May 12,1923 
Rhizopus Responsible for Sweet Potato Decay 
449 
others were inoculated while in the condition in which they came from the 
storage house. Washed and unwashed potatoes, wounded in the same 
manner as those inoculated, were used as controls in all the experiments. 
The following species were employed in these experiments: R. nigricans, 
R. reflexus, R. artocarpi, R. tritici, and R. oryzae. Washed and unwashed 
potatoes were inoculated with each of the species alone, except R. nigri¬ 
cans. Inoculation in each case was by dipping them into a spore suspen¬ 
sion. Washed and unwashed potatoes were inoculated with a mixed 
spore suspension of R. reflexus and R. nigricans. Washed potatoes were 
inoculated with a mixed spore suspension of R. artocarpi and R. nigricans, 
and unwashed potatoes with R. txitici and R. nigricans. 
In the preceding experiments R. nigricans was shown to be the principal 
organism causing decay at the usual storage temperatures. This species 
has also been shown to cause decay of many fruits and vegetables. {4) 
It was found in the second type of experiments that when the sweet 
potatoes were inoculated with R. tritici and R. nigricans, nigricans caused 
all the decay below 20° C., notwithstanding the fact that R. tritici is not 
only capable of infecting potatoes at these temperatures, but apparently ^ 
to the same degree where the “ weU’' method of inoculation is employed. 
In view of these facts it was thought desirable to make competitive 
tests between R. nigricans and some of the other species as to their 
capacity to infect sweet potatoes. 
R. tritici was selected, first, because it is the species most commonly 
isolated from sweet potatoes next to R. nigricans, and, second, because 
with the exception of R. nigricans it is the species most commonly 
received in response to requests to other investigators for cultures of 
Rhizopus. It has been sent in, on a number of occasions, as R. nigricans. 
R. oryzae was selected because Hanzawa {2) placed it in the high- 
temperature group {R. tritici falling into the intermediate and R. nigri¬ 
cans into the low-temperature group) with the idea of studying one 
organism of each thermal group. 
R. reflexus was chosen because it belongs to the same thermal group 
as R. nigricans, its upper temperature limit being slightly higher than 
that of R. nigricans and its lower temperature limit slightly lower. 
Furthermore, it decays sweet potatoes (where the ‘ ‘ well meSiod is used) 
fully as rapidly as R. nigricans. 
R. artocarpi was likewise chosen because it belongs to the same thermal 
group as R. nigricans and is able to decay sweet potatoes over tnuch the 
same temperature range, where the “ well” method of inoculation is used. 
The spore suspensions employed in these experiments were made by 
introducing spores from cultures of the respective species into battery 
jars containing water. In every case except that of R. nigricans the 
concentration was so great as to render the water almost black. 
The reason for using such highly concentrated spore suspensions was to 
insure ample competition for R. nigricans, which, in the first and second 
series of experiments, successfully excluded R. tritici from infecting 
potatoes at temperatures below 20® C. and which in the course of these 
experiments proved to be able to compete against odds with the other 
species. 
7 It is not practical to directly compare the amount of decay caused by one species with that of another 
because of the indeterminable factors involved, but the indications are that the rate of decay by R. tritici 
is as rapid as that of R. nigricans at most temperatures below 20* C. 
