BULLETIN OF THE 
No. 212 
Contribution from the Bureau of Plant Industry, Wm. A. Taylor, Chief. 
May 26, 1915. 
(PROFESSIONAL PAPER.) 
OBSERVATIONS ON THE PATHOLOGY OF 
THE JACK PINE. 
By James K. Weir, 
Forest Pathologist, Office of Investigations in Forest Pathology. 
INTRODUCTION. 
A discussion of the fungous diseases of a particular forest tree is 
incomplete unless the general habitat in which the tree grows and 
which influences the occurrence and virulence of its diseases is con- 
sidered. In general, a description of the characteristic home of the 
jack pine (Pinus divaricata (Ait.) Du Mont, de Cours.) is essentially 
that of the sandy plains in the region of the Great Lakes, where it 
attains its greatest size. Here the sand deposits are usually of great 
thickness and heavily mixed with glacial drift. The soil is composed 
chiefly of the same materials. With the exception of some of the 
lower plains and old lake levels the humus soil is very thin. In most 
regions within the range of the jack pine there is practically no humus. 
Where humus does exist in any appreciable thickness it is so much a 
part of the underlying sand and gravel that it dries out very rapidly, 
affording no opportunity for a luxuriant and uniform forest cover. 
Exceptions to this occur in parts of Minnesota and Canada. The 
improvement in the quality of the soil is at once reflected by the larger 
size of the jack pine and incidentally in the nature and virulence of 
the diseases attacking it. Observations show that a continuous and 
sustained growth in the case of the jack pine is not conducive to much 
injury from wood-destroying fungi. 
Owing to the rapidity with which the soil of the jack-pine "plains" 
dries out and to the inflammable nature of the slight ground cover, 
favorable conditions are furnished for forest fires. This, in turn, 
likewise greatly influences the presence of fungous diseases as a result 
of injuries caused by the fires. Severe and rapid changes in temper- 
ature and a fluctuation of the mean annual precipitation are other 
factors characteristic of the jack-pine habitat. The susceptibility of 
forest trees, and likewise of the fungi attacking them, to the influence 
of soil and climate directly or indirectly produces conditions favor- 
85752°— Bull. 212—15 
