July 14, 1923 
Degeneration Diseases of Irish Potatoes 
49 
mosaic appeared on the top leaves on one of the plants. At this time 
the vines varied from 30 to 40 cm. in height. On the vines in the 
second generation, however, this and two additional cases developed mild 
mosaic. These three mottled vines—that is, 37 per cent of the treated 
plants—included all the plants, except one, kept in a moist chamber 
for 24 hours after treatment. None of the inoculations outside of moist 
chambers were effective. The three successful inoculations were per¬ 
formed on three of the four plants that were 5 cm. or more high when 
inoculated. 
On December 21, 1921, eight healthy plants were treated as those 
inoculated on December 19 had been, with the exception that only two 
of the plants were kept in moist chambers until 24 hours after inocula¬ 
tion. One of these plants developed mild mosaic before harvest, and 
in the second generation the other showed mild mosaic. Hence only 
the two vines which were placed in moist chambers developed mosaic. 
In each of the foregoing series only a single inoculation was performed. 
Leaf-mutilation inoculation (not repeated) with mild mosaic had no 
effects when made in the Orono (Me.) greenhouse in the winter of 1921-22, 
in 10 hills from five tuber units. The 10 progeny of the sources of inocu¬ 
lum and the 27 progeny of the inoculated hills were all grown in the 
same greenhouse in the following summer, when neither group showed 
any mosaic symptoms. If infection took place, it was masked in the 
second generation. This was in contrast to parallel inoculations with 
the rugose type of mosaic, where symptoms appeared in both generations 
(P- 52 ). 
Fifteen healthy plants representing six different tubers were inoculated 
four times at approximately weekly intervals, with inoculum taken from 
a mild mosaic vine and applied by the leaf-mutilation method. The first 
inoculation was made when the plants were from 3 to 9 cm. in height, 
February 1, 1922, and the last on February 23, in the Washington 
(D. C.) greenhouse. On March 1 the first mild mosaic symptoms ap¬ 
peared on the young leaves in 2 plants and by March 21, 10 additional 
plants, or 80 per cent, showed mottling. The same number of vines 
were mild mosaic on all leaves in the second generation. The character 
of these symptoms was like those on the vines with a single inoculation 
with mild mosaic made in December, previously described. Accordingly, 
repeated inoculations produced a higher percentage of infected plants 
than a single inoculation. All uninoculated controls from the same 
tubers as the inoculated plants remained healthy in both generations. 
It may be pointed out that the three unsuccessful inoculations and one 
with incomplete current-season symptoms (on only one shoot) were made 
on plants among the 7 which were 5 cm. or less in height at the time 
of the first inoculation. 
Leaf-mutilation inoculations with mild mosaic were performed again 
in 1922, both in the open field and inside of insect cages, with current- 
season symptoms in the cages (PI. 2, A, 2). (For details see later section 
on “ Inoculations performed in 1922.”) 
These results show that leaf-mutilation inoculation is sometimes, but 
not necessarily, an effective means of infection with mild mosaic and 
might explain the skepticism of certain workers in Holland regarding 
this method were it not that this skepticism is apparently not based on* 
any trial of this method, but rather on a needle : prick method (4, p. 19), 
which the writers long ago discarded as useless (discussion on streak, 
P- 53 ). 
