May 1, 1925 
Infection and Decay of Sweet Potatoes by Rhizopus 
805 
phenomenon the loss due to the disease 
is more nearly represented by the 
area covered by the spot than by the 
diameter. 
FOURTH TYPE OF EXPERIMENTS 
These experiments were devised to 
determine the upper and lower tem¬ 
perature limits at which infection by 
the various organisms will take place, 
where the “well” method of inocula¬ 
tion was used. 
Upper temperature limits. —The 
upper temperature limits for infection 
by Rhizopus tritici, R. oryzae, and R. 
maydis are recorded in Table V, and for 
R. nigricans, R. reflexus, and R. arto- 
carpi in Table III. R. tritici was 
isolated from each of eight potatoes at 
42° C. but was not isolated at 44°. 
The upper limits found by Weimer and 
Harter (7) for mycelial growth were 
42° to 45.5°. 
any infection in the experiments in 
which it was used was 28.8° C. (Table 
III). Some infection took place by 
strain b at 30° after two days’ time, 
but none at 34° (Table IV). This latter 
strain is the same as strain 4652, em¬ 
ployed by Harter and Weimer in their 
physiological studies of strains of 
Rhizopus nigricans (4)- They did not 
obtain growth on culture media of this 
strain at 30° in one day, but did ob¬ 
tain germination of spores at 31°. 
Here is a case where the maximum 
temperature for infection exceeds that 
of mycelial growth on culture media, 
but it is entirely possible that the 
difference in the time element is suffi¬ 
cient to account for this apparent dis¬ 
crepancy especially since a vigorous 
culture (48 hours old) was used in the 
infection experiment as contrasted 
with a spore suspension drop in the 
mycelial growth experiment. It is 
possible, too, that the sweet potato 
Table V.— Upper temperature limits for infection of sweet potatoes by Rhizopus 
tritici, R. oryzae and R. maydis 
Organisms isolated 
Num¬ 
Organism with 
which potatoes 
Time 
after 
Num¬ 
ber of 
pota¬ 
toes 
inocu¬ 
lated 
Num- ! 
ber of 
pota¬ 
toes 
de¬ 
cayed 
Num¬ 
ber of 
isola¬ 
Tem¬ 
pera¬ 
ber of 
times 
organ¬ 
ism 
were inoculated 
inocu¬ 
lation 
tions 
! made 
ture 
used 
in 
inocu¬ 
lation 
Other organisms isolated 
was 
1 
iso¬ 
lated 
Days 
°C. 
Two plates were sterile and 5 gave 
Rhizopus tritici _ _ 
2 
8 
8 
7 
44 
0 
bacteria. 
Do_ 
2 
8 
8 
8 
42 
7 
1 Rhizopus tritici plus bacteria. 
Do_ 
2 
8 
8 
8 
34 
8 
Rhizopus oryzae.. 
2 
8 
8 
6 
44 
0 
4 plates sterile, bacteria in 2. 
Do_ 
2 
8 
8 
7 
42 
3 
3 plates bacteria, 1 bacteria and 
Rhizopus. 
Do_ 
2 
8 
8 
8 
34 
7 
Rhizopus maydis. 
i 2 
6 
! 6 
6 
44 
0 
Bacteria in all plates. 
Do_ 
2 
8 
1 8 
8 
42 
1 
Bacteria in 5 plates. 
Do_ 
2 
8 
8 
8 
34 
6 
Bacteria ini, 1 sterile. 
Rhizopus oryzae was isolated in 3 
or 4 out of 8 potatoes at 42° C., but it 
was not isolated at 44°. The max¬ 
imum found for mycelial growth by 
Weimer and Harter (7) was from 42° to 
45.5°. Rhizopus maydis was iso- 
ated from 1 potato out of 6 at 42°, 
indicating that this temperature is near 
the maximum. It was not isolated at 
44°. The maximum temperature for 
mycelial growth recorded by Weimer 
and Harter (7) was 40° to 44.5°. 
The highest temperature at which 
Rhizopus nigricans, strain a, caused 
may form a better medium for growth 
than potato agar, the culture medium 
used by Harter and Weimer. It has 
been shown (7) that the maximum 
temperature may be affected by the 
culture medium. 
Rhizopus reflexus caused some decay 
at 29.8° C. but none at 34.5° (Table 
III, and fig. 7). Weimer and Harter 
(?) found mycelial growth at 33°. It 
is possible. that infection may have 
taken place at temperatures between 
29.8° and 34.6°, but the drop in 
the amount of decay between 28.1° 
