Sept, a, 1918 
Pinon Blister-Rust 
4 i 5 
and two of Ribes odoratum were selected from the stock beds. One half 
of these plants were inoculated with C. occidentale by Hunt and the 
others with C. ribicola by Mr. G. F. Gravatt, of this Office. All plants 
were covered with bell jars and put under a side bench of the greenhouse. 
A tight glass and wood partition separated the two lots of plants, but 
the most widely separated plants were less than 6 feet apart. 
On November 21 the plants of Grossularis inermis inoculated with 
Cronartium ribicola were producing urediniospores. On November 22, 
urediniospores were abundant on these plants, and some were being 
produced by part of the numerous uredinia on the plant of Ribes odora¬ 
tum . A careful examination with a hand lens failed to reveal any ure¬ 
dinia in process of formation on the plants inoculated with C. occi¬ 
dentale , although these plants later developed a heavy infection, first 
producing a few spores on November 27. Abundant uredinia and telia 
were produced on all of the plants inoculated. The difference in the 
length of the incubation period is variable but marked, and this fact 
supports the view that the two forms are distinct species. 
To summarize briefly, Cronartium occidentale differs essentially from 
C. ribicola as follows: In the Peridermium or aecial stage the former 
bears its thicker-walled aeciospores in a few large cavities under the bark 
and bordered with thin and evanescent peridia, while those of the latter 
are borne in numerous small cavities beneath thicker-walled, protruding, 
persistent peridia; the pycnial stage of the former has an orange-chrome 
pycnial exudate, the latter a honey-yellow one; the incubation period 
from the aecial stage to the uredinial stage is longer in the former than in 
the latter; the telial stage of the former infects Grossularia leptantha 
abundantly and Ribes nigrum rarely and sparsely, while the reverse is 
true in the case of the latter. 
DISTRIBUTION OF CRONARTIUM OCCIDENTALE 
DISTRIBUTION OF THE ^ECIAL STAGE 
The secial stage is here designated * Peridermium occidentale” to dis¬ 
tinguish it from the secial stages of other species of the form-genus 
Peridermium. It has been found only in two States, Arizona and 
Colorado (fig. 1), as follows: 
Arizona: 
On Pinus monophylla .— Near Prescott , 1 2 HEdgcock and Bethel, October 28.* 
Colorado: 
On Pinus edulis— Bayfield, HEdgcock and Bethel, May 25 (first collection of 
aecial stage), Bethel and Gaymon, June 23, 28, and 30, July 12 and 13, Bethel, 
August 26 and 31; Cedar Creek, Montrose County, Bethel, July 31; Glenwood Springs, 
Payson, August 31, and Hunt and Bethel, October 3; Mancos, Bethel and Payson, 
August 16, Bethel, August 17, and Bethel and Hunt, September 19; Mesa Verde 
National Park, Bethel and Hunt, September 21. 
1 All data on distribution are supported by numbered specimens in our collection. 
2 For data with the year omitted refer to the year 1917. 
