116 U.S. DEPT. OF AGRICULTURE MISC. PUB. 939 



the swelling, the youngest tips growing outward near the poles of the 

 spindle. Flowering shoots first appear near the original court of en- 

 trance, but succeeding ones emerge in concentric zones of increasing 

 diameter. The center of such an infection usually deteriorates and 

 forms a court of entrance for various decay-producing fungi and bark 

 beetles. 



Some dwarfmistletoes, but only on certain host species, call forth 

 the production of a witches'-broom in which suppression of lateral 

 branches is largely released, resulting in a dense growth of abnormal 

 appearance. In many of such brooms the endophytic system of the 

 dwarfmistletoe pervades all branches, even to penetration of the apical 

 meristems of the host, and these mistletoe shoots are present in pre- 

 dictable patterns along these host branches. In most instances the un- 

 infected part of the tree deteriorates badly, leaving only the broomed 

 portion alive. 



Since the susceptibility of many exotic conifers has been demon- 

 strated and some native hosts are planted extensively abroad, the 

 North American species of Arceuthobium pose a considerable threat. 

 Fortunately, the seeds are not long-lived and lose their ability to 

 adhere to new objects after some time, therefore the chance of estab- 

 lishment on other continents by means of seed is small. A greater 

 danger, perhaps, lies in the export of trees of nursery size, since these 

 may bear invisible infections. Experience in North America has 

 shown complete elimination of an infected stand to be the only possi 

 bility of eradication. 



Dwarfmistletoes are difficult to evaluate as to their economic im 

 portance. There is general agreement, however, on the severity of 

 losses in certain areas. Field observations point to significant losses 

 in the following instances : 



1 



Hosts: Pinaceae Dwarfmistletoes Area 



Abies spp Arceuthobium campy- Sierra Nevada, California. 



lopodum. 



Picea spp A. pusillum Northeastern United States 



and across Canada to 

 Manitoba. 



Pinus spp _■ A. campylopodum Eastern Oregon south 



through most Pacific and 

 Southwestern States. 



P. banksiana, P. con- A. americanum Northern Alberta to Colo- 



torta. rado. 



Pseudotsuga menziesii- A. douglasii Interior southern British Co- 

 lumbia to northern Cali- 

 fornia and the South- 

 western States. 



Tsuga heterophylla A. campylopodum Pacific Coast, Alaska to 



Oregon. 



A more complete listing of hosts is provided in Kuijt (1955). 



Literature : 



Gill, L. S. Arceuthobium- in the United States. Trans. Conn. Acad. 

 Arts & Sci. 32: 111-245. 1935. 



Kuijt, Job. Dwarfmistletoes. Bot. Kev. 21 : 569-628. 1955. 



. Morphological aspects of parasitism in the dwarfmistle- 

 toes (Arceuthobium). Univ. Calif. Pub. Bot. 30: 337-436. 1960. 



