Dwarf Mistletoes 



461 



from the branches of a tree. That is 

 because often a lag of several years 

 occurs between the time infection takes 

 place and the time the first shoots ap- 

 pear. These latent infections will be 

 missed at the initial pruning. Because 

 the seeds require 2 years to mature, the 

 interval between pruning can be 2 

 years without danger of self-infection. 

 Assuming that the trunk is not in- 

 fected, one should be able to prune out 

 all mistletoe at 2-year intervals in a 

 period of 8 years or less. It is doubtful 

 if heavily infected trees should be 

 pruned even though the trunk has not 

 been invaded. If the control operation 

 requires the removal of more than one- 

 third to one-half of the crown, one 

 must consider the adverse effect of 

 pruning alone on the physiology of the 

 tree. 



IN MANAGED FORESTS, in the case 

 of infected stands subject to their first 

 cut, mistletoed trees are marked for 

 cutting wherever possible. Where such 

 trees are not merchantable, they 

 should be eliminated as a sanitary pre- 

 caution, a practice that tends to re- 

 duce the source of mistletoe seed and 

 thus protect the oncoming young trees 

 from infection to some degree. It is 

 most effective in lightly infected stands 

 where practically all mistletoe trees 

 can be cut without sacrificing other 

 silvicultural principles. 



In more heavily infected stands, 

 where only the worst cases can be cut 

 and a large number of lightly mistle- 

 toed trees remain, less protection is 

 offered to the residual stand. Increased 

 light and the stimulating effects of re- 

 lease tend to favor the production of 

 mistletoe shoots; the result is that 

 there is likely to be heavy self-infec- 

 tion and spread to the younger stories 

 of the stand. 



With more intensive management 

 than is possible in virgin stands, 

 mistletoe will demand more drastic 

 control measures in order to attain 

 maximum yields of wood. 



Eradication of the parasite in sec- 

 ond growth through a series of weed- 



ing, pruning, and thinning operations 

 appears to be one effective method. 

 Another would be to establish barrier 

 zones around heavily infected centers 

 in an effort to protect surrounding un- 

 infected timber and keep the mistle- 

 toe confined to a small area, possibly 

 until such time as it could be clear-cut 

 and, if necessary, planted. In the case 

 of A. vaginatum, recent studies indi- 

 cate that a pine-free strip 60 feet wide 

 should keep the parasite confined. 



As the problem appears today, 

 there is a need for developing more 

 effective direct-control methods than 

 the present one of physically removing 

 the parasite by pruning and cutting. A 

 selective substance that would kill it 

 without deleterious effects on the host 

 would be highly desirable if it could be 

 produced and applied economically. 

 Also needed is more knowledge of the 

 behavior of the dwarf mistletoes in 

 order that the best silvicultural-man- 

 agement practices can be developed in 

 forests where the parasite is taking its 

 annual toll of wood substance and tree 

 life. 



LAKE S. GILL is in charge of the 

 field headquarters of the Division of 

 Forest Pathology in Albuquerque, N. 

 Mex. He has studied the problem of 

 mistletoe infestation for more than a 

 decade and has developed much basic 

 information on its development and 

 spread. In cooperation with the Forest 

 Service, Dr. Gill has also conducted 

 experiments on the control of mistletoe 

 in ponderosa pine in the Southwest. 



JESS L. BEDWELL is in charge of the 

 field headquarters of the Division of 

 Forest Pathology in Portland, Oreg. 



For further information on dwarf 

 mistletoes and other diseases, insects, 

 and parasites of forest and shade trees, 

 the reader may consult publications 

 listed in the later section, "For Further 

 Reference." The publications are gen- 

 erally available in libraries; some of the 

 bulletins listed can be obtained from 

 the Office of Information, the United 

 States Department of Agriculture. 



