MISTLETOE INJURY TO CONIFEES. 



13 



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With the conchision of this general statement of mistletoe injury 

 a more detailed discussion of the various types of infection will 

 now be taken up. 



RESULT OF INFECTION ON THE BRANCHES. 



One of the first effects of infection, either of stem or branch, is 

 the formation of a fusiform swelling (fig. 10). Sometimes this 

 swelling is very pronounced and may resemble the enlargements 

 caused by some species 

 of Peridermium (fig. 

 11). The swelling is 

 the first stage of the 

 future hypertrophy 

 commonly known as 

 witches'-brooms. The 

 absence of any pro- 

 nounced brooming 

 from early infections 

 has led some observers 

 to the conclusion that 

 brooms are never pro- 

 duced on some conifers. 

 Any change from the 

 normal branching is 

 here considered a 

 broom. Still it is not 

 necessary to draw such 

 sharp lines, as the 

 brooms produced by all 

 mistletoes of the geniis 

 in question are quite 

 typical. It may re- 

 quire several years for 

 the broom to form. If 

 young trees are gen- 

 erally, infected they 

 sometimes assume an open, ragged appearance, which to the casual 

 observer would not be considered a broom (PI. Ill, fig. 2). Never- 

 theless, the tree is no longer excurrent. A similar condition is 

 sometimes noted in more mature larches, where the infection is so 

 generally distributed throughout the entire crown that no typical 

 brooms are produced for years. Heavily infected branches of old 

 trees of all species are seldom without brooming of some kind, and 

 in most cases typical brooms are formed. The mistletoe plant may 

 die, out entirely on very old brooms, especially those of yellow pine 



Fig. 9. — Western hemlock (Tsuga heterophylla) infected 

 by itazoumofskya tsugensis. These trees do not possess 

 a single normal branch. All are broomed. The trees in 

 the background are spike topped. The tree in the fore- 

 ground has had its growth in height arrested by an 

 immense terminal broom. 



