794 
Journal of Agricultural Research 
Vol. XXX, No. 9 
by itself, especially when it is con¬ 
sidered in connection with the species 
Weimer and Harter (?) employed in 
their experiments. 
The experiments recorded in the 
present paper had for their purpose 
(1) the determination of the optimum, 
maximum, and minimum temperatures 
for the infection and decay of sweet 
potatoes; (2) the measurement of the 
amount of decay with rise in tempera¬ 
ture and after different periods of 
time; and (3) the determination of the 
time required for infection to take 
place, by six species of Rhizopus. 
These species are, R . tritici, R. oryzae, 
R. maydis , R. reflexus, R. artocarpi, 
and R. nigricans strains a and 5. 3 
The reason for eliminating three of 
the species (Rhizopus nodosus, R. dele- 
mar, and R. arrhizus) found by Harter, 
Weimer, and Lauritzen (3) to be para¬ 
sitic on the roots of sweet potatoes, and 
employing only the remaining six 
species, will now be considered. R. 
tritici and R. nodosus are almost iden¬ 
tical in their temperature responses on 
culture media (?). The temperature 
responses of R. oryzae and R. delemar 
are likewise almost identical according 
to the data of Weimer and Harter (?) 
and there is scarcely any difference ac¬ 
cording to Hanzawa’s data ( 1 ), ex¬ 
cepting in fruiting, and here the differ¬ 
ence is not great. Since these two 
pairs likewise behaved similarly in in¬ 
fection experiments (5), it was decided 
to eliminate one of each pair. R. 
arrhizus was not available at the time 
these experiments were conducted. R. 
maydis was selected because it is rather 
distinct in some of its characters, par¬ 
ticularly in that it produces very few 
spores. 
Rhizopus nigricans, R. reflexus, and 
R . artocarpi belong to the same thermal 
group, but show some variation in their 
temperature responses ( 6, ?). For ex¬ 
ample, there is a slight variation in the 
maximum, minimum, and optimum tem¬ 
peratures for spore germination, my¬ 
celial growth, and fructification of these 
three species (?). R. artocarpi is more 
successful in competition with R. nigri¬ 
cans at 14° C. than at 12°, and R . 
reflexus is more successful at 12° than 
at 14° (6). 
Rhizopus nigricans is by far the most 
important organism of the group from 
the standpoint of the amount of decay 
it causes (6); in fact, in storage, it 
causes most of the soft rot decay of 
sweet potatoes. 
MATERIALS 
Single-spore strains of the above 
listed species were used in these experi¬ 
ments. The variety of sweet potatoes 
used was Little Stem Jersey. The po¬ 
tatoes were cured for 10 days at from 
25° to 30° C. and stored at tempera¬ 
tures ranging mostly between 10° and 
15°. 
In these experiments it is important 
that the size and shape of the potatoes 
should be as nearly uniform as possible, 
otherwise any irregularities at the sur¬ 
face of the potatoes around the point 
of infection would alter the amount of 
decay. To be comparable in any two 
cases, the decay must be able to pro¬ 
ceed within equal radii, which it can 
not do for any length of time if the 
potato is small and irregular in shape. 
It is not possible to obtain potatoes of 
identical size and shape, but it is pos¬ 
sible to approach this condition suffi¬ 
ciently to make the results comparable. 
If one is comparing the amount of 
decay at several temperatures, there 
must be enough tissue available for the 
decay to proceed at the higher tem¬ 
peratures over sufficiently long periods 
to give opportunity for decay to take 
place at the lower temperatures. 
APPARATUS 
The apparatus described by Lauritzen 
and Harter ( 6 ) was used throughout 
these experiments. 
Four types of experiments form the 
basis of this work, the methods em¬ 
ployed varying with the type. 
FIRST TYPE OF EXPERIMENT 
This type of experiment was designed 
to determine the time required for in¬ 
fection, where infection depends on the 
organisms normally present on the po¬ 
tatoes. The potatoes in the condition 
in which they were received from the 
storage house, were wounded 4 to the 
same degree on the side of the roots at 
the point of the greatest diameter. 
It is believed that wounding is the 
chief predisposing condition permitting 
infection by Rhizopus, and the methods 
employed in these experiments enable 
one to obtain some idea of the time re¬ 
quired for infection to take place. They 
3 Strain as used here merely means that the organisms used were obtained from different sources and 
isolated from different hosts. Strain a was isolated from tomato by F. C. Meier, and strain b from sweet 
potato by one of the writers. 
4 The wounding was done with a special instrument with two blades, 1 cm. long and 0.2 cm. wide. The 
blades were mounted parallel to each other 1 mm. apart on a brass rectangular plate 1.5 by 1.5 cm., which 
served as a shoulder. On the opposite side of the plate was a handle used to shove the blades into the 
potatoes. The blades were pushed into the potatoes twice, as far as the shoulder permitted, the second 
time at right angles to the first. 
