460 



Yearboo^ of Agriculture 1949 



serves the double purpose of holding it 

 fast to the medium on which it alights 

 and of gathering and holding moisture 

 for the protection of the primary root 

 upon germination. With proper con- 

 ditions of moisture and temperature, 

 mistletoe seed will germinate on prac- 

 tically any substratum, but only those 

 that happen to be on the young, tender 

 branchlets of suitable host plants can 

 survive. The primary rootlet then 

 forces its way into the tender bark and 

 from there establishes an absorption 

 system inside the host; after 2 years or 

 more it may produce many crops of 

 aerial shoots. 



The explosive nature of the seed dis- 

 persal tends to intensify the mistletoe 

 on a tree once it is infected and leads 

 also to a slow but steady encroachment 

 of the parasite into the forest once it 

 is established on a single tree. In that 

 respect it differs fundamentally from 

 the leafy or Christmas mistletoes, which 

 are spread only by birds, with the re- 

 sult that trees are seldom infected until 

 they are large enough to provide at- 

 tractive roosting places. The distribu- 

 tion of the dwarf mistletoes indicates 

 that they, too, may be carried long 

 distances, presumably by birds. 



THE ECONOMIC IMPORTANCE is great. 

 Damage by mistletoe in the forest is of 

 four general categories: Increased 

 mortality, the lower timber quality, re- 

 duced increment, and predisposition to 

 other diseases or insect attack. 



Mistletoe is not a killing parasite in 

 the sense that great numbers of trees 

 may suddenly die from it. Except in 

 the case of young seedlings that become 

 infected, death due to the parasite is 

 gradual. Nevertheless, infected mer- 

 chantable trees do have a lower life 

 expectancy than healthy ones. In an 

 investigation that continued 30 years, 

 it was concluded that mistletoe was 

 the greatest single cause of loss in pon- 

 derosa pine in the Southwest. 



Probably greater than mortality is 

 the loss from degrade or cull in logs 

 caused by mistletoe. Long-standing in- 

 fections of the parasite frequently re- 



sult in witches' -brooms and trunk 

 cankers which either directly, or 

 through the aid of secondary organ- 

 isms, render a part of the stem useless 

 or less valuable for lumber. Excessively 

 large knots are commonly associated 

 with mistletoe infection and in the 

 case of old trunk infections the wood 

 itself is brash, weak, and often dis- 

 colored or pitch-soaked. 



Mistletoe-infected trees are poor 

 seed producers. Stands that are at- 

 tacked by the parasite therefore do not 

 reproduce so abundantly as healthy 

 ones. Besides, mistletoe retards the 

 growth of its host tree. A number of 

 studies in Western States indicated that 

 mistletoe may reduce the lumber pro- 

 duction of a tree by 30 to 50 percent. 



Besides those direct losses from 

 mistletoe, the parasite tends to weaken 

 its host physiologically. Bark beetle 

 outbreaks may easily originate in in- 

 fected trees. Heart-rotting fungi find 

 favorable ports of attack through the 

 exposed wood in cankers or through 

 the excessively large branches that are 

 associated with infection. Root diseases 

 that healthy trees could withstand fre- 

 quently kill mistletoe-weakened trees. 



THE ONLY EFFECTIVE METHOD 



known so far for controlling mistletoe 

 is to prune it out and thereby eliminate 

 the absorption system and the sources 

 of reinfection. In the case of A. vag- 

 inatum, if an infected branch is cut 18 

 inches or more behind the mistletoe 

 shoots the entire mistletoe plant is 

 usually removed from the tree. Where 

 shoots appear on a branch within 18 

 inches of the trunk, the chances are 

 rather high that the absorption system 

 will have invaded the trunk and that 

 shoots will develop on it after pruning, 

 usually at the cut. There is no sat- 

 isfactory method of eliminating mistle- 

 toe from the trunk once it becomes 

 established there. Young infections, 

 where the shoots appear only on 3- to 

 4-year-old wood, can be safely cut less 

 than 18 inches from the trunk. 



Several pruning operations are us- 

 ually required to eliminate mistletoe 



