Sept a, 1918. > 
Pinon Blister-Rust 
421 
yards; but the season was dry, and this factor may have limited the 
area of infection. 
The uredinial stage (PI. 57) of the rust on species of Ribes follows 
the infection by seciospores at the end of 12 to 15 days, or in cool weather 
a few days longer. In nature the reinfection of the species of Ribes 
by urediniospores may follow in about two weeks, the time being largely 
influenced by physical conditions, thus continuing the reinfection until 
the end of the growing season. It is not uncommon to find an abundance 
of uredinia on R. aureum and R. odoratum, with viable urediniospores 
in late autumn, even after freezing weather begins. Both the seciopores 
and unrediniospores undoubtedly are chiefly wind-disseminated. Insects 
may carry them to some extent. Cattle, sheep, and goats feed on 
R. aureum, brushing against the diseased leaves. During moist weather 
they are probably instrumental in spreading the disease. 
The telial stage (PI. 57) in nature ordinarily follows the uredinial 
stage in one to two weeks, varying somewhat with the temperature of 
the air and the age of the infected leaves, the controlling conditions not 
being well known. The mature teliospores germinate in situ t the telia 
assuming a silvery tinge, owing to the presence of sporidia. Pines in 
turn are infected by the sporidia, the pycnial and secial stages following 
in succession after a period of one or probably more years, the time 
probably varying as in other species of Cronartium (4, 6 ). 
Cronartium occidentale seems to be able to overwinter on Ribes aureum . 
This is indicated by collections of the disease by one of the writers 
(Bethel) from the same clumps of R. aureum , and even from the same 
bushes, year after year in the parks of Denver and Boulder. The first 
collections of the disease at these places were made in 1909. It was 
subsequently found in the same places each year until 1917, when a 
severe drouth seems to have killed the organism in all of the clumps 
on which the disease had previously been found. The only known 
natural occurrence of the disease in either city during 1917 was a scatter¬ 
ing infection found by Mr. Hunt in a group of plants of R. aureum in 
Washington Park, Denver. Conditions were such that it is quite possible 
the disease will not reappear there. It seems inconceivable that 
seciospores could be blown 50 miles or more and infect the same plant or 
groups of plants year after year, while many plantations near by remained 
free from infection. 
Mcial infections usually occur on more or less scattered leaves all 
over the plants infected, and not on a single well-protected branch in the 
center of a clump. The greater part of the collections of the uredinial 
and telial stages of the fungus in 1917 were made in localities too remote 
from pinons for any probable secial infection, and the manner of de¬ 
velopment indicated that the rust had in some way lived over the winter 
on the plants. In a few cases remote from pinons in 1917 the uredinial 
stage was observed at first on a single well-protected branch of a large 
