THE HOST 



The scientific name of Douglas-fir [Pseudotsuga menziesii (Mirb.) 

 Franco] honors the Scotch physician and naturalist Sir Archibald 



Menzies (1754-1842), who discovered the tree in 1791 at Nootka 

 Sound on Vancouver Island, British Columbia (32). In the "Check 

 List of Native and Naturalized Trees of the United States," the 

 United States Forest Service (32) recognizes two varieties of Douglas- 

 fir: (1) Pseudotsuga menziesii var. menziesii (Mirb.) Franco, the 

 typical green or coast variety, known in the European literature as 

 variety "viridis" and (2) Pseudotsuga menziesii var. glauca (Beissn.) 

 Franco, which includes not only the blue or "glauca" variety, but 

 also the grey "caesia" variety of Europe. The latter two are also 

 commonly known as Rocky Mountain and Intermountain Douglas- 

 fir, respectively. 



Douglas-fir comprises approximately 60 per cent of the standing 

 timber of our western forests, and furnishes about one-fifth of the 

 total annual cut of timber in America (22). The natural range of 

 the tree is the western United States and British Columbia. The 

 hardy Rocky Mountain form has been grown successfully in the 

 Northeast for Christmas trees. 



As first suggested by Wilson and Wilson (70) the habit of Rhab- 

 docline attack on the Douglas-fir in Europe seems to indicate that 

 the viridis, glauca, and caesia varieties are actually distinct strains 

 or races. The three strains of the Douglas-fir in Europe show marked 

 differences in growth rate and susceptibility to various diseases (2, 

 13, 70). The effects of Rhabdocline pseudotsugae in reducing height 

 growth of the grey and blue strains when contrasted to immune 

 green strains on Van Vloten's estate in Holland were reported by- 

 Murray (39). The silvicultural differences of the green and blue 

 forms were studied by Frothingham (17) for western United States. 

 He concluded that the coastal green Douglas-fir was faster growing 

 and of better form than the blue Rocky Mountain strain. 



Although racial resistance to the disease is not known to be 

 present in the United States, certain individuals show marked 

 resistance to R. pseudotsugae. Two adjacent trees of the same age 

 in a plantation may differ in resistance from complete susceptibility 

 to complete or apparently complete immunity (5). 



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