BIRCH DISEASES 113 



only briefly from a few collections in northeastern United States, 

 Canada and Greenland, they may be more generally distrib- 

 uted and common on other species of birch. The lesions 

 are confined to the leaves and consist of large reddish brown ■ 

 blistered areas, which may cause the leaf to curl. The mycelium 

 is confined to the space which it makes for itself between the 

 cuticle and the epidermal cells. Due to the parasitic activities 

 of these fungi, the tissues of the leaf are stimulated to an in- 

 crease both in number and size of the cells. This results in the 

 bulging and curling of the leaf between the more rigid veins. 



For the control of these diseases, the same methods that are 

 used for peach leaf-curl should give results (Hesler, L. R., and 

 Whetzel, H. H., Manual of fruit diseases, p. 277). 



Powdery Sapwood-Rot 



Caused by Polyporus betulinus Fries 



Many species of birch are subject to this sapwood-rot through- 

 out the northern hemisphere. Yellow, white and paper birch 

 are commonly affected in the United States. Although the 

 fungus causing this rot is very common on injured and dead 

 birch trees, its importance in causing serious damage to healthy 

 trees is questioned. The fungus does not enter through branch 

 wounds and other injuries where heartwood is exposed and, 

 therefore, never causes a heartwood-rot of the living tree. This 

 rot is similar to the yellowish sapwood-rot caused by Fomes 

 fomentarius. In the case of both of these fungi, badly injured 

 or weakened trees are attacked and the sapwood is the first 

 part of the trunk decayed. Later, the fungi extend their 

 activities into the heartwood and the entire woody cylinder 

 of the trunk is destroyed. These wood-rots are nevertheless 

 important in the forest since the timber value of the species 

 they affect rapidly deteriorates as soon as the trees become 

 mature or injured. 

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