UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 360 



Contribution from the Bureau of Plant Industry 

 WM. A. TAYLOR, Chief 



SU^"^^U 



Washington, D. C. 



PROFESSIONAL PAPER 



June 17, 1916 



MISTLETOE INJURY TO CONIFERS IN THE 

 NORTHWESTo^ 



By James R. Weir, 

 Forest Pathologist, Office of Investigations in Forest Pathology. 



CONTENTS. 



Introduction 1 



General nature of the mistletoe injury 2 



Result of infection on the branches 13 



Result of Infection on the trunk 20 



Relation of mistletoe injury to fungous at- 

 tack 25 



General suppression and fungous attack 27 



Page. 



Relation of mistletoe injury to insects 28 



Influence of mistletoe injmy on the seed pro- 

 duction of the host 30 



Host afOnities in relation to silvicultiu'e 31 



Suggestions for control 33 



Summary 37 



Literature cited 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 occidentaUs (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), R. campylopoda (Engelm.) Piper (PI. II, fig. 2), 

 R. americana (Nutt.) Kuntze (PL I, fig. 2), and R. douglasii (En- 

 gelm.) Kuntze (PL II, fig. 1). 



1 Thanks are due Mr. E. E. Hubert for assistance in the preparation of the graphs and 

 a number of the other illustrations used in this bulletin. 



24182° — Bull. 360 — 16 — — 1 1 



