SOME PHYSIOLOGICAL VARIATIONS IN STRAINS 1 OF 
RHIZOPUS NIGRICANS a 
By L. L. Harter and J. L. Weimer, Pathologists, Office of Cotton, Truck and Forage 
Crop Disease Investigations, Bureau of Plant Industry, United States Department of 
Agriculture 
Several investigators have demonstrated that Rhizopus nigricans 
Ehrnb. causes a soft rot of the sweet potato ([Ipomoea batatas Lam.). 
Recent investigations (2 ) 3 have shown that this organism when isolated 
from the sweet potato will cause decay of a number of different fruits 
and vegetables. Likewise an organism has been isolated many times 
from other crops which appears to be identical with the one associated 
with the sweet potato soft rot. The decay of a number of fruits and 
vegetables has frequently been attributed to Rhizopus nigricans by 
other investigators. In view of these investigations and the more 
recent ones by Lauritzen and Harter (5), there appears to be no doubt 
that Rhizopus nigricans is the principal cause of the sweet potato soft 
rot and of a similar decay of many fruits and vegetables. So far as the 
writers are aware, no one has attempted to study the physiological 
similarity or dissimilarity of the different isolations or strains of the 
organism which is taxonomically called Rhizopus nigricans; that is, of 
the strains of R. nigricans isolated from various hosts. Being conscious 
of the fact that it would be impossible to make a detailed investigation 
of any considerable number of the possible physiological relations of 
these strains, the writers decided to study (1) their parasitism, (2) the 
influence of temperature on their spore germination and subsequent 
growth, (3) their ability to produce pectinase, and (4), their influence on 
tiie hydrogen-ion concentration of the substrate. * 
This organism has been reported from various parts of the world, hence 
the question naturally arises as to whether or not R. nigricans from Cuba, 
for example, differs parasitically and physiologically from R. nigricans 
from Maine. The problem, then, in part at least, is concerned in a 
study of the physiological specialization within the species. Matsumoto 
(6) found that different isolations of Rhizoctonia solani varied consider¬ 
ably physiologically, and La Rue and Bartlett (4) concluded that by a 
sufficiently refined technique a nominal species such as Pestalozzia 
guepini might be resolved into an indefinite number of demonstrably 
distinct strains, the number depending only upon the precision of the 
methods. 
Out of a large number of different strains in the writers’ possession, 
eighteen were selected for study, which were obtained from a variety of 
hosts widely separated taxonomically. Care was also exercised to 
select those which were obtained from hosts widely separated geograph¬ 
ically, in which it ’was hoped that any influence of climate might be 
detected. The isolation key number, the host, the locality, and the 
name of the person who made the isolation are given for each of the 
eighteen strains employed in these investigations, in Table I. 
1 Strain as here used does not imply any difference morphologically, but different isolations of the same 
species. 
* Accepted for publication Aug. 16, 1923. 
3 Reference is made by number (italic) to “Literature cited/’ p. 371. 
Journal of Agricultural Research, 
Washington, D. C. 
ahx 
(363) 
Vol. XXVI. No. 8 
Nov. 24, 1923 
Key No. G-338 
