414- 
Journal of Agricultural Research 
Vol. XIV, No. 10 
For the convenience of those who wish to compare Cronartium occi - 
dentate with C. ribicola the principal points of difference are shown 
below in parallel columns. The characteristics of C. ribicola are fur¬ 
nished by Dr. Perley Spaulding and Dr. R. H. Colley, of this Office. 
Cronartium occidentale differs from C. ribicola in its pycnial and aecial 
stages, in* its aecial hosts, and in its behavior on some telial hosts as 
follows: 
CRONARTIUM RIBICOLA 
O.— Pycnial exudate honey-yellow; 
Pycnial spots numerous and con¬ 
spicuous. 
I.—^Ecial areas usually showing a 
marked fusiform swelling, espe¬ 
cially on younger trees (PL 55). 
iBciospOREs borne within closely 
aggrerated, prominently protrud¬ 
ing peridia (Pl. 55). 
Peridium, thick, persistent; 
Spores released by irregular break¬ 
ing of the peridia. 
Spores 18 to 20 by 22 to 33 /*, wall 2.0 
to 2.5 ju thick. 
O and I.—On white (5-needled) pines. 
I and II.—Incubation period on Ribes 
spp. 5 to 14 days. 
II and III.— Ribes nigrum best host. 
Grossularia leptantha poorest host in 
inoculations. 
Native of Old World. 
The striking differences in the appearance and morphology of the 
aecial stages of Peridermium occidentale and Peridermium strobi might be 
thought to be due, in part at least, to differences in the bark of the hosts, 
since Pinus edulis has a rather thick bark while Pinus strobus and other 
white pines have a thin bark. However, Peridermium harknessii , which 
sloughs off the overlying bark tissues in much the same way as Perider¬ 
mium occidentale , has the same characteristics whether found on the 
thin-barked Pinus contorta or the thick-barked Pinus ponderosa . Peri¬ 
dermium pyriforme , which produces numerous protruding peridia much 
like those of Peridermium strobi , also occurs on Pinus contorta and on 
Pinus ponderosa , with no apparent modification resulting from the varia¬ 
tion in the thickness of the bark. Peridermium filamentosum on the 
same hosts behaves similarly. 
Although it was soon evident that the incubation period on Ribes 
spp. was longer for Cronartium occidentale than for C. ribicola y only one 
test has been made for the specific purpose of comparison on this 
point. On November 14 four very similar plants of Grossularia^ inermis 
CRONARTIUM OCCIDENTALE 
O.—Pycnial exudate orange chrome; 
Pycnial spots uncommon and not 
conspicuous. 
I. —^cial areas showing slight, if any 
hypertrophy (Pl. 54). 
JSciospores borne in large cavities, 
peridia seldom prominent, often 
not visible (Pl. 54). 
Peridium, thin, evanescent; 
Spores usually released through ir¬ 
regular cracks in the overlying 
bark; 
Spores 12 to 28 by 22 to 38 ju; wall 1 
to 5 Ht averaging 3.0/t thick. 
O and I.—On pinon (1- and 2-needled) 
pines. 
I and II.—Incubation period on Ribes spp. 
12 to 36 days. 
II and III .—Ribes nigrum, one of the 
poorest hosts in inoculations. 
Grossularia leptantha best host in at 
least one locality in Colorado. 
Native of America. 
