Jan. 6, 1923 
Parasitism of Sclerotium rolfsii on Irish Potatoes 
43 
The various symptoms of the disease followed each other in succession. 
Some seed tubers decayed so rapidly that the sprouts never reached the 
surface of the ground. Thus, the missing hills, a little over 25 per cent 
of the total number planted, gave the first indication of the activities 
of the parasite. About 16 per cent of those that did come up were able 
to form only a few first leaves, as the stems became thoroughly infested 
with the fungus, turned dark and water-soaked at and a little below the 
surface of the ground, and died. The appearance of such plants was 
fully typical of damping-off. The next distinct stage of attack may 
quite properly be characterized as wilt. It was observed with older 
plants, when not only the roots but also the stems were so seriously 
injured by the parasite that the activity of the conducting vessels was 
greatly impaired. A little later the stems were so corroded that the 
plants died. In some cases the foliage symptoms were so typical of wilt 
that it would be difficult on casual observation to distinguish this form 
from those caused by Fusarium, Verticillium, or bacteria (PI. 2, A). It 
is very easy, however, to ascertain the actual cause of the trouble on 
closer examination of the base of the stem, where usually a weft of white 
mycelium and the brown sderotia of Sclerotium rolfsii can be found as 
well as a stemrot in place of the discoloration of the vascular system. 
In some cases the infestation of the stem was not deep enough to sever 
completely the vascular connections; and then the symptoms of the wilt 
were not so pronounced, but the plants presented a general unhealthy 
appearance with partly drying, partly wilting foliage. This form may 
readily come under the conception of blight. This condition, too, led to 
the ultimate death of the plants (PI. 2, B). The number of wilted and 
blighted plants in the foregoing experiment constituted a little over 40 
per cent. 
A repetition of this experiment was made in 1920 on another piece of 
land on the same farm. Only the Irish Cobbler variety was used this 
time, but there were two sets of plantings—an early one, on April 8, and 
a later one, on May 6. Twelve hills to each strain of the fungus in each 
series were planted, or 48 hills in all. The remainder of the procedure 
was identical with that of 1919. The progress of the disease was one of 
great interest. Up to the middle of June all plants in the earlier set 
inoculated with culture No. 126 remained healthy, and of those inoculated 
with culture No. 127 two showed wilt. On the same date in the later 
set there were 2 hills missing and 7 wilting in the lot inoculated with 
culture No. 126, while not a single hill inoculated with culture No. 127 
showed any sign of infection. During the next two weeks the disease 
made a general advance in all the sets, and the conditions on June 29 
were as follows: 
EARLY PLANTING 
(a) Inoculations with culture No. 126: 4 hills showed infection. 
(b) Inoculations with culture No. 127: 7 hills more or less infected, in some hills 
only one stem showing the disease. 
LATER PLANTING 
(c) Inoculations with culture No. 126: 10 hills were affected, of which 5 decayed 
completely and disappeared. 
(d) Inoculations with culture No. 127: No hill showed infection. 
At harvesting time, on July 20, 3 hills remained alive in lot (a), an 
increase of 75 per cent in affected plants, 6 in lot (b) showing that 1 hill, 
partially affected, had recovered during the last three weeks before the 
harvest, 4 in lot (c), indicating another partial recovery, and 11 in lot (d). 
20516—23-5 
