214 MANUAL OF TREE DISEASES 



Mistletoe Buel and Witches'-Bkoom 



Caused by Razoumofskya laricis Piper 



The mistletoe disease of larch is common and destructive 

 in northwestern United States. It is especially abundant in 

 open stands and causes but little damage in the dense forests. 

 In moist and fertile areas, the larch attains full development 

 and is only rarely deformed by the mistletoe. On the other 

 hand in regions of light rainfall, variable temperature, low 

 humidity, dry soil and especially in open stands, the growth of 

 the mistletoe is favored and the tree suppressed. Measure- 

 ments made of unaffected and badly infested trees show that 

 the rate of growth of the larch may be reduced to one-half the 

 normal. 



Symptoms. 



Young and old larches are affected. The seeds of the mistle- 

 toe produce infection if they fall on the green twigs. Burls 

 are produced in the trunk and larger branches due to the irri- 

 tation caused by the roots of the mistletoe. Infected younger 

 branches are stimulated to produce abnormal twigs, forming 

 witches'-brooms. The foliage area of the tree is reduced by 

 the deforming of the branches and the general development 

 of the tree is suppressed. 



Cavse. 



This burl and witches'-broom disease is caused by one of 

 the species of dwarf mistletoe, Razoumofskya laricis. The 

 roots of the parasite penetrate the bark and wood and grow 

 down the branch for some distance, sometimes entering larger 

 branches or the trunk. The irritation caused by the roots 

 of the parasite results in an increased growth of the affected 

 parts. The parasitic plants are inconspicuous, never being 

 more than about four inches tall. A . cluster of yellowish 



