THE WESTERN" YELLOW PINE MISTLETOE. 35 



Mistletoe infection causes a marked decrease in the rate of growth 

 of the host, which continues until the virulent parasite ultimately 

 causes the death of the tree. The rate of decrease varies directly with 

 the degree of infection from little or no decrease in the growth of 

 lightly infected trees to a very marked falling off in the current 

 growth of heavily infected trees. Many trees are killed annually by 

 this pest alone or in conjunction with other causes, such as insect 

 infestations or porcupine injury. 



The decrease in the rate of diameter and height growth and the con- 

 sequent current increment of trees infected with mistletoe are accom- 

 panied by a reduction of the leaf surface of the host. 



Trees of all age classes are subject to mistletoe infection, provided 

 the seeds of the parasite fall on parts of the tree which are not pro- 

 tected by the bark. Young seedlings and saplings usually die com- 

 paratively soon after becoming severely infected, while older trees 

 may remain alive for a much longer time. 



The quantity and quality of the seed produced by trees infected 

 with mistletoe is below that of normal, healthy trees. Heavily 

 infected trees are practically worthless for seed production and should 

 not be left as seed trees 



The most practical method of controlling mistletoe is to remove 

 the infected trees while cutting operations are in progress. All heav- 

 ily infected trees should be marked for cutting. Moderately infected 

 trees should be marked for cutting except where others are not 

 available for seed trees. On areas of light to moderate infection the 

 marking rules should require the removal of all mistletoe-infected 

 trees possible without breaking up the continuity of the stand or 

 materially interfering with the silvicultural requirements of the forest. 

 Exceptional care should be taken to leave as heavy a stand of healthy 

 trees as possible in order to compensate for the removal of diseased 

 trees and to maintain better forest conditions. 



Areas on which the entire stand is too heavily infected with mistle- 

 toe to permit adequate sanitation measures without very material 

 interference with the silvicultural requirements of the forest should 

 be marked for clear cutting under a mistletoe-control project. 

 Although in actual practice such a condition will seldom be encoun- 

 tered, relatively small areas will probably be found on which forest 

 planting may be desirable. 



When proposed timber-sale areas contain a great amount of mistle- 

 toe, a sanitation clause should be inserted in the timber-sale contract 

 requiring the cutting of all heavily infected trees, whether merchant- 

 able or unmerchantable, and certain other moderately infected trees 

 which may be marked. 



