BULLETIN OF THE 



wMIOI 



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 R. 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 j ack 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 



