May i, 1925 Infection and Decay of Sweet Potatoes by Rhizopus 795 
at least permit of a comparison of the 
time required for infection at the vari¬ 
ous temperatures, because the wound¬ 
ing was the same at all temperatures 
and all the potatoes were grown and 
stored Under identical conditions. These 
experiments were conducted the latter 
part of December and the first part of 
January so that there had been oppor¬ 
tunity for some accumulation of spores 
on the potatoes, there being more 
spores ( 6 ) on the potatoes with the 
advance of the storage season. 
the potatoes were wounded as in the 
first type of experiment and inoculated 
by dipping them in a spore suspension 
made from an equal number of cultures 
of Rhizopus tritici and R. nigricans 
(strain a). 
Potatoes of uniform size and shape 
were selected, washed in tap water, 
dried in the laboratory for 24 hours, 
and divided into lots according to size 
and shape, each lot being weighed, 
wounded, and then inoculated by 
dipping them in a spore suspension. 
Table I.— Time required for the infection of sweet potatoes hy Rhizopus tritici and 
R. nigricans at various temperatures 
Temper¬ 
ature 
(°C.) 
Time re¬ 
quired for 
infection 
: 
Number 
of pota¬ 
toes in¬ 
oculated 
Number 
of pota¬ 
toes in¬ 
fected 
Remarks 
32.5_ i 
i 
43 hours... 
20 
9 
Size of lesions varied from 1 to 6 cm. in diameter. No in¬ 
fection in 24 hours. 
27.5_ 
...do_ 
20 
7 
Size of lesions varied from 0.5 by 1 to 4 by 5 cm. in diame¬ 
ter. No infection in 24 hours. 
25.5_ 
...do_ 
20 
19 
Size of lesions varied from 0.3 by 1 to 4 cm. in diameter. No 
infection in 24 hours. 
23.5_ 
...do_ 
: 20 
14 
Size of lesions varied from 1 to 3 cm. in diameter. No infec¬ 
tion in 24 hours. 
19.5_ 
...do_ 
20 
14 
Infection just started. No infection in 24 hours. 
18.0_ 
—do_ 
20 
6 
Do. 
15.0_ 
3 days_ 
1 20 
12 
Infection just evident. No infection evident after 2 days. 
13.5_ 
4 days_ 
20 
15 
Lesions from 1 to 2 cm. in diameter. 
9.0. 
7 days_! 
1 
20 
20 
Lesions varied from 3 to 10 cm. in diameter. No infection 
evident in 5 days. 
Table I shows the number of days 
required for infection to take place at 
the various temperatures by Rhizopus 
tritici and R. nigricans. At tempera¬ 
tures of 18° C. and above infection 
took place in 43 hours and less. At 
18° and 19.5° infection had just started 
in 43 hours. Above these tempera¬ 
tures infection started earlier, but in 
no case as early as 24 hours. At 15° 
three days were required for infection, 
and at 9° five to seven days. The 
time required for infection to take 
place at any of the temperatures is 
comparatively short. There will be 
some variation from this time, depend¬ 
ing upon the degree of wounding, the 
time of season, and the conditions 
under which the potatoes are stored. 
When infection has once been estab¬ 
lished the time required for the pota¬ 
toes to decay completely is compara¬ 
tively short, four or five days at most. 
If potatoes are to be utilized after they 
have been badly wounded, they must 
be consumed within the first few days. 
SECOND TYPE OF EXPERIMENT 
These experiments were devised to 
measure quantitatively the amount of 
decay at various temperatures, where 
After having been subjected to this 
treatment the potatoes were placed at 
the various temperatures and left for 
infection to take place and decay to 
develop. After the decay had de¬ 
veloped to a suitable degree, the lots 
were weighed again, the decay removed 
by a spoon or some other blunt instru¬ 
ment, and the remaining undecayed 
portion weighed. The weight of the 
decay was obtained by subtracting the 
latter weight from the first weight. 
The reason for making the second 
weighing was to ascertain the loss or 
gain of water during the infection and 
incubation periods. There was a slight 
loss of water, especially at the higher 
temperatures, which was correlated to 
some extent with the amount of decay. 
There was a very slight gain of water 
at the lower temperatures in some cases 
where the humidity was exceptionally 
high. The humidity, however, was 
fairly comparable throughout the 
chambers, being slightly lower at the 
higher temperatures. The variation 
in loss of water was not sufficient to 
alter the general relations obtained. 
It will be seen from the results re¬ 
corded in Table II that the amount of 
decay increases rapidly with the rise in 
temperature, more rapidly than one 
