May 12, 1923 
Rhizopus Responsible for Sweet Potato Decay 
445 
during the early part of the season than later in the year. There are, 
however, a sufficient number of Rhizopus spores present during the early 
part of the season to cause a large percentage of infection when the 
potatoes are wounded by the method mentioned above. 
Fresh wounding contributes materially to infection by Rhizopus.® 
It has been found, as will be shown later in this paper, that fresh wound¬ 
ing is all that is required to permit of infection under the conditions of 
these experiments. 
Table I contains the results of isolations from potatoes that were 
wounded as mentioned above, but not inoculated. Inspection of the 
table reveals that two species, R. nigricans and R. triticiy^ are probably 
the species chiefly responsible for the decay of sweet potatoes. The 
absence of the other species would explain why there was no infection 
by them, but it is not known whether or not they were present. The 
presence of R. tritici and R. nigricans in greater quantity (either as to 
spores or mycelium) than the other species might explain these results. 
Later data will indicate to some degree how far this is the case with 
R. nigricans. 
Table I.— Isolations^ from uninoculaie^ wounded sweet potatoes held at different 
temperatures 
37 --- 
32.. . 
28.5 
27-5 
26.5 
25.. . 
23.. . 
20.. . 
19.. . 
18.. . 
15.. . 
14.. . 
12.. . 
11.. . 
10.. . 
3 * 3 - 
Organisms isolated. 
Temperatures. 
Rhizopus. 
tritici. 
Rhizopus nigricans. 
°C. 
60 
62 
8 
30 
21 
4 
3 
3 
5 
19 
14 
47 
17 
6 
19 
19 
78 
74 
4 
20 
4M.,2 2 M. and R. n. 
1 The figures in the columiis under the heading “Organisms isolated” represent the number of isolations 
of the particular organism at the various temperatures. They have no relation to the number of infections, 
which was always nearly 100 per cent, except between 20** and 30° C., where it was somewhat less. The 
figures in the succeeding tables under the above heading likewise refer only to the munber of isolations. 
* M.=Mucor. R. n.=Rhizopus nigricans. 
Only R. nigricans was isolated at temperatures below 20° C. It will 
be seen, therefore, that R. nigricans is the species involved in the decay 
of sweet potatoes at the usual storage temperatures. The importance 
of R. nigricans on storage will be taken up more fully later. 
® A paper on wounding as a factor in storage of sweet potatoes is in course of preparation. 
® Although it is thought by the authors that R. tritici is the predominating species at the higher temper¬ 
atures, it is impossible at present to be certain that R. nodosus, R, oryzae, and R. delemar are not responsible 
for decay in some instances, and even to the same degree as R. tritici. As far as the author are concerned, 
there are no characters by which these four species can be definitely separated. R. tritici, therefore, will 
be employed as the name to designate this group of species. 
