involved with natural uncontrolled field infection, with inoculations in 
the field, and with inoculations in the greenhouse. 
general observations in The FIELD 
In 1919, small lots of different varieties were grown in rows alternat¬ 
ing with rows of mild mosaic Green Mountains. As the result of in¬ 
fection in 1918, all the 6 Triumph lots and 12 of the 14 Green Mountain 
lots thus planted were mosaic in over 19 per cent of the hills and were 
discarded. The other 2 Green Mountain lots and 1 lot each of three 
other varieties in the same group {48) —namely, Carman No. 1, Gold 
Coin, and Norcross—contained no mosaic in 1919, but were sufficiently 
infected, presumably by insects from the alternating mosaic rows, to 
have 48 per cent in the Gold Coin lot in 1920, and from 70 to 87 per cent 
in the other lots. Twenty-one lots representing the Cobbler, Early 
Michigan, Rose, Early Ohio, Burbank, and Rural groups were grown in 
the same plot in similar alternation with mosaic Green Mountain rows. 
Ten of these, consisting of some of those from the Rural group, were 
discarded in 1920 for lack of room, when they showed no mosaic. The 
rest were grown between Bliss Triumph mosaic rows in 1920, and by 
1921, after two years of alternate-row proximity to mosaic plants, showed 
from none to 12 per cent mosaic. Either the Green Mountain and 
Triumph groups were much more susceptible to mosaic, or they dis¬ 
played different symptoms from the other groups mentioned. 
intervarietal inoculations in the FIELD 
Results previously reported consist of the transmission of mosaic 
(probably the rugose type) by leaf mutilation with current-season 
symptoms (1919) from Bliss Triumph to Green Mountain, from Irish 
Cobbler to Green Mountain and to Bliss Triumph, and from Green 
Mountain to Bliss Triumph and to Irish Cobbler (40, p . 324-26 ). The 
progeny of the inoculated hills were all “mosaic or mosaic dwarf” in 
1920—that is, infected with mild mosaic or rugose mosaic and com¬ 
binations. As indicated elsewhere, the presence of disease in the second 
season, following inoculation in the open field with insects uncontrolled, 
often is not significant because of the controls having also contracted the 
disease from the neighboring inoculated hills. That was the case here, 
showing that sometimes observations on the first generation in an ex¬ 
periment, including the controls, are more valuable than those on the 
second generation. 
