45 ^ Journal of Agricultural Research voi. xxiv, no. 6 
unsuccessful in competition with these two species. When the potatoes 
were inoculated as above, the competition between the species present, 
whether introduced or occurring normally, was more nearly equal than 
it was between the species present on the potatoes as they came from 
storage and the inoculum used in connection with the “well’’ method. 
It should be realized from a comparison of the results obtained by 
the two methods, that because a fungus produces a disease under one 
set of experimental conditions, it does not necessarily do so under another. 
The factors involved in determining infection under experimental con¬ 
ditions may vary greatly from those under normal conditions, depending 
upon, first, whether or not the experiments are designed to approximate 
normal conditions, and, second, whether or not the circumstances and 
our knowledge enable us to recognize and control the factors involved. 
There seems to be some unknown factor or factors, either as a part 
of the capacity to decay, or associated with it, on the part of R, nigricans 
that enables it under normal conditions or the conditions of these experi¬ 
ments to infect sweet potatoes to the exclusion of the other species. 
SUMMARY 
(1) Rhizopus nigricans and R, tritici are the species primarily respon¬ 
sible for the decay of sweet potatoes known as softrot, R, nigricans at 
temperatures between 6® and 20® C., and R, tritici at 30^ and above, 
the two pverlapping between 20° and 30®. 
(2) The temperature range of infection by R. tritici, R. nigricans^ and 
Mucor is so wide as to exclude the possibility of storing sweet potatoes 
beyond the limits of this range. 
(3) R, tritici, R, oryzae, R, reflexus, and R, artocarpi can not compete 
successfully with R, nigricans at the temperatures of 12°, 14°, and 18® C. 
UTERATURE CITED 
(1) Clark, W. Mansfield. 
1920. THK dbtisrmination OP HYDROGEN IONS . . . 317 p., 38 fig. Baltimore. 
Bibliography, p. 239-302. 
(2) Hanzawa, Join. 
1914-15. STUDiEN UBBR RiNiGE RHizopus-ARTEN. In Mycol. Centbl., Bd. 5, 
p 230-246, 257-281, 12 fig. 
(3) Harter, L. E., and Weimer, J. E. 
1921. A COMPARISON OP THE PECTINASE PRODUCED BY DIPPERENT SPECIES OP 
RHIZOPUS. In Jour. Agr. Research, v. 22, p. 371-377, 2 fig. 
(4) -. 
1922. DECAY OP VARIOUS VEGETABLES AND PRUITS BY DIPPERENT SPECIES OP 
RHIZOPUS. In Phytopathology, v. 12, p. 205-212. Eiterature cited, 
p. 212. 
(s)- and Adams, J. M. R. 
1918. SWEET-POTATO STORAGE-ROTS. In JovLT. Agr. Research, v. 15, p. 337-368, 
pi. 21-27. Eiterature cited, p. 366-368. 
(6) -and Eauritzen, J. I. 
1921. THE DECAY OP SWEET POTATOES (iPOMOEA BATATAS) PRODUCED BY DIPPER¬ 
ENT SPECIES OP RHIZOPUS. In Phytopathology, v. ii, p. 279-284. 
Eiterature cited, p. 284. 
(7) Thompson, H. C. 
1918. sweet-potato storage. U. S. Dept. Agr. Farmers’ Bui. 970, 27 p., 
15 fig- 
