MISTLETOE INJURY TO CONIFERS. 37 



mistletoes. Through a proper appreciation of the need of adopting 

 control measures in all sales areas where the percentage of infection 

 is high and in all replanting projects in mistletoe regions, with the 

 free-use privileges of mistletoed trees and the cutting of all infected 

 growth in the vicinity of forest-improvement stations, a good be- 

 ginning could be made toward the eradication or the lessening of 

 the ravages of these parasites. 



SUMMARY. 



The conifers in the Northwest most subject to injury by mistle- 

 toes of the genus Razoumofskya are Larix occidentals, Pinus con- 

 torta, Pseudotsuga taxifolia, and Plum ponderasa. In the order 

 of the above-named hosts the mistletoes most responsible for the 

 greatest damage are Razoumofskya laricis, R. americana, R. doug- 

 lasii, and R. campylopoda. 



The general nature of the injury by these mistletoes is expressed 

 in a gradual reduction of the leaf surface of the host, which causes 

 a great reduction of growth in height and diameter. 



New infections take place only through the agency of a germinat- 

 ing seed, which reaches the point of infection through the natural 

 expelling force of the seed capsule, which may be made more effec- 

 tive in point of distance traveled by the aid of strong winds, by 

 falling from branches above after they have been loosened from 

 their original resting place by rains, and by animal life. 



Trees of all age classes are liable to infection provided the mistle- 

 toe seeds fall on parts of the host not yet protected by the mature 

 cortex. The parasite may spread from the original point of infec- 

 tion into older cortical tissues, which are not liable to infection 

 from without. The spread of the cortical stroma in the reverse 

 direction from the line of growth of the branch may continue until 

 the outer cortex becomes too thick for the aerial shoots to penetrate 

 it. After this, the cortical roots become suppressed and eventually 

 die, or they may become wholly parasitic. 



Excessive mistletoe infection of the lower branches of a tree may 

 cause the upper portion of the crown to die, giA^ing rise to the phe- 

 nomenon commonly called staghead or spiketop. Severe infection 

 throughout the entire crown often results in the death of the tree. 

 Young seedlings from 3 to 6 years old are often killed within a com- 

 paratively short time after infection. 



Infection on the branches in practically all cases causes the forma- 

 tion of large brooms, which seriously interfere with the life function 

 of the tree. The same is true in the case of infection on the trunk, 

 whereby burls are formed. 



The weakening effect of the formation of burls and brooms by 

 mistletoe on forest trees is often responsible for serious depredations 

 by fungi and forest-tree insects. 



