EFFECTS OF MISTLETOE ON YOUNG CONIFERS 



By James R. Weir 



Forest Pathologist, Investigations in Forest Pathology, Bureau of Plant Industry, 



United States Department of Agriculture 



During the years 191 1 to 1917 the writer has been engaged in the 

 investigation of the injurious effects of mistletoes of the genus Razou- 

 mofskya (Arceuthobium) on conifers. A part of these investigations 

 have already been published.* During the course of the work, a number 

 of studies were made which have not yet been reported. It seems that 

 these studies are of sufficient importance to be presented at this time. 



The fact that these parasites are a cause of suppression ^ in forest 

 trees is readily appreciated after the injury has become acute. In 

 middle-aged and older trees where accumulated injury has resulted in 

 small diameters and broomed scraggly crowns with reduced leaf surface, 

 the effect of the parasites is quite evident (PI. 37, A). The effects of 

 mistletoe on its host is by no means so apparent on trees ranging in age 

 from 4 to 10 years. Trees which have become infected early in life 

 may begin to react in marked degree, but the extent of the suppression 

 is not always apparent to the eye. It may be noticed that the branches 

 of young trees are broomed along with other types of infection of a more 

 general nature, but the retardation of the excurrent or elongated growth 

 of the main stem may not be readily recognized. This is where detailed 

 measurements are of value and prove or disprove the early suppression 

 of trees of the younger age classes by mistletoe. 



In' order to demonstrate the suppression in young trees, a series of 

 measurements of the height growth of w^estern yellow pine (Pinus ponde- 

 rosa) were made in Spokane County, Washington. Two representative 

 plots of I acre each, consisting of infected and uninfected reproduction 

 and representing all age classes, were selected. The site was level bench 

 land with a sparse stand of merchantable-sized trees. Some of these trees 

 were severely infected with mistletoe {Razoumofskya campy lopoda 



' Weir, J. R. larch mistletoe: some economic considerations of its injurious effects. U. S. 

 Dept. Agr. Bui. 317, 25 p., 13 fig. 1916. 



mistletoe injury to conifers in the northwest. U. S. Dept. Agr. Bui. 360, 39 p., 27 fig. 



X916. literature cited, p. 39.' 



some suggestions on the control of mistletoe in the N.^TION.^L forests op the northwest. 



In Forestry Quart., v. 14, no. 4, p. 567-577. 1916. 



' The term "suppression" is not used here in the ordinary forestry sense, but refers to a retardation of 

 growth induced by parasitic organisms. 



Journal of Agricultural Research, Vol. XII, No. n 



Washington, D. C. (/^S) VltiT. 18, 1918 



ml Key No. G— 138 



