Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief 
Washington, D. C. PROFESSIONAL PAPER June 17, 1916 
MISTLETOE INJURY TO CONIFERS IN THE 
NORTHWEST.’ 
By James R. WEIR, 
Forest Pathologist, Office of Investigations in Forest Pathology. 
CONTENTS. 
Page. Page. 
PItrOG EHO NM se aes see tee Saale See nace we 1 | Relation of mistletoe injury to insects....... 28 
General nature of the mistletoe injury.....-- 2 | Influence of mistletoe injury on the seed pro- 
Result of infection 6n the branches..-......-- 13 ductionyofthe hoste.- 2). 22-2) fan- see eee 30 
Result of infection on the trunk. ........-.-.- 20 | Host affinities in relation to silviculture. .--..- 31 
Relation of mistletoe injury to fungous at- Suggestions foricontrols - <2 5.2 sete ae ee 33 
DOIG Gee Oe CHAOS RBOC OS an aaa se eee esi 25>). SUMMA yee see ee nate elace aber sce 37 
General suppression and fungous attack..... ACA ANCIENT OURS OUiC Ly oka Sa secenok oucaocadauneos= 39 
INTRODUCTION. 
It is not generally known that the injury by the mistletoes to 
coniferous trees in the northwestern United States is such as to 
assume in many regions the nature of a serious forest problem. 
The aim of this bulletin is to point out some of the direct and 
indirect results of this injury. The species of trees most subject to 
injury are Larix occidentalis (western larch), Pinus ponderosa 
(western yellow pine), Pinus contorta (lodgepole pine), and 
Pseudotsuga taxifolia (Douglas fir). Each of these trees is attacked 
by a particular species of mistletoe of the genus Razoumofskya 
(Arceuthobium). With a few exceptions, these species very rarely 
occur in nature on any other than their common hosts. In the 
order of the above-named hosts they are Razoumofskya laricis Piper 
(Pl. I, fig. 1), 2. campylopoda (Engelm.) Piper (Pl. II, fig. 2), 
R. americana (Nutt.) Kuntze (PI. I, fig. 2), and RP. douglasit (En- 
gelm.) Kuntze (PI. IT, fig. 1). 
1 Thanks are due Mr. E. E. Hubert for assistance in the preparation of the graphs and 
&@ number of the other illustrations used in this bulletin. 
24182°—Bull. 360—16 1 if: 
