A PHYSIOLOGICAL STUDY OF MUCOR RACEMOSUS 
AND DIPLODIA TUBERICOLA—TWO SWEET POTATO 
STORAGE-ROT FUNGI 1 
By L. L. Harter 
Pathologist f Office of Cotton, Truck, ancl Forage Crop Disease Investigations, Bureau 
of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
Most of the investigations conducted 
thus far by the writer and his col¬ 
leagues on the storage-rot fungi of 
sweet potatoes have been conducted 
with the various species of the genus 
Rhizopus, a group of fungi generally 
conceded to be very destructive to 
many vegetables and some fruits (7) . 2 
Inoculation experiments showed that 
27 different hosts were susceptible to 
decay by some of the species, two of 
which, Rhizopus microsporus and R. 
chinensis , attacked the hosts somewhat 
weakly. Likewise, the same species, 
with the exception of R. microsporus 
and R. chinensis, were found to cause 
a soft rot of sweet potatoes (6). Nu¬ 
merous isolations from various hosts 
infected naturally in storage and on 
the markets gave almost exclusively 
either R. nigricans or R. tritici, the 
former greatly predominating. Sub¬ 
sequent investigations (11) demon¬ 
strated that these two species were 
primarily responsible for the decay of 
sweet potatoes in storage, and at differ¬ 
ent temperatures in infection chambers, 
even when brought into competition 
with other species of the same genus. 
Rhizopus nigricans predominated at 
temperatures between 6° and 20° C. 
and R. tritici at 30° and above, with 
some overlapping at temperatures from 
20° to 30°. 
These organisms produce a watery 
soft rot and cause the host to decay 
quickly. Under favorable conditions 
sweet potatoes of an average size may 
be decayed in from three to five days. 
The cells are not entered by the fungus, 
at least in the earlier stages. Investi¬ 
gations (4, o) have shown that an en¬ 
zyme, peetinase, is secreted and has 
the ability to dissolve the middle 
lamellae so that coherence of the cells 
is lost. A portion of this cell wall 
dissolving enzyme was shown to be 
exuded into the culture media (8), the 
remainder being retained by the myce¬ 
lium. When the fungus is grown two 
or three days on a suitable medium 
such as sweet potato decoction, raw 
sweet potato disks 1 mm. thick are 
completely disintegrated in from two 
to four hours when immersed in a 
substrate after freed from the mycelial 
growth. 
The organisms of the Rhizopus group 
studied both from the standpoint of 
their parasitism and physiology pro¬ 
duce decay at temperatures from about 
16° to 30° C. or higher, and cause a 
rapid destruction of their hosts. They 
differ essentially in this respect from 
such fungi as Mucor racemosus and 
Diplodia tubericola, the two organisms 
which form the basis for the discussions 
in this paper. 
Mucor racemosus, although not so 
common or destructive as Rhizopus 
nigricans or R. tritici, is nevertheless 
frequently found on vegetables, espe¬ 
cially sweet potatoes (3), when held at 
temperatures a little above freezing. 
As a matter of fact, this fungus seems 
to be of no economic importance ex¬ 
cept at low temperatures. If, for ex¬ 
ample, sweet potatoes are held for 
several weeks at low temperature, from 
one to several infection centers of M. 
racemosus may develop. This fungus 
causes a slow progressive rot of sweet 
potatoes. The tissue is rendered gray 
and somewhat stringy, but not so wet 
as by the rot caused by R. nigricans. 
A microscopic examination of the de¬ 
cayed tissue shows that the cells are 
separated along the line of the middle 
lamellae in a manner similar to cells 
of tissue decayed with R. nigricans 
and R. tritici. 
Diplodia tubericola differs parasiti- 
cally and physiologically from both 
Mucor raceynos us and the various species 
of Rhizopus. It appears to have a 
great variety of hosts and to grow 
Received for publication July 29, 1924; issued July, 1925. 
J Reference is made by number (italic) to “Literature cited,” p. 968. 
Journal of Agricultural Research Vol. XXX, No. 10 
Washington, D. C. May 15, 1925 
Key No. G-487 
(061) 
