Jan. 6,1923 
Parasitism of Sclerotium rolfsii on Irish Potatoes 
45 
curved, or irregular in shape. The constant relative difference in the size 
of the sclerotia of the two strains was observed on the following media: 
Raw potato, cooked potato plugs, potato agar, corn-meal agar, oatmeal 
agar, rice agar, and rice. Other media were not included in the studies. 
Considering the characters of the two cultures stated in the preceding 
paragraphs, the writers believe the conclusion to be quite warranted 
that the difference between Sclerotium No. 126 and Sclerotium No. 127 
is that of, at least, varietal character. Having no authentic culture of 
Sclerotium rolfsii Sacc. they are unable to determine which, if either, of 
the two is the true species. 
CHARACTER OF SCLEROTIUM TUBER-ROT 
The potato tubers infected with Sclerotum rolfsii in the field or arti¬ 
ficially inoculated in the laboratory are subject to a rapid progressive 
decay. This decay is practically odorless and colorless in its earlier 
stages but takes on a yellowish appearance in older portions, particularly 
if there is a considerable development of the fungus hyphae. The affected 
host tissues become usually more or less porous. This was exactly the 
type of rot from which culture No. 126 was isolated. It was also re¬ 
peatedly reproduced in laboratory inoculations. The term “white rot’' 
would seem to be quite appropriate to and descriptive of this stage of 
the disintegration. Under sufficiently favorable moisture and temperature 
conditions the decay may develop into what has been termed “melter” 
type, when the affected portions become exceedingly soft and watery. 
Most of the laboratory inoculation experiments were carried on with 
the Irish Cobbler variety, but a small quantity of Bliss Triumph and a 
seedling variety were also tested with no apparent difference in the 
response of these varieties to the action of the parasite. Inoculum was 
inserted either in needle pricks or in shallow scalpel wounds. Both 
relatively young sclerotia and young mycelium of the fungus were used 
with practically the same success. The inoculated tubers were placed in 
ordinary glass moist chambers or stone jars with layers of wet filter 
paper below and above and were left in the laboratory for two weeks at 
the temperature of 20° to 22 0 C. The difference in extent of rot pro¬ 
duced under these conditions by the two Sclerotium cultures was only 
slight and may not alone be regarded as significant. The diseased areas 
resulting from inoculation with culture No. 126 averaged 23 to 25 mm. 
in diameter and 14 to 15 mm. in depth; those resulting from inoculation 
with culture No. 127 averaged 18 to 20 mm. in diameter and 10 to 12 
mm. in depth. The “melter” type of decay did not develop during this 
period and under these conditions. Liquid, however, separated from 
tubers kept in moist chambers for a longer period of time. 
The destructive effect of the fungus on potato tuber tissue was very 
clearly shown when sterile raw potato blocks were inoculated in large 
quantities in Erlenmeyer flasks. There first developed an abundant 
growth of pseudo-parenchymatous mycelium emveloping the blocks and 
practically filling the flasks. Abundant sclerotial formation followed 
with the development of large compact aggregates of these bodies which 
were frequently an inch or even more in diameter. When the mycelial 
development had reached its maximum there appeared a gradually in¬ 
creasing accumulation of light amber liquid in the bottom of the flask, 
sometimes amounting to as much as 500 cc. in a single 2-liter flask. 
Since no additional water was placed in the flask with the raw potato 
blocks, it is evident that this liquid resulted from the action of the fungus 
