446 
Journal of Agricultural Research 
Vol. XXIV. No. 6 
It is interesting to note that R, tritici in these experiments did i^ot 
cause decay of sweet potatoes over its entire temperature range of growth. 
(Range of growth and infection for R, tritici is about 5^ to 44-45C.) 
Infection by R, tritici did not take place by the method employed here, 
at temperatures at 20° C. and below notwithstanding the fact that it 
takes place readily where the “well” method of inoculation is used. 
The temperature ranges of infection by R, tritici and R, nigricans 
overlap between 20° and 30° C. Infection by R. nigricans becomes 
progressively less with the increase in temperature above 20°. R. tritici 
shows a similar decrease in the amount of infection as the temperature 
passes from 30^^ to 20°. Infection by the two species becomes nearly 
equal at 26.5^ (Table III). 
Results in connection with these and other experiments show that 
a smaller percentage of the potatoes decay at temperatures between 
20® and 30° C. than at higher or lower temperatures. Decay also will 
start in a large number of cases, then stop. Whether or not these 
temperatures are more favorable for the potatoes and less favorable 
for the pathogens, or both, is not clear, except that temperatures near 
30° are less favorable for R. nigricans. 
Table II shows the results of isolations made from wounded but 
uninoculated potatoes used in experiments in the storage house and 
in the infection chambers at temperatures at which sweet potatoes are 
usually stored. The top row of figures represents the average of three 
experiments, run at different times of the year in the storage house. 
Most of the isolations proved , to be R. nigricans. However, in two out 
of the three experiments, R. tritici was isolated in nine instances. Re¬ 
cords show that the temperatures were above 20° C. for ^ few hours 
during the early part of these experiments. These results indicate that 
R. tritici may be a factor in sweet-potato storage if the temperatures 
are high enough. R. nigricans is, however, the principal cause of decay 
in the storage house. The four lower rows of figures in Table II rep¬ 
resent the results obtained from four experiments run in the infection 
chambers at temperatures (10° and 14°) corresponding closely to those 
used in storage. R. nigricans was invariably isolated. 
Tabi^E II .—Isolations from wounded uninoculated sweet potatoes held in the storage 
house and in the infection chambers at storage temperatures 
Temperatures. 
10-25 
14. , 
12 . , 
II. , 
10. .. , 
Organisms isolated—Number of cultures 
isolated. 
Rhizopus 
nigricans. 
Rhizopus 
tritici. 
Mixed cultures 
of Rhizopus 
tritici and 
Rhizopus 
nigricans. 
140 
9 
3 
78 
0 
0 
74 
0 
0 
4 
0 
0 
3 
0 
0 
^ The figures 10-25 indicate that the temperature varied between 10 and 25®. 
