May 1, 1925 
Infection and Decay of Sweet Potatoes by Rhizopus 
807 
Lower temperature limits. —The 
data relating to the lower temperature 
limits for all six species are recorded 
in Table VII. R. tritici will be seen 
to infect sweet potatoes readily at 5° C. 
by this method of inoculation, it being 
obtained 38 times out of 40 isolations. 
At 3.4° it was only obtained 5 times 
out of 14 isolations. This temperature 
is probably very near the lower limit 
for infection by this organism because, 
although Weimer and Harter (?) found 
that R . tritici spores germinated at 1.5°, 
no growth of mycelium in culture 
medium took place at this temperature, 
but mycelial growth did take place at 
6.5°. No observations were made on 
mycelial growth between these tem¬ 
peratures. 
There was no infection by Rhizopus 
oryzae after 51 days at 5° C. In 
Table III there is some evidence of 
infection at 5°, but the loss in weight 
(5.5 gm.) recorded after nine days 
may have been due to maceration, 
brought about by the enzyme that may 
have been present in the cultures used 
for inoculation and a slight amount of 
sweet potato decoction that was re¬ 
maining in the “well.” 6 Some decay 
(Table III) did take place at 9°. One 
is therefore safe in assigning some 
temperature between 5° and 9° as the 
lower temperature limit for infection. 
Weimer and Harter (?) found germina¬ 
tion of spores at 9° but none at 7° and 
growth of mycelium at 11° but none 
at 7.5°. 
It will be observed that the lower 
temperature limit for infection by 
Rhizopus oryzae is somewhat higher 
than that for R. tritici (Table VII). 
These results correspond to those ob¬ 
tained by Hanzawa and Weimer and 
Harter for spore germination, mycelial 
growth, and fructification ( 1 ), (?'). 
Rhizopus maydis caused no infection 
at 3.4° C. or at 5° after 51 and 52 days, 
respectively. Although the lower tem¬ 
perature limit has not been accurately 
determined in this case, it can be 
definitely said that no infection took 
place at and below 5°. According to 
the data recorded in Table III there 
is a wide range over which scarcely no 
infection takes place. What would 
have happened if more time (two days) 
had been employed can not be said. 
In any case the behavior exhibited is 
unusual. Even at temperatures from 
18.5° to and including 28° scarcely 
any infection had taken place after 
two days. Weimer and Harter (?) 
obtained growth of mycelium at 7.4 
but not at 1.5°. 
Rhizopus nigricans strain a infected 
26 or 27 potatoes out of 29 at 5° C. 
Nine of these potatoes were held 25 
days and 20 for 51 days at this tem¬ 
perature. It infected only 3 potatoes 
out of 9 at 3.4° after 52 days, indicat¬ 
ing that this temperature is near the 
lower temperature limit, because at the 
higher temperatures it was not difficult 
to obtain 100 per cent infection. These 
results are confirmed by earlier results 
obtained by the writers (6) with this 
strain where very slight infection was 
obtained at 3.5° but none below. 
Weimer and Harter (?) obtained ger¬ 
mination of R. nigricans spores at 1.5° 
and mycelial growth at 6.5°. They 
give no records of mycelial growth 
between these temperatures. 
Rhizopus reflex ns infected 12 out of 
16 potatoes at 5° C. Rhizopus was 
isolated from the other four potatoes, 
but being contaminated with bacteria 
their identity was not certain, but it 
seems probable that they were R. 
reflexus. Another lot showed 100 per 
cent infection, but the decay at the 
time of observation was too advanced 
for isolation work. At 3.4° R. reflexus 
was isolated from every potato out of 
18 held at this temperature. It will 
thus be seen that the lower limit for 
infection by R. reflexus is probably 
lower than that of R. nigricans. 
Weimer and Harter (?) found that not 
only would the spores of R. reflexus 
germinate at 1.5° but the mycelium 
would likewise grow at this tem¬ 
perature. 
Rhizopus artocarpi caused no infec¬ 
tion at 5° C. Whether or not the 
fungus had actually established host 
relations at 5° and 9° in the experi¬ 
ments recorded in Table III can not 
be said definitely, because no isolations 
were made. Twelve degrees is the 
lowest temperature at which infection 
has been recorded, but it seems probable 
that it can be obtained at a lower 
temperature. 
DISCUSSION AND CONCLUSIONS 
Knowledge of the time required for 
infection to take place is often an 
important factor in the loss due to 
diseases. Such knowledge is especially 
valuable in connection with diseases 
of fruits and vegetables in storage and 
6 This statement likewise applies to some of the weighings recorded in Tables III and IV in connection 
with the other species at the lower temperatures. Where the weighings are 8 gm. and less the loss of weight 
can be explained on this basis. At the upper limiting temperatures the decoction dries up sooner and the 
action otf the enzymes is less evident unless infection actually takes place. 
