796 
Vol. XXX, No. 9 
Journal of Agricultural Research 
would expect from the vaivt Hoff law. 
The quantity of decay varies from 
nearly six times to many times as much 
at one temperature as at 10° below, 
except between 20.5° and 21.5° C. and 
at temperatures near the upper limit 
for infection. 
Table II.— Amount of decay at various 
temperatures by Rhizopus tritici and 
R. nigricans 
1 
Temperature 
Time 
after 
inocu¬ 
lation 
N T umber 
of po¬ 
tatoes 
inocu¬ 
lated 
Number 
of po¬ 
tatoes 
decayed 
Weight 
of de¬ 
cayed 
tissue a 
0 c. 
37.0_ 
Days i 
4 
50 
48 
Grams 
3,511 
32.5_ 
4 
50 ! 
46 
5,906 
29.0_ 
4 
50 j 
42 
4,600 
26.5_ 
4 
50 
29 
2, 626 
21.5_ 
4 
50 
37 
925 
20.5_ 
4 
50 
40 
885 
14.5_ 
7 
50 
1 34 
1,878 
11.5_ 
7 
50 
| 43 
254 
6.5....- 
12 
50 
50 
680 
4.0__ 
12 
i 
50 
50 
109 
° These weights were calculated on the basis of 
30 potatoes from the decayed potatoes at each 
temperature. 
There was less decay between 20.5° 
and 21.5° C., but these temperatures 
correspond to the lower limit of infec¬ 
tion for Rhizopus tritici when this 
method of wounding and inoculation 
are used (6), and there is often a de¬ 
crease of decay observed at these 
temperatures. It is not clear whether 
this is due to some factor associated 
with the pathogens or to resistance on 
the part of the host. 
The increments of decay per degree 
are particularly high at temperatures 
between 11.5° and 14.5° C. and be¬ 
tween 4° and 6.5°. 
No isolations were made from these 
potatoes, but previous experience ( 6) 
would justify one in concluding that 
the organisms involved were Rhizopus 
tritici 5 and R. nigricans, R. tritici ex¬ 
clusively at 30° C. and above, and R. 
nigricans at 20° and below, the two 
overlapping between 20° and 30°. 
THIRD TYPE OF EXPERIMENT 
The procedure in these experiments 
was the same as in the second, except 
that the “well” method ( 2 ) of inocula¬ 
tion was employed. Forty-eight-hour 
old cultures of the organisms used, 
grown on 2^ c. c. of sweet potato de¬ 
coction in test tubes at room tempera¬ 
ture (20 to 25° C.), were introduced 
into “wells” 1 cm. in diameter and 
4}^ cm. deep. The “wells” were 
deep enough so the inoculum reached 
approximately the center of the pota¬ 
toes. It was desired to start infection 
at or near the center so that the decay 
might proceed for some time before 
reaching the surface or skin of the 
potato as it proceeded outward from 
the point of infection. An indetermin¬ 
able error results if the decay is per¬ 
mitted to reach the surface, because as 
soon as it reaches the skin at any point 
it stops in that direction, thus limiting 
the amount of decay. 
From the data presented in Tables 
III and IV the six species studied can 
be separated into two groups according 
to their temperature responses: First, a 
high temperature, and second, a low 
temperature. Rhizopus tritici, R. oryzae 
and R. maydis belong to the former and 
R. nigricans, R. reflexus and R. artocarpi 
to the latter. 
This relation is illustrated in Figures 
1 and 2. Curves representing all of 
the organisms except Rhizopus oryzae 
are shown in Figure 1. R. oryzae and 
R. tritici are shown in Figure 2. The 
curves in Figure 1 were drawn from 
weighings (Table III) of decay at 
different temperatures after two days 
and those in Figure 2, from weighings 
made after three days. It will be seen 
from Figure 2 that R. oryzae belongs to 
the same group as R. tritici. The opti¬ 
mum temperatures for infection for the 
three species of the high temperature 
group lie between 32° and 35° C. 
(Table III and figs. 1 and 2), and those 
for the three species of the low tempera¬ 
ture group between 18.5° and 23° C. 
(Table III and fig. 1). 
In case of the high temperature group 
there is a close correspondence to the 
optimums obtained by Weimer and 
Harter (7) ( R , tritici, 33° to 35° C., 
R. oryzae, 31° to 34°, and R. maydis, 
30.5° to 32.5°, for mycelial growth on 
culture media. The optimum tem¬ 
peratures obtained by Weimer and 
Harter (7) for R. nigricans (23° to 26°), 
R. reflexus (26° to 28°), and R. arto¬ 
carpi (26° to 28°) are somewhat higher 
than the optimum temperatures for in¬ 
fection obtained in the present experi¬ 
ments ( R . nigricans, strains a and b, 
18.5° to 23.5°, R. reflexus, 18.5° to 23°, 
and R. artocarpi, 23° to 24° (Table 
III). The data, in connection with the 
last three species correspond to those 
recorded by Lauritzen (5) for Puccinia 
graminis Pers. var. tritici Erick, on 
wheat and Colletotrichum lindemuth - 
ianum (Sacc. and Magn.) Bri. and Cav. 
on beans, where the optimum temper¬ 
atures for infection were slightly lower 
than optimum temperatures for ger¬ 
mination of spores and growth of my¬ 
celium. 
5 Rhizopus tritici here is used in a collective sense and may include R. nodosus, R. oryzae and R. delemar. 
It is not possible from present knowledge to distinguish morphologically between any of these species 
