DIAGNOSING WHITE-PINE BLISTER-RUST FROM ITS 
MYCELIUM 
By Reginald H. Colley, 
Assistant Pathologist , Investigations in Forest Pathology , Bureau of Plant Industry , 
United States Department of Agriculture 
INTRODUCTION 
The mycelium of Cronartium ribicola Fischer, the causal organism of 
the white-pine blister-rust, can be found among the cells of Finns strobus 
L. for some time before the spores of the fungus are produced. The 
examination of thousands of sections of diseased bark has convinced the 
writer that the morphological characters of the mycelium and its method 
of parasitism are sufficiently distinctive to warrant the assertion that the 
blister-rust can be recognized from the mycelium alone. 
The stains generally employed in the preparation of slides for diagnosis 
are safranin and lichtgruen. The use of these two stains is, of course, 
already well established in both zoological and botanical work, and their 
employment in combination is by no means new. They are particularly 
valuable in the study of the blister-rust mycelium because they not only 
stain host and parasite at the same time but they also bring out extremely 
well the anatomical features of the host tissue. It has always seemed to 
the writer that more detailed accounts of the experiences of investigators 
in handling stains would be of very great value to students. Accordingly 
the method which has been employed is given below, step by step, with the 
hope that it may be useful and suggestive to investigators of both normal 
and pathological anatomy. Throughout, the writer has kept in mind 
the fact that there has been very little published in this country on the 
use of the stains in combination. 
METHOD OF DOUBLE STAINING 
(a) Sections. —Free-hand sections of white-pine bark can be made to 
answer very well if no microtome is available, but many more sections of 
a better quality can be obtained with an ether or carbon-dioxid freezing 
attachment and a small microtome. Radial longitudinal sections are 
best, for in them the medullary rays are exposed, and in infected bark 
the mycelium of the blister-rust is generally closely associated with the 
cells of the rays. The sections should include, where practicable, the 
whole width of the bark, the cambium region, and three or four annual 
rings of wood. Such sections are, of course, larger than can be conve¬ 
niently cut with a section razor by hand, but it is perfectly possible to 
(281) 
Journal of Agricultural Research, 
Washington, D. C. 
km 
Vol. XI, No. 6 
Nov. 5, 1917. 
Key No. G—125 
