May 15, 1925 
967 
Study of Two Sweet Potato Storage-Rot Fungi 
Table II.— Hydrogen-ion concentration 
of the controls and the used solu¬ 
tions after 10 days’ growth of Diplodia 
iubericola. Average of two experi¬ 
ments 
Solution 
Control 
(Ph) 
Inocu¬ 
lated 
solution 
(Ph) 
Prune decoction_ 
3. 86 
3.98 
Sweet-potato decoction_ 
4. 66 
8. 49 
Carrot decoction_ _ . 
5. 11 
8. 62 
Potato decoction__ 
6.04 
8. 72 
Turnip decoction_ 
4. 98 
8.47 
String-bean decoction_ 
4. 75 
8. 62 
Beef bouillon_ _ 
7.15 
8. 59 
Czapek’s solution_ . 
3. 77 
2.52 
Pfeffer’s solution_ 
3. 62 
2. 42 
Richard’s solution__ 
3. 65 
2. 11 
DISCUSSION 
The foregoing data show that within 
the limits of these experiments Diplo¬ 
dia Iubericola does not produce a 
macerating principle at any of the 
temperatures tried. On the other 
hand, M ucor racemosus secretes an 
enzyme which will completely disin¬ 
tegrate the tissue of raw sweet potato 
disks in from 6 to 8 hours. The same 
media were used in these experiments 
as were employed in studying the in¬ 
fluence of the substrate on pectinase 
production by Rhizopus tritici (8). 
In the case of the latter organism dis¬ 
solution of the middle lamellae was 
complete in a shorter period of time. 
However, a comparison of the results of 
the investigations of these two organ¬ 
isms with respect to the time elements 
is not strictly justifiable, in view of the 
known differences in the growth habits 
of the two fungi. It is interesting to 
note, however, that both fungi are 
alike influenced by the substrate on 
which they grow. 
Pectinase was produced in all of the 
vegetable media tried except prune and 
potato decoction. None was produced 
by either R. tritici or M. racemosus 
when grown on synthetic media (Cza¬ 
pek’s, Pfeffer’s, and Richard’s), or on 
beef bouillon where glucose was used 
as a source of carbon. 
The only conclusion seemingly to be 
drawn from these results is that there 
is a regulatory influence of the sub¬ 
strate. Were it not for the results 
obtained from prune and potato decoc¬ 
tions it might be assumed on theo¬ 
retical grounds that the regulatory 
substance may be some of the pec tic 
compounds. In the article cited above 
it was shown that although no pec¬ 
tinase was produced by R. tritici when 
grown in Czapek’s nutrient solution 
with glucose as a source of carbon, it 
was secreted if pectin was supplied as 
the only source of energy. On the 
other hand, if the pectin were combined 
with glucose as the available supply of 
carbon, the action on raw sweet potato 
disks was very feeble. 
The volume of growth can hardly be 
urged as a possible explanation, inas¬ 
much as it has been found that when 
the macerating principle is secreted, 
this occurs very early in the growth of 
the fungus, before any considerable 
amount of mycelium has been pro¬ 
duced. A demonstrable amount of 
pectinase is secreted by R. tritici in 
6 to 7 hours of growth and by Mucor 
racemosus in 3 days. Although the 
growth on prune decoction was less 
than on the other vegetable media em¬ 
ployed, it was nevertheless fairly good, 
and on potato decoction it was not 
exceeded to any considerable extent 
except on sweet potato and turnip. 
The dry weight of mycelium produced 
on the synthetic media was equal on 
an average to that on the vegetable 
media, and no pectinase was produced. 
A comparison of Tables I and II 
shows that both Mucor racemosus and 
Diplodia tubericola have somewhat simi¬ 
lar action on the hydrogen-ion concen¬ 
tration of the substrate. Prune decoc¬ 
tion was not materially changed, but 
the acidity of all the other vegetable 
decoctions and of beef bouillon was 
decreased, and in all cases to the alka¬ 
line side of neutrality. The hydrogen- 
ion concentration of Czapek’s, Pfeffer’s, 
and Richard’s solutions was increased 
by D. tubericola . M. racemosus caused 
no appreciable change in the hydrogen- 
ion concentration of Pfeffer’s and Rich¬ 
ard’s solutions, but increased the acid¬ 
ity of Czapek’s solution. These results 
agree only in part with those obtained 
with Botrytis cinerea (13), which in¬ 
creased the hydrogen-ion concentration 
of some of these vegetable media and 
decreased that of others. 
A study of these data and those of 
Rhizopus tritici show that different fungi 
act differently under what may be 
considered similar conditions. One 
organism may increase and another de¬ 
crease the hydrogen-ion concentration 
of the same medium. From these 
results it is evident that no sweeping 
generalizations can be made for all 
fungi from the results obtained from a 
few. Each organism must be assumed 
a priori to be physiologically different 
from all others. 
The investigations already carried 
out and discussed above show that 
pectinase was produced when Mucor 
