May 1,1925 
Infection and Decay of Sweet Potatoes by Rhizopus 799 
Table III.— Amount of decay of sweet potatoes in grams produced by six species of 
Rhizopus at various temperatures and after different periods of time a 
Tempera¬ 
ture 
Rhizopus tritici 
Rhizopi 
oryzae 
as 
9 
days 
Rhizopus 
maydis 
Rhizopus 
nigricans 
strain a 
Rhizopus Rhizopus 
reflexus artocarpi 
1 
day 
2 
days 
3 
days 
5 
days 
« 
days 
1 
day 
3 
days 
l 
day 
2 
days 
2 
days 
4 
days 
5 
days 
2 
days 
3 1 
days day 
2 
days 
° C. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
Gm. 
39.5.. 
155 
729 
1,051 
22.5 
31 
39.0. 
164.5 
660 
34.5_ 
212 
1,162 
1,607 
G 
0 
34.0_ 
91 
534 
33.0.._ 
228.8 
1, 555 
0 
32.0_ 
222 
702 
1, 399 
40.5 
624 
3. 5 
29.8.. 
28 
28.8.._ 
61 
443 
1,121 
0 
6 
0 
28.0.._ 
122.2 
1,295 
21 
45 
149.1 
100 
43 
40 
24.5_ 
77. 7 
1,119 
36 
245 
526 
122 5 
191 
24.3_ 
50 
211 
909 
24.0_ 
1 
17 
23 
135 
408 
23.0.... 
23.5 
83 
462 
1,196 
888 
72 
356 
18. 5 
25 
36 1 
578 
770 
231 
273 
85 
516 
20.0___ 
32 
127 
329 
1,176 
698 
24 
188 
11. 5 
12 
79 
362 
264 
224 1 
348 
68 
432 
18.5.. 
39 
62 
272 
780 
890 
_ 
13.5 
16 
72 
508 
635 
194. 5 
650 
18.0.._ 
19.6 
151_ 
40 
281.1 
14.5_ 
25 
56 
321 
415 
4 
27.7 
7 
8 
82. 6 
352. 
7 
73 
14.3.. .. 
_ _ 
20 
264 
474 
12.0_ 
0 
25 
275 
11 
8 
15 
242 
321 
33 5 
231 
4 
32 
9.O.._ 
12 
24 
68 
72 
0 
16 
20. 0 
7 
27 
59 
12 1 
56 
o 
10 
5.0_ 
8 
14 
10 
0 
0 
5.5 
0 
7 
0 
0 
0 
12.5 
0 
9 
° The data recorded in Table III are the average of two or more experiments of all the species (strain a of 
Rhizopus nigricans), except that of R. maydis where only one experiment was conducted, due to the diffi¬ 
culty of obtaining sufficient spores for a spore suspension from which to inoculate the tubes of sweet potato 
decoction. The weighings given represent the decay of four potatoes and in most instances they are the 
averages of two or more experiments. In some instances more than four potatoes were used, but the 
weighings given were calculated on the basis of four potatoes as a unit. 
In genera], it might be expected that 
the optimum temperature for infection 
would fall below the optimum for 
growth on culture media, because of the 
resistance offered by the hosts and the 
direct availability of the food in the 
culture media. 
It has been noticed that at a tem¬ 
perature of 33° C. and above a larger 
percentage of unwounded sweet pota¬ 
toes normally decay with Rhizopus soft 
rot than at lower temperatures. In 
other words, the resistance to Rhizopus 
is apparently broken down at the higher 
temperatures. This may explain why 
the optimum for decay bv R. tritici R. 
oryzae and R. maydis is as high as for 
growth on artificial media. 
Figures 3, 4, 5, 6, 7, and 8 show tem¬ 
perature curves of the six species indi¬ 
cated ( Rhizopus nigricans , strain a) and 
represent the amount of decay after 
different periods of time. It will be 
noted from these curves (see also figs. 
1 and 2) that the amount of decay does 
not increase in the same proportion as 
the rise in temperature in the early 
stages of decay in case of any of the 
organisms, there being a lag at the 
lower temperatures. There is a partial 
recovery of this lag with the lapse of 
time. The additional time required for 
the fungus to become established at the 
lower temperatures before decay actu¬ 
ally begins may be a factor in producing 
the lag. 
Table IV. — Amount of decay of sweet 
potatoes caused by Rhizopus nigricans 
strain b, two days after inoculation a 
Temperature 
Weight 
of decay 
Number 
of pota¬ 
toes in¬ 
oculated 
Number 
of 
potatoes 
decayed 
°C 
Grams 
38_ 
20 
34.._ 
20 
30_ 
103.2 
37 
12 
28._ 
138.0 
47 
28 
24. 5_ 
312.0 
47 
40 
22. 5_ 
379. 6 
47 
39 
19. 7_ 
296.0 
47 
42 
18.8.._ 
234.4 
46 
43 
13.0_ 
38.4 | 
47 
47 
11.8.._ 
24.0 j 
47 
47 
10. 0_ 
9. 2 
47 
46 
7. 0_ 
5. 2 
46 
46 
5. 0... 
47 
a In this table the results of two experiments are 
given and the weighings are calculated on the basis 
of four potatoes as in Table III. Controls were 
run in connection with these experiments. Sterile 
sweet potato decoction was introduced into the 
“well” instead of cultures of the organisms. The 
potatoes remained sound in all cases, indicating 
that the decay in case of the inoculated potatoes 
was due to the organism employed in the inoculum. 
Further proof of this indication is the fact that 
the decay of the respective organisms was always 
within their temperature growth limits on culture 
media (7). 
