May It, 1933 
Rhizopus Responsible for Sweet Potato Decay 
447 
TYPK TWO 
In the second type of experiments the sweet potatoes were wounded 
as above and dipped in a spore suspension made by introducing the spores 
of an equal number of cultures of R, tritici and R. nigricans^ grown under 
identical conditions, into 10 gallons of water. Prom 8 to lo cultures of 
each organism were usually employed. 
It was discovered in the first type of experiments that R. tritici and 
R. nigricans were the species chiefly responsible for the softrot of sweet 
potatoes at Washington, D. C. R. nigricans was responsible for all the 
decay at temperatures below 20° C., quite to the exclusion of R. tritici, 
notwithstanding the fact that R. tritici, as shown by unpublished data, 
has the capacity of causing decay between 6^ and 44°, where the “ welP' 
method of inoculation is used. Both R, tritici and R, nigricans decayed 
sweet potatoes between the temperatures of 20° and 30° and R. tritici 
above 30°. It is possible that the amount of infection by these species, 
as well as their temperature ranges, may have been influenced by the 
number of spores present on the potatoes. The number of spores present 
of both species, in turn, may have been influenced by local conditions 
and the conditions under which the potatoes were stored. The storage 
temperatures were nearly always below 20°, except during the curing 
period of from 10 days to 2 weeks. Since R. nigricans is essentially a 
low-temperature form, these temperatures are more favorable for its 
development than for R, tritici, which grows better at high temperature. 
The second series of experiments, therefore, were designed to determine 
whether or not the range of infection discovered in the first series would 
be altered in any way, especially the lower limit at which R, tritici 
caused infection, when the sweet potatoes were inoculated with approxi¬ 
mately the same number of spores of each species. 
Table III shows the results of 920 isolations made during two years 
in connection with 15 different experiments, most of which were designed 
to determine the distribution of decay by R. tritici and R. nigricans 
at different temperatures. In 12 of these experiments, wounded pota¬ 
toes were inoculated with a spore suspension of R. tritici and R. nigricans. 
In the other 3 experiments the potatoes were wounded but not inocu¬ 
lated, and the data include all the isolations made from potatoes held 
at 14° C. and 74 out of the 81 isolated from potatoes held at 12®. Some 
of the experiments in which the isolations were made from potatoes 
held at low temperatures (7° and below), were designed to determine 
the lower limit at which these organisms would decay sweet potatoes. 
In the latter experiments the potatoes were wounded and inoculated as 
described in the first part of this paragraph. In these experiments the 
two species are distributed according to temperature, in much the same 
manner as shown in Table I. The upper and lower limits for infection 
by R. tritici are approximately 44° and 20°, respectively. The highest 
and lowest temperatures at which R. nigricans caused infection were 
27° and 3.5°, respectively. 
These results tend to indicate that the range of temperatures at which 
these species will infect sweet potatoes is so wide as to leave little possi¬ 
bility of adjusting temperatures in which to store the potatoes which 
will be outside the range of infection by these organisms. Although it 
has been found that these organisms do not cause any appreciable amount 
of decay at temperatures below 6 ^ C. it was shown by Harter, Weimer, 
and Adams (5), and later verified by the authors, that if potatoes were 
