BODY AND BRANCH DISEASES AND INJURIES 55 



Spruce, pine, larch, fir and hemlock are attacked by more 

 or less restricted species of Razoumofskya. These parasitic 

 plants are very small and not easily recognized among the 

 green needles of the host. When the mistletoe seed germinates, 

 root-like sinkers are pushed down into the wood of the branch. 

 The young mistletoe plant then obtains its water and food 

 materials by robbing the host. The damage done by the 

 dwarf mistletoes is largely due to the stimulus reactions set 

 up. Excessive growth occurs in the part of the tree attacked, 

 and large numbers of short bushy branches are produced which 

 form what are commonly known as witches'-brooms. These 

 brooms are often very large, and the excessive growth results 

 in lessened vigor of the portion of the limb beyond the broom. 

 When several brooms grow on a tree, its lumber value de- 

 preciates because of the diminished annual growth. Another 

 damaging effect is due to the' weight of the brooms when 

 covered with ice and snow, causing them to break off, leaving 

 wounds which are easily infected by various wood-rot fungi. 



Following is a list of the dwarf mistletoes (species of Ra- 

 zoumofskya) which grow parasitically on conifers in the western 

 United States : — 



On flr (Abies) 



R. Douglasii abietina (Engelm.) Piper. Rocky Mountains and west- 

 ern United States 



R. occidenialis abietina (Engelm.) Coville. Pacific Coast and Utah 

 On fir (Pseudotsuga) 



R. Douglasii (Engelm.) Kuntze. Rocky Mountains and Pacific 

 Coast 

 On liemlock 



R. tsugensis Rosend. Northwestern United States 

 On larch 



R. laricis Piper. Northwestern United States 

 On pine (five needle soft pines) 



B. cyanocarpa (A. Nelson) Rydberg. Western United States 



R. Blumeri (A. Nelson) Standley. Arizona 

 On pine (pinon, nut pines) 



R. divaricata (Engelm.) CoviUe. Central and southern Rocky 

 Mountains and California 



