July 14, 1923 
Degeneration Diseases 0} Irish Potatoes 
97 
It makes it desirable to inspect the fields that produce the seed tubers, 
since the condition of the parent plants has more effect on the progeny 
than the conditions of seed-tuber storage or of the growth of the progeny. 
Other phases of tuber perpetuation will be discussed in connection with 
the question of tuber selection. 
DIFFUSION OF THE CONTAGIUM 
The spread of the contagium or virus within the plant will be termed 
“ diffusion.'’ Stocks assuredly mosaic, that have shown the symptoms 
of mild mosaic for several seasons, usually do not show the symptoms 
in the first part of the season (several days to several weeks) and often 
lose them in the last part. This seasonal cycle is considered by the 
writers to be the result not of diffusion of the contagium to and from the 
leaves, but rather of the effect of the degree of maturity upon leaves con¬ 
taining the virus, and frequently shows variations due to temperature 
and other environmental factors. Diffusion of the virus from the point 
of inoculation in the vines to the tubers may occur without the appear¬ 
ance of symptoms, even after the normal period of incubation, if the leaves 
have ceased growing. The time needed for this diffusion is of importance 
in relation to the possibility of tuber infection being reduced by an early 
death of the aerial parts of the plants, after the latter have been inocu¬ 
lated by insects. 
Knowledge of virus diffusion may explain the disease division of hill 
lots, tuber units, hills, and shoots, where the progeny of a hill or of a 
tuber, and even a single shoot, may be only partly diseased. Although 
such disease division may be caused by recent insect inoculation in some 
cases at least, it occurs in conditions where such insect inoculation is not 
a satisfactory explanation. Incompleteness of diffusion is of interest to 
those who test hills or tubers by sampling one eye of each, and to those 
who plant a seed plot by tuber units. If its occurrence in a seed tuber 
is sometimes the cause of the late appearance of symptoms in the upper 
part of the plant, it is also important to those who attempt to remove 
all diseased hills early in the season. 
In the winter of 1920-21, at Orono, the plants of nine tuber units were 
inoculated at different times by the leaf-mutilation method with inoculum 
from mild-mosaic Bliss Triumph plants, and were dug within a certain 
period of time after inoculation. Three different intervals elapsed for 
parts of each tuber unit. The progeny were grown in the field in 1921, 
and showed symptoms as described in Table XXV, for Series 1. In 
another series (No. 2 of Table XXV) the plants of three tuber units were 
inoculated with a reliable method similarly at a given time and four dif¬ 
ferent intervals elapsed for parts of each tuber unit. In each series, 
10 days allowed much less diffusion of the virus to the tubers than 20 
days or longer. This helps to explain why digging seed tubers within 
10 days after the maximum infestation by transmitting insects avoided 
the maximum amount of disease (p. 104). It also indicates how early 
autumnal killing frosts in northern regions, following the usual late spring 
hatching of aphid eggs and consequent late development of infestation, 
might contribute to the general less abundance of degeneration diseases 
in the north, resulting in the well-known preference for northern-grown 
seed (io, p. 162-63 ). 
