808 
Journal o f Agricultural Research 
Vol. XXX, No. v 
transit, where environmental conditions 
are or can be under partial control. 
It may serve as a means of preventing 
infection, reducing the amount of 
decay, and affording a basis for pre¬ 
dicting the amount of loss that might 
be expected to take place under certain 
conditions. Sweet potatoes exposed 
to cold storage conditions at temper¬ 
atures between 0°C. and 5° for a 
period of two to six weeks will in¬ 
variably become infected with Mucor 
racemosus Fes. ( 6 ). They can be 
allowed, however, to remain at these 
temperatures for a period of 10 days 
if they are removed to temperatures 
ranging from 10° to 15° without 
becoming infected with this organism. 
Where sw~eet potatoes show some 
infection with black rot, it can be 
predicted with certainty that if they 
are exposed to temperatures of 18° to 
30° C. the loss will be greatly ag¬ 
gravated. If these potatoes are held 
at temperatures ranging from 10° to 
12° the advance of the decay will be 
greatly checked. 
The time required for infection with 
Rhizopus is not only short, but there 
is another discouraging feature that 
makes it difficult and generally im¬ 
possible to prevent infection where the 
conditions are favorable to infection. 
Although sweet potatoes generally 
must be freshly wounded to permit 
of infection by Rhizopus, it is only 
under special conditions, namely, that 
the wounding be shallow and clear cut 
and that the humidity be high, 96 to 
100 per cent, that infection can be 
prevented after potatoes have been 
wounded. 
It is possible to retard infection five 
or six days after the potatoes have 
been wounded (Table I) by placing 
them at 9° C. Losses can thus be 
prevented if the potatoes are consumed 
within this time. This period may be 
extended where the potatoes are not as 
severely wounded as they were in 
these experiments, which probably is 
usually the case. The period might 
also be extended if few spores w T ere 
present or shortened if the spores were 
abundant. After infection takes place 
there is little hope of saving the pota¬ 
toes unless they are used immediately, 
because the decay progresses very 
rapidly even at the lower temperatures. 
In Table VI is given a summary of 
the optimum, minimum, and maximum 
temperatures for the six species of 
Rhizopus studied. 
The optima for R. tritici, R. oryzcie, 
and R. maydis are almost identical 
with those obtained by Weimer and 
Harter (?) for mycelial growth, while 
the optima for R. nigricans , R. reflexus, 
and R. artocarpi fell from 3° to 5° below 
those for mycelial growth. 
This relation of the optima for infec¬ 
tion to the optima for mycelial growth 
seems to be correlated with the resis¬ 
tance of the host which apparently 
breaks down at the higher tempera¬ 
tures. 
There is a close correspondence be¬ 
tween the temperature limits for infec¬ 
tion and those for mycelial growth on 
culture media recorded by Weimer and 
Harter (7), particularly the upper 
limits. In some instances (R. maydis , 
R. nigricans , strain b, at high tem¬ 
perature and R. tritici , R. oryzae 
and R. nigricans at low temperatures) 
infection was obtained at temperatures 
beyond the limits where mycelial 
growth is recorded; but the next 
higher or lower temperatures employed 
in their experiments amounted to 
several degrees, (except in the case of 
R. nigricans , strain &,) and it is quite 
possible that had they employed 
intervening temperatures mycelial 
growth may have been obtained. Only 
in the case of R. nigricans , strain b, 
was infection obtained as high or low 
as the temperature recorded for the 
absence of mycelial growth. R. nigri¬ 
cans , strain b, caused infection at 30° 
C. in two days but did not exhibit 
mycelial growth at this temperature 
in one day. Spore germination was 
obtained at 31°, so that difference in 
time in the two instances may account 
for these results. R. maydis is the 
exception that does not show a close 
correlation between the temperature 
limits for infection and those for 
mycelial growth. It shows a wide 
range at the lower temperatures over 
which it causes no infection. The 
explanation of this behavior is not 
apparent. 
The maximum temperature recorded 
here for infection for Rhizopus tritici 
is not quite so high as that given by 
Lauritzen and Harter ( 6 ), the former 
being 42° C. and the latter 44° C. 
It will be noted however in the latter 
case that R. tritici was isolated only three 
times out of 34 isolation's. In any 
case the cardinal temperatures given 
are not to be regarded as definite as 
the figures may indicate. One must 
take into consideration the variations 
of the pathogen, host, and the ex¬ 
perimental conditions in connection 
with these experiments, as well as 
with any physiological experiments. 
The temperature range for infection 
by any of the low-temperature species 
combined with any of the high- 
temperature species is wide, as was 
