56 DISEASES OF DECTDUOTS I'olIKST TREES. 



presence on the outside until the wood has become sufficiently 

 decayed to permit of the formation of sporophores, which then usually 

 form by growing out through the season checks to the outside air. 

 It is on account of its ability to produce decay in the interior of the 

 stick of wood that this fungus is so very destructive, and it is for 

 this reason that the greatest care should be taken to guard against its 

 possible entrance. In moist climates the fruiting bodies will form 

 above the ground on wood -which may be several feet above the soil. 

 Where wood has a chance to have air circulate around it continuously, 

 however, the possibilities that it will become infected with this fungus 

 are remote. 



The decay which Polystictus versicolor brings about in wood is 

 usually confined for a year or more to the sapwood, and in many 

 species it is wholly confined to the sapwood. This is true of such 

 trees as have their heartwood sharply differentiated from their sap- 

 wood, as in the oaks (Quercus spp.), black walnut (Juglans nigra L.), 

 red gum (Liquidambar styraciflua L.), etc. In woods where the 

 differentiation between heart and sap wood is indistinct, as in the 

 tulip poplar (Liriodendron tulipifera L.), willows (Salix spp.), and 

 cotton wood (Populus deltoides Marsh.), the fungus brings about the 

 destruction of the sapwood with great rapidity, and even destroys 

 the heartwood, although this takes place more slowly. Between 

 these two extremes there are many gradations. Sapwood of the 

 white ash (Fraxinus americana L.), yellow birch (Betula lutea 

 Michx. f.), and beech (Fagus atropunicea (Marsh.) Sudworth) is 

 rapidly destroyed, but the heartwood resists the attacks of this 

 fungus for a considerable period of time, although not so long as does 

 the heartwood of the oaks. The rate at which the sapwood of differ- 

 ent broadleaf species decays presents little variation. It may be 

 stated as a general rule that the sapwood of all trees is very suscepti- 

 ble to the attacks of Polystictus versicolor and that where any differ- 

 ence in the resisting power of such woods to this fungus occurs it will 

 be in favor of the heartwood. 



Measures for pro vent ing or arresting the development of Polystictus 

 versicolor will be considered later, together with other sap-rotting 

 fungi, because preventives which apply to this fungus will apply to 

 practically all of the sap-rotting forms. 



SAP-ROT CAUSED BY POLYSTICTUS PERGAMENUS 



A second form of sap-rot of great importance is caused by Poly- 

 stictus pergamenus Fr. This fungus, while usually found on dead 

 wood, may now and then occur on living trees where those have been 

 severely injured. It is one of the forms which frequently appear 

 on standing trees after forest fires have killed certain parts of the 



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