SN le a his i aie ne 
Ry Se FE rn ed 
THE WESTERN YELLOW PINE MISTLETOE. 39 
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. 
