MISTLETOE INJURY TO CONIFERS. 



23 



infected, however, may continue to produce aerial branches of the 

 mistletoe to a very advanced age. True mistletoe burls are probably 

 of less frequent occurrence on Douglas fir than on any other economic 

 tree species. Burls do occur, however, with sufficient frequency to be 

 characteristic of mistletoe infection on the trunk of this tree. Large 

 elongated mistletoe burls, including the entire circumference of the 

 trunk, occasionally occur in heavil}^ infected trees in many parts 

 of Idaho and Montana (fig. 21). More frequently there is a series 



Fig. 21. — Large mistletoe burl on Douglas fir 

 caused by Razoumofskya douglasii. This 

 burl is approximately 10 feet long and 2 

 feet in diameter at its widest part. 



Fig. 22. — A Douglas fir, showing numer- 

 ous burls caused by Razouniofskt/a 

 douglasii. The branches are heavily 

 broomed. A high degree of infection, 

 but a common condition, is shown. 



of individual burls, more or less confluent, on one trunk (fig. 22), 

 each burl representing the seat of an old infection, from which the 

 aerial parts of the parasite have long since disappeared. Longitu- 

 dinal and cross sections through these burls show the characteristic 

 fan-shaped areas of infection (fig. 23). In numerous cases the burls 

 originate from infections at the base of branches. If the branch 

 dies or is broken off, an open wound is formed in the center of the 

 burl. Very peculiar swellings or small burls frequently occur on 



