452 
Journal of Agricultural Research voi. xxiv, No. 6 
washing in connection with these experiments* and those with R. tritici 
are so small as to fall within the limits of experimental error. 
The results recorded in Tables IV, V, VI, and VII show: First, that R. 
tritici and R. oryzae can not compete with R. nigricans at temperatures of 
12° and 14° C. in the infection of sweet potatoes, even where high concentra¬ 
tions of spores of these organisms are used in the absence of R. nigricans 
from the inoculum; second, although R. artocapi and R, reflexiu are more 
successful than R. tritici and R. oryzae in competition with R, nigricans, 
they can not compete when the latter is included in the inoculum; third, 
R. tritici, R. oryzae, R. artocarpi, and R, reflexus do not infect sweet po¬ 
tatoes at these temperatures when infection depends upon the organisms 
present on the potatoes, while R, nigricans does. 
Tabi^E VII .—Organisms isolated fropi sweet potatoes inoculated with Rhizophus oryzae, 
or not inoculated and held as controls 
Number of cultures isolated. 
Potatoes inoculated with Rhizopus 
oryzae. 
Temperature. 
Washed before inocu¬ 
lation. 
Unwashed. 
Remarks. 
Rhizopus 
nigricans. 
Rhizopus 
oryzae and 
Rhizopus 
nigricans 
mixed. 
Rhizopus 
nigricans. 
®C. 
14 . 
14 
2 
13 
Same control as in Table V. 
12. 
13 
14 
When comparing the results from experiments with R. tritici and/?. 
oryzae with those obtained with R, artocarpi and R. reflexus some reser¬ 
vations must be made because it is not known that the concentration 
of the spore suspensions are equal or comparable The differences in the 
amount of infections are so great that they would seem to be due to 
differences in capacity to infect, rather than to differences in concentra¬ 
tion. The concentration was so great in every case that it would seem 
that a small variation in concentration would alter but little the amount 
of infection by the particular species used in the inoculum. In fact, 
very little infection occurred under any circumstances with R. tritici 
and R, oryzae. It is reasonable to expect that R. reflexus and R. artocarpi, 
especially the former, would cause more infection at these temperatures 
than R. tritici and R. oryzae, since the former are low, while the latter 
are high temperature forms. 
It will be seen from Tables V and VI that it may be important in studies 
of resistance and susceptibility to take into consideration the tempera¬ 
tures at which comparisons are made. For instance, R. artocarpi is 
more successful in competition with R. nigricans at 14° than at 12° C., 
while R. reflexus is more successful at 12° 3 ian at 14°. These results are 
consistent with other temperature relations of these organisms, which 
show that the lower temperature limit for infection with R. reflexus is 
lower than for R. artocarpi. 
In only one instance has R. tritici (Table V) been isolated from sweet 
potatoes at storage temperatures when the potatoes became infected, 
in the absence of this organism in the inoculum. It is to be expected 
that R. tritici would be responsible for a small percentage of infections, 
