962 
Vol. XXX, No. 10 
Journal of Agricultural Research 
saprophytically on almost any woody 
substance. This fungus is most preva¬ 
lent in the southern part of the United 
States, where it is a common cause of 
a storage rot of sweet potatoes known 
as the Java black rot. It produces a 
slow, comparatively dry rot of sweet 
potatoes, rendering them finally black, 
liard, and mummified. Investigations 
have shown that it will cause decay 
where the humidity of the surrounding 
air is relatively low. For example, 
inoculated sweet potatoes will decay 
if laid on a shelf in a room heated with 
warm air. 
Mucor racemosus and Diplodia tuberi- 
cola seem to differ physiologically and 
pathologically from the Rhizopus group, 
with which much of the work thus far 
has been done. The former causes 
decay only at low temperatures, and 
the latter at rather high temperatures 
and in a relatively dry atmosphere. 
In view of the striking dissimilarity 
parasitically between these two organ¬ 
isms and the different species of Rhi¬ 
zopus, they were studied physiologi¬ 
cally with the hope of obtaining some 
facts useful or applicable to the keeping 
of the sweet potato in storage. 
METHODS OF EXPERIMENTATION 
The detailed methods employed in 
conducting the experiments are dis¬ 
cussed later, certain modifications and 
variations being imperative to meet 
certain phases of the problem, there¬ 
fore only a cursory discussion of the 
methods used is given here, certain 
details being left for discussion when 
the experimental data are presented. 
The culture of Mucor racemosus 
used in these experiments was obtained 
from sweet potatoes kept at a low 
temperature at Arlington Farm, near 
Rosslvn, Va. After this organism had 
fruited it was “ pure-lined ” by cul¬ 
turing from a single spore. This 
pure-lined culture was employed in all 
the experiments. 
The culture of Diplodia tubericola 
was obtained from some decayed sweet 
potatoes sent to the writer from Ala¬ 
bama in 1922, This organism fruited 
abundantly in culture and was later 
proved to be parasitic. It was “pure- 
lined” in a similar manner to that of 
Mucor racemosus. Both of these 
organisms were kpet in an active state 
of vegetative growth by frequent 
transfers to suitable culture media. 
For the enzymatic work several dif¬ 
ferent culture media were employed at 
various times. The one used for most 
of the investigations was sweet potato 
decoction, which medium was prepared 
according to the following formula: To 
peeled potatoes add double their weight 
of water, steam for 1 hour, then squeeze 
out the liquid through gauze; steam a 
second time, filter by suction through 
absorbent cotton and autoclave 20 min¬ 
utes at 15 pounds pressure. The result¬ 
ing solution is practically free of cellu¬ 
lose structures, but it does contain some 
starch and some sugars. These organ¬ 
isms, especially Mucor racemosus, make 
a good growth on this medium. 
In studying the secretion and macer¬ 
ating action of pectinase, both the solu¬ 
tion on which the fungus grew and the 
mycelium itself were employed. The 
fungus was usually grown in 2-liter 
Erlenmeyer flasks on whatever media 
were selected for study. The mycelium 
was carefully removed from the flasks 
and washed in running water for about 
10 minutes, after which the water was 
squeezed out, the mycelium being sub¬ 
sequently treated with acetone and ether 
according to a method previously em¬ 
ployed by Dox (2). 
The mycelium after treatment was 
spread out in order to allow the ether 
to escape, after which it was bottled 
and stored until required for use. The 
mycelium was ground in fine quartz sand 
before it was employed for macerating 
experiments. The solution on which the 
fungus had grown was filtered through 
absorbent cotton, which removed any 
mycelium that might be suspended in 
it, and many of the spores. Toluene was 
used as an antiseptic. This procedure 
was frequently unnecessary, inasmuch 
as maceration was usually complete in a 
few hours or before any contaminating 
organisms could alter the results. If the 
experiment was continued for several 
hours, toluene was added to the solution. 
The preparation was then thoroughly 
shaken and the flask tightly stoppered. 
Controls prepared by steaming for 10 
minutes in an Arnold sterilizer were run 
with all experiments. 
All reducing sugars were determined 
according to the method of Clark ( 1 ). 
Raw sweet-potato disks were em¬ 
ployed in determining the macerating 
action of the enzyme pectinase. These 
were cut 1 mm. thick and 15 mm. in 
diameter from as near the center of a 
sound sweet potato as possible. Fur¬ 
ther details of the methods employed 
may be found in an article by Harter 
and Weimer (5) published in 1921. 
EXPERIMENTAL DATA 
Enzymes Studied 
No attempt has been made to study 
all or any considerable number of the 
enzymes which might be produced by 
