May 13 , X923 
Rhizopus Responsible for Sweet Potato Decay 
455 
DISCUSSION OF RESULTS 
R. tritici and R, nigricans are the two species chiefly responsible for 
the decay of sweet potatoes, kno\\Ti as softrot. The former is responsible 
for decay at the higher and the latter at the lower temperature, while 
the two overlap between 20° and 30° C. Although other species are 
capable of causing softrot, they do not seem to do so under the storage 
conditions at Washington, D. C., and in the infection chambers at the 
different temperatures. R. tritici^ R. oryzae^ R. reflexus, and R, artocarpi 
cannot compete successfully with R. nigricans at temperatmes of 12° 
and 14° when sweet potatoes are inoculated with any one of these 
organisms along with R, nigricans. Even though high concentrations of 
spores of R. tritici, R. oryzae, R. reflexus, and R. artocarpi are used alone, 
R. nigricans nearly always causes more decay than any of them, and R, 
tritici and R. oryzae cause very little under such conditions. 
When a mixed spore suspension of R. tritici, R. oryzae, R. reflexus, and 
R. artocarpi is used in inoculating the potatoes held at temperatures of 
14° and 18® C., R. artocarpi, R. reflexus, and R. nigricans, with R. nigricans 
greatly predominating, were isolated at 14°, and R. artocarpi and R. 
nigricans at 18°, R. artocarpi greatly predominating. When R. nigricans 
was added to the inoculum just mentioned, R. nigricans and R. artocarpi 
were the only species isolated, with the former greatly predominating. 
These results show that R. nigricans, except in the case of R. artocarpi 
at 18° C. (when R. nigricans was not included in the inoculum), was the 
chief agent of decay, even in the presence of the highly concentrated 
spore suspension of the other species used in these experiments. 
Why R. nigricans should cause so much decay when such high con¬ 
centrations of spores of the other species were employed, especially in 
the cases of R. reflexus and R, artocarpi, cannot be explained now. This 
seems rather strange when it is considered that R, reflexus, in particular, 
seems to decay sweet potatoes fully as rapidly as R. nigricans when the 
‘‘well” method of inoculating is employed at the same temperatures. 
The two cases are hardly comparable, however, since in the present 
experiments we are dealing with the percentage of infection, while in 
the case cited we have to do with the rapidity of decay after infection 
has occurred. When the “well” method is used there is nearly always 
100 per cent infection, irrespective of the species employed. This 
method is not at all comparable, however, to the one employed in these 
experiments. 
When these results are compared with those obtained by Harter, 
Weimer, and Lauritzen (6), who showed that several species of Rhizopus 
have the capacity to decay sweet potatoes, one should take into considera¬ 
tion the methods employed in ^e two cases. In the former case the 
particular species involved in infection was not only present as a con¬ 
centrated mass of active mycelium, but had the additional advantage 
of having access to the available food still remaining in the decoction. 
The species concerned also had access to the food rendered available 
soon after inoculation by the action of the pectinase, present in the 
decoction, upon the middle lamellae (j). In the latter case, infection 
depended either upon the spores present on the potatoes as they were 
removed from storage, or upon the spores introduced as a suspension in 
water. Where the potatoes were not inoculated, the species present may 
have been limited to those {R. tritici and R. nigricans) isolated in the 
first type of experiments. If other species were present, they were 
