May 1, 1925 
Infection and Decay of Sweet Potatoes by Rhizopus 809 
the case with Rhizopus tritici and R. 
nigricans (Type I) (6), covering tem¬ 
peratures varying from 3.4° to 12° C. 
at the lower limits to 42° at the upper. 
R. tritici has the widest range of any 
of the species; in fact, is as wide as 
the combination of any two of the 
low and high temperature species. 
It is quite possible that R. reflex us 
may be able to infect at a lower tem¬ 
perature than R. tritici , but these 
data do not show that it can do so. 
The maximum and optimum tem¬ 
peratures of Rhizopus tritici , R. oryzae, 
and R. maydis are several degrees 
higher than those of R. nigricans , R. 
reflexus, and R. artocarpi. These six 
species can thus be separated into a 
high and low temperature group. 
The former group corresponds to the 
intermediate-temperature group of 
Weimer and Harter (7) and the latter 
to their low-temperature group. 
The two strains of Rhizopus nigricans 
show a slightly different response to 
temperature. Strain b has a some¬ 
what higher maximum after a given 
time than strain a. The optimum of 
strain b falls within the optimum range 
of strain a, the latter having a slightly 
wider range. The lower temperature 
limit of strain b was not determined 
except for the period of two days, 
which was too short to be accurate. 
Considerable time is required for 
infection at temperatures below 5° C. 
The increments of decay with the 
rise in temperature are too large to 
claim any relation with the van’t 
Hoff law, at least directly. In some 
instances the amount of decay is 10 
times as great at one temperature as 
at 10° below. 
Notwithstanding the fact that a 
comparison frequently is made between 
the rate of chemical reaction and the 
rate of growth, there seems upon 
careful analysis of the phenomena 
involved in two cases to be no real 
justification for such a comparison. 
In the first case pure chemical reac¬ 
tions are under consideration, while 
in the second numerous reactions, not 
all chemical, too complex for perfect 
analysis with present knowledge are 
involved. 
In the present experiments the 
surface exposed to the action of the 
fungus is never the same at the various 
temperatures at any moment after 
the decay begins. The decay radiates 
outward in all directions from the 
point of infection where the “well” 
method of inoculation is employed, 
and in all directions where there is 
any tissue to decay where infection 
begins on the side of the potato form¬ 
ing a spherical mass of decayed tissue. 
The area of attack corresponds to the 
surface of this sphere. Not only 
does the formation of this sphere at 
the higher temperatures commence 
earlier than at lower temperatures but 
it enlarges more rapidly because of the 
greater rate of decay due to the 
effects of temperature, so that the 
discrepancy in the area exposed to 
the action of the fungus at the various 
temperatures becomes larger and larger 
with time. 
Where the decay is a surface phenom¬ 
enon the area exposed to the action 
of the fungus likewise increases with 
the lapse of time. In this instance 
the area exposed conforms more to the 
area of a circle instead of that of a 
sphere. 
There is no reason to expect any 
correlation between linear measure¬ 
ments of growth and chemical reactions. 
In the latter case one has to do with 
mass; in the former, with distance. 
The mass in case of any decay will vary 
with the host and, the character of the 
disease, in other words, where two spots 
are of the same dimensions in two dif¬ 
ferent tissues, the mass may be totally 
different. 
Other factors, such as the amount of 
enzymes secreted at the various tem¬ 
peratures, the passing of the hyphae 
of the fungus out of the area of the end 
products, the nature of the decay, and 
the character of the host and the fungus, 
play a part in these reactions. These 
factors are absent in pure chemical 
reactions. 
SUMMARY 
The data regarding the influence of 
temperature upon infection and decay 
of six species of Rhizopus are presented 
in this paper. 
The time required for Rhizopus to 
infect sweet potatoes wounded but not 
artificially inoculated varies from five 
to seven days at 9° C. to 43 hours and 
less at 18° to 32°. This time would be 
expected to vary with the amount of 
wounding and the number of spores 
present on the potatoes. 
The six species of Rhizopus studied 
can be placed into two groups accord¬ 
ing to their temperature responses, as 
follows: A high-temperature group, R. 
tritici , R. oryzae , R. maydis , and a low- 
