May 12. 1923 
Rhizopus Responsible for Sweet Potato Decay 
453 
since it can infect at these temperatures and probably is nearly always 
present on the potatoes. (Table I.) 
The fact that R. nigricans was always obtained from the controls as 
well as inoculated potatoes in previous experiments when the tempera¬ 
ture conditions were right indicates that this species is usually, if not 
always, present on the potatoes. 
TYPH I^OUR 
In the fourth type of experiments the potatoes were wounded as in the 
preceding experiments, but inoculated differently. The first inoculation 
was made with a suspension of spores of R. tritici, R. oryzae, R. reftexus, 
and R. artocarpi, taken from two cultures of each species of tiie same age 
and grown under identical conditions, suspended in 3 liters of water; 
the second inoculation was with a supension of spores of R. tritici, R, 
oryzae, R, reflexus, R. artocarpi, and R. nigricans, two cultures of each of 
the first four species and one of R, nigricans. The same amount of water 
as in the first case was used in making the inoculum. 
The object of these experiments was to determine which species would 
infect sweet potatoes when several species were in competition with each 
other at storage temperatures. The number of spores of each species 
present in the inoculum only approximated that of the other species, and 
in the case of R. nigricans was probably much less. Of course, as has 
been shown in previous experiments, R. nigricans is usually, if not always, 
present, but it is believed that, at least when the potatoes were not 
inoculated with R. nigricans, the number of spores of Ais species present 
per unit area was less than that of the other species, because the concen¬ 
tration of the spores in the inoculum of the other species was fairly high. 
It did not seem practical at the time to attempt to use exactly equiva¬ 
lent concentrations because of the difficulties involved. The most serious 
difficulties were, first, to obtain an equal number of spores of each species, 
second, the number of spores may not be a measure of the germinating 
and infecting capacity of the spores, which in turn may vary with the 
species, and, latterly, some variation in the germinating and infecting 
capacity of the spores of the different species may be expected under 
different conditions, as, for example, at different temperatures, and yet 
a comparison between species must be made under identical conditions 
to be valid. This is shown to be the case by the fact that, although the 
temperature ranges of R. reflexus and R. artocarpi run almost parallel, 
R. reflexus is the more successful in competition with R. nigricans at 12° 
C., while at 14° R. artocarpi is the more successful (Tables V and VI). It 
is true that the temperature range of R. reflexus is wider than that of R. 
artocarpi, but its maximum is about the same number of degrees above 
that of R. artocarpi as its minimum is below. This difference in tempera¬ 
ture ranges is, perhaps, sufiScient to account for their difference in infec¬ 
tive power. It probably would be difficult to find two fungi of identical 
temperature ranges. To make a quantitative comparison these factors 
must be taken into consideration. 
It is believed, however, that the results from the methods employed 
in these experiments will show which species of Rhizopus are important 
in the decay of sweet potatoes. 
Table VIII shows the number of isolations obtained from potatoes 
inoculated with R. tritizi, R. oryzae, R. reflexus, and R. artocarpi, and from 
potatoes inoculated with the same organisms plus R. nigricans, at the 
temperatures 14^^ and 18° C. It will be noted that at 18° R. artocarpi 
