450 
Journal of Agricultural Research 
Vol. XXIV, No. % 
The spore suspensions of R. nigricans used in connection with spme of 
the other species was not so highly concentrated, because it so happened 
that there was not a sufficient number of cultures of R. nigricans available. 
Table IV shows the results of competitive tests of infection by R. 
tritici and R. nigricans, first when potatoes were washed and inoculated 
with R. tritici, second, when unwashed potatoes were inoculated with 
R, tritici, and third, when unwashed potatoes were inoculated with a 
spore suspension of R. tritici and R. nigricans. Controls of both washed 
and unwashed potatoes were also included. 
R. nigricans alone was isolated from the unwashed and control potatoes 
and greatly predominated when either the washed potatoes were inocu¬ 
lated with R. tritici or unwashed potatoes were inoculated with a mixed 
spore suspension of R. tritici and R. nigricans. These results indicate 
that R. tritici can not compete with R. nigricans and is not much of a 
factor in the decay of sweet potatoes at these temperatures. The number 
of spores of R. nigricans on the potatoes as they were obtained from the 
storage house or when the potatoes were washed, was sufficient to cause 
nearly all the infection, even when the potatoes were inoculated with a 
highly concentrated spore suspension of R. tritici'. Washing reduces 
infection by R. nigricans but slightly. 
Table V shows that R. artocarpi can compete more successfully with 
R. nigricans than R. tritici. When R. artocarpi is used alone as the 
inoculum, the percentage of infection is fairly high, especially at 14° C. 
The washing of the potatoes seems to have no effect. When the inocula¬ 
tions were made wiffi R. nigricans and R. artocarpi together, no infection 
by the latter organism took place. R. nigricans alone was isolated from 
the controls. 
Although these results show that R. artocarpi may cause considerable 
decay if spore suspensions of high concentration are used, it seems 
probable that under storage and transit conditions the number of spores 
of R. artocarpi would rarely if ever be sufficient to be a factor in the decay 
of sweet potatoes, because R. nigricans seems to be universally present 
and would likely grow and produce spores as readily as R. artocarpi under 
most, if not all, conditions. 
R. reflexus, as is shoivn in Table VI, was capable of causing considerable 
decay when it was used alone as the inoculum. 
TABiyi® IV .—Organisms isolated from sweet potatoes inoculated with Rhizopus tritici or 
with a mixed spore suspension of Rhizopus tritici and Rhizopus nigricans, or not inocu¬ 
lated and held as controls 
Inoculated with lUiizopus tritici. 
Inoculated with 
Rhizopus tritici 
Control. 
Tempera¬ 
ture. 
Washed. 
Unwashed. 
and Rhizopus ni¬ 
gricans (im- 
washed). 
Washed. 
Un¬ 
washed. 
Rhizopus 
tritici. 
Rhizopus 
nigricans. 
Rhizopus 
tritici 
and 
Rhizopus 
nigricans 
mixed. 
i Rhizopus 
tritici. 
^izopus 
nigricans. 
Rhizopus 
tritici. 
Rhizopus 
nigricans. 
Rhizopus 
nigricans. 
Rhizopus 
nigricans* 
•c. 
14. 
2 
1 
9 1 
Q 
5 
14 
13 . 
I 
Q 1 
Q 
r‘7 
17 
r 6 
*‘0 
w 1 
