July 14,1923 
Degeneration Diseases of Irish Potatoes 
99 
way were all diseased)—or were healthy then and partly mosaic later. In 
these 37 tuber units, often different stalks in a hill were affected with 
different severity. Whether healthy or partly mosaic on July 1, these 
units became more diseased as the month progressed. They could be 
arranged in a series of gradually increasing amount and severity of 
disease. They seemed to be either the last hills to show mosaic as the 
result of 1919 infection or hills that showed the first symptoms resulting 
from 1920 infection. If the last to show 1919 infection, they would 
seem to be so because of incompleteness of infection of the tubers in 1919. 
If they were mosaic as the result of 1920 field infection, the identity of 
the transmitting agents is in doubt, since aphids were not discovered 
on June 26. On July 26 they were limited to colonies each of about a 
dozen consisting of a mature potato aphid individual and her young ones. 
A shoot in the Orono greenhouse, where aphids were absent, was ob¬ 
served having one side mosaic, the line of distinction between diseased 
and healthy even bisecting two leaves lengthwise. 
Whatever the cause of delayed and partial manifestation of symptoms, 
the effects are troublesome. About a fourth of these 37 tuber units 
became affected with streaking between July 1 and 26. This streaking 
was either accompanied or preceded shortly by the appearance of brittle¬ 
ness and mottling. A few more became thus affected between July 25 
and August 27, one alone with no mosaic mottling. This single apparent 
exception to the rule of association of mottling and streaking was not 
surprising since mottling had become reduced in distinctness or entirely 
effaced in over one-third of the tuber units. Thus, in certain tuber 
units streaking was an accompaniment of mottling that appeared later 
than normal and that was found in only part of each unit. More¬ 
over, in many of these units mottling became reduced during August; 
even though the variety was Green Mountain, one that retains this 
symptom better than most varieties, and the streaking was followed in 
part by progressive necrosis. As a result, a combination of streaking, 
brittleness, and leaf dropping could be seen in August with no mottling 
evident in the plants, and were it not for the wrinkling present, such 
plants would have been considered by anyone without access to previous 
records as being affected only with streak. Furthermore, at one time 
it was possible to arrange the streaked hills in a series with complete 
gradation from a mottled-streaked condition to a typically streak con¬ 
dition. It seems possible that this change and gradation from mosaic 
to streak was due in part to the presence of two viruses—rugose mosaic 
and streak, in part to variation in the time of infection (or to variation in 
diffusion of the virus), and in part to the increase in maturity of the 
plants. However, in this and similar cases, it is also possible that a. 
single virus (rugose mosaic) produces streaking and leaf dropping in the 
conditions described. At any rate, correct diagnosis and the early 
removal of diseased hills seemed difficult under such conditions. 
response to environmental factors 
Environment influences both the symptoms and the rate of spread of 
degeneration diseases. New symptoms following the transferring of a 
given lot to another region can not be regarded as the effect of the new 
environment unless either there is evidence that there was no new 
infection during the last season in the first region, or a similar part of the 
stock is kept in the first region as a control. Loss of symptoms following 
