May 15, 1925 
963 
Study of Two Sweet Potato Storage-Bot Fungi 
these two fungi. At the outset it was 
merely planned to study pectinase pro¬ 
duction, or the secretion and action of 
the enzyme responsible for the dissolu¬ 
tion of the middle lamellae. However, 
since there was no information available 
as to any of the enzymes produced by 
these two fungi, an attempt was made 
to demonstrate the presence of a few of 
the more common ones. 
PECTINASE 
Mucor racemosus. —A series of pre¬ 
liminary pectinase experiments with 
Mucor racemosus grown on a modifica¬ 
tion of Czapek’s nutrient solution at 
room temperature failed to demonstrate 
the secretion of the enzyme cytase. 
However, an examination of sweet po¬ 
tatoes decayed by this fungus at tem¬ 
peratures a little above freezing showed 
that the middle lamellae were dissolved 
somewhat in advance of the growing 
hyphae. The fact that decay occurred 
only at such low temperatures suggested 
the possibility that either the macer¬ 
ating principle was secreted only at low 
temperatures, or that it was produced 
only on certain substrates, or that both 
hypotheses were true. 
Another preliminary experiment in 
which sweet potato decoction was em¬ 
ployed showed that a substance which 
would completely macerate raw sweet- 
potato disks in from 6 to 8 hours was se¬ 
creted into the substrate. A number 
of different media were later employed 
as substrates, the results of which will 
be discussed elsewhere. 
A series of experiments was planned 
subsequently in which after inoculation 
2-liter Erlenmcyer flasks containing 
about 500 c. c. of sweet potato decoc¬ 
tion were incubated at a temperature of 
5° C. Immediately upon the removal 
of the cultures from the incubator the 
substrate and the mycelium were pre¬ 
pared for use in accordance with meth¬ 
ods already discussed. As soon as the 
raw sweet-potato disks were added to 
the system incubation was carried out 
at a constant temperature of 35° C. 
In the first experiment the organism 
was grown for 37 days, at the end of 
which time the flasks were removed 
from the incubator and the macerating 
power of the enzyme in the solution on 
which the fungus grew and that re¬ 
tained by the mycelium was determined. 
The results showed that maceration of 
the disks suspended in the substrate was 
evident in 3 hours, well advanced in 6 
hours, and practically complete in 7 
hours. The disintegration by the myce¬ 
lium was very much slower, maceration 
being only started at the end of 41 hours. 
In another series of experiments eight 
flasks were inoculated at one time and 
incubated at a constant temperature of 
5° C. Three of these flasks were re¬ 
moved from the incubator at the end 
of 16 days, 3 after 24 days, and 2 at 
the end of 44 days. The results 
showed that there was no measurable 
difference in the time required to com¬ 
pletely macerate raw sweet potato disks 
in the solutions removed from the 
incubators at the end of the three 
different periods of growth. Macera¬ 
tion started in about 3 hours and was 
practically complete at the end of 7 or 
8 hours. But the enzyme in 0.25 gm. 
of mycelium suspended in 25 c. c. of 
water required 18 to 24 hours to dis¬ 
integrate the middle lamellae com¬ 
pletely. These experiments were re¬ 
peated several times by growing the 
mycelium at the same temperature, 
with no measurable difference in the 
results. 
It has already been pointed out that 
decay of sweet potatoes by Mucor 
racemosus has been observed to occur 
only at temperatures a little above 
freezing. In view of this fact, experi¬ 
ments on pectinase production were 
first carried out by growing the myce¬ 
lium at a temperature of 5° C., in which 
the enzyme was shown to be produced. 
Some of the enzyme was exuded into 
the substrate and some was retained by 
the mycelium. 
Inasmuch as decay by this fungus 
does not usually occur at higher tem¬ 
peratures, experiments were designed to 
determine whether or not pectinase was 
produced when the organism was grown 
at a temperature of 30° C. The 
methods of experimentation were the 
same as those already presented for 5°. 
Without discussing these experiments 
in detail it may be said that although 
pectinase was produced at 30°, it was 
somewhat less active than when the 
fungus was grown at the lower tem¬ 
perature, for 1 to 2 hours longer were 
required to macerate raw sweet potato 
disks completely when immersed in the 
solution. The action of the enzyme 
from mycelium suspended in water, 
however, was considerably slower; in 
some cases 48 hours or more were 
required to completely dissolve the 
middle lamellae so that coherence was 
lost. It was further found that a 
considerable amount of the macerating 
principle was produced during a three 
days’ growth of the fungus, resembling 
in this respect some of the species of 
Rhizopus (9). 
The results of these investigations 
show that Mucor racemosus when 
grown in sweet potato decoction at 5° 
