JOURNAL OF AflaCUnRAL RESEARCH 
Vol. XXX Washington, D. C., April 1 , 1925 No. 7 
RELATION OF WEATHER CONDITIONS TO THE SPREAD 
OF WHITE PINE BLISTER RUST IN THE PACIFIC 
NORTHWEST 1 
By L. H. Pennington 2 
Collaborator , Office of Investigations in Forest Pathology, Bureau of Plant Industry, 
United States Department of Agriculture 
INTRODUCTION 
Weather conditions to a large extent 
determine the spread and severity of 
fungous diseases of plants. White 
pine blister rust is no exception to the 
rule. The direction and rapidity of 
spore distribution over long distances 
depend upon winds. Favorable mois¬ 
ture and temperature conditions are 
necessary for germination of the spores 
and consequent infection of the host 
plants. 
Moisture conditions are more vari¬ 
able than temperature and seem to 
determine the amount of infection 
which may occur. If moisture con¬ 
ditions are favorable, temperature 
conditions also are practically certain 
to be favorable. Particular attention 
has, therefore, been given to the cor¬ 
relation between the spread of the rust 
and the winds and moisture conditions. 
The western white pine, Pinus 
monticola Dougl., is practically the 
only pine to be taken into consider¬ 
ation. There are several species of 
Ribes in the region under discussion. 
(See fig. 1.) Some qualifications will 
be necessary because of differences in 
both abundance and susceptibility to 
infection of Ribes in different localities. 
It has not been possible to follow 
the course of the disease from year to 
year in the Northwest. It has, there¬ 
fore, been necessary to determine, as 
nearly as possible, by field studies 
during the seasons of 1922 and 1923 
the year of infection as well as the 
amount and direction of spread of the 
rust. This has been done, and it is 
now possible to determine in a general 
way how often heavy infections may 
be expected to occur in places contain¬ 
ing both pines and Ribes. It is pos¬ 
sible also to make some estimate of 
the length of time which will be re¬ 
quired for the disease to spread by 
natural means into regions now free 
from it. 
DETERMINATION OF SEASON OF 
INFECTION 
Special efforts have been made to 
determine the year of infection in all 
places in which the rust has been stud¬ 
ied. In young and vigorous trees it is 
practically always possible to deter¬ 
mine the internode in which infection 
began. Until recently infections were 
classified according to the year’s inter¬ 
node in which they first appeared. 
This method was found to be inaccu¬ 
rate. Earlier tabulations in which no 
reference to the age of the canker was 
given are not good indicators as the to 
year of infection. 
During the season of 1922 a large 
number of incipient infections were 
found in the internodes of 1917, 1918, 
1919, and 1920. Table I shows the 
distribution of 176 of these infections 
upon a few pines near Bold Point, 
British Columbia. (For location of 
infection centers see fig. 2.) There 
were many Ribes near these pines. 
1 Received for publication June 11, 1924; issued June, 1925. 
2 The writer wishes to express his gratitude to J. S. Boyce, forest pathologist for Washington and Oregon, 
and to G. B. Posey and S. N. WyckofT, pathologists in charge of blister-rust control in 1922 and 1923, re¬ 
spectively, for the Western States. Each has been most cordial in his cooperation and has aided in the col¬ 
lection of data in every possible way. Particular credit is due to Harry G. Lachmund, junior pathologist, 
who most ably and faithfully assisted during the summers of 1922 and 1923 in the collection and preparation 
of field data. The writer acknowledges the cordial cooperation of the meteorologists and other Government 
and State officials in the Western States and British Columbia. He is especially indebted to the following: 
A. T. Davidson, in charge of blister-rust scouting for the Dominion Government, who has furnished valu¬ 
able data secured by himself and his scouts and who has aided materially in other ways; and F. Napier 
Denison, superintendent, Gonzales Heights Observatory, Victoria, British Columbia, who furnished all 
the available meter or ological data for British Columbia. 
Journal of Agricultural Research. 
Washington, D. C. 
19977—25f-1 
Vol. XXX, No. 7 
Apr. 1, 1925 
Key No. G-429 
(593) 
