A BIOMETRIC COMPARISON OF THE UREDINIOSPORES 
OF CRONARTIUM RIBICOLA AND CRONARTIUM OCCI¬ 
DENTALS 1 
By Reginald H. Colley 
Pathologist , Office of Investigations in Forest Pathology , Bureau of Plant Industry , 
U, S. Department of Agriculture 2 
INTRODUCTION 
It appears to be impracticable to 
differentiate between Cronartium ribi - 
cola Fischer and Cronartium occi - 
dentale Hedgcock, Bethel, and Hunt 
in the uredinial stage by any ordinary 
means of visual inspection or low 
power microscopical examination. Both 
rusts attack a large number of species 
of the genus Ribes. Superficially and 
structurally the uredinia resemble each 
other very closely; and the uredinio¬ 
spores of the two species differ only 
slightly—so slightly in fact that the 
differences escape observation unless 
special technique is used. The fact 
that C. ribicola is already present in 
the northwestern United States makes 
the problem of differentiation some¬ 
thing more than a matter of academic 
interest, on account of the necessity of 
recognizing advance infections on 
Ribes. 
Hedgcock, Bethel, and Hunt 3 give 
the size of the urediniospores of C. 
ocddentale as 18.5 to 32 by 13.5 to 
20/i averaging 24 by 16/x, and state 
that the wall is 2 to 3ju thick. Colley 4 
has described the size of the uredinio¬ 
spores of C . ribicola roughly as 19 to 
45 by 10 to 20/t. These two range 
descriptions are not comparable, be¬ 
cause the measurements on which the 
figures are based were not made by 
similar methods. Granting that they 
are comparable, however, the range 
descriptions would not be a sufficiently 
sound basis for diagnosis, for there is 
no means of knowing the distribution 
of the more common spore sizes within 
the limits of the ranges. Of the many 
hundreds of spore measurements made 
on the urediniospores of the two 
species within the last few years, some 
have been made on fresh spores and 
some on dry spores, with and without 
the use of special mounting media. 
Obviously these results also are more 
or less unsatisfactory. The object of 
this paper is the presentation of an 
analysis of strictly comparable meas¬ 
urements made on 3,000 urediniospores 
of each species. 
METHODS 
SELECTION OF MATERIAL 
Herbarium material was selected 
for the measurement study. Experi¬ 
ence had shown clearly that measure¬ 
ments made on fresh spores were not 
comparable with measurements made 
on dry spores; and it was obvious that 
herbarium specimens had one thing at 
least in common—they were all dry. 
Furthermore, it was possible to select 
specimens from the herbarium cover¬ 
ing the widest possible range for host, 
locality, and time of collection. The 
spores were taken from specimens 
which appeared to be clean and well 
preserved, and from sori which ap¬ 
peared to be mature. 
The selected specimens were grouped 
in three series. In each series there 
were 10 sets of spores—each set con¬ 
sisting of 100 urediniospores of Cronar¬ 
tium ribicola and 100 urediniospores of 
Cronartium ocddentale. The descrip¬ 
tion of the three series follows: 
Field series. —A selection of 50 speci¬ 
mens of each species from collections 
made in the field; 5 specimens from 
each species in a set; set numbers 1-10, 
inclusive. 
Block Island series. —A selection of 10 
specimens of each species from collec¬ 
tions made in experimental plots 
located on Block Island, off the coast 
of Rhode Island; one specimen of each 
species in each set; set numbers 11-20, 
inclusive. 
1 Received for publication May 16, 1924; issued April, 1926. 
2 The writer wishes to acknowledge his indebtedness to Dr. George G. Hedgcock, Glenn G. Hahn, and 
Rush P. Marshall of the Office of Investigations in Forest Pathology, and to Miss Minnie W. Taylor, 
formerly of the office, for their generous assistance during the course of this study. 
3 Hedgcock, G. G., Bethel, E., and Hunt, N. R. piAon blister-rust. Jour. Agr. Research 14: 
411-424, illus. 1918. 
4 Colley, R. H. parasitism, morphology, and cytology of cronartium ribicola. Jour. Agr. 
Research 16: 619-660, illus. 1918. 
Journal of Agricultural Research, 
Washington, D. C. 
(283) 
Vol. XXX, No. 3 
Feb. 1, 1925 
Key No. G-439 
