66 MANUAL OF- TREE DISEASES 



this reason such trees are usually free from wood-rots until 

 heartwood is formed. The larger part of the deciduous trees 

 form no such protective wound exudations, and injuries which 

 cause the exposure of the sapwood often result in infection by 

 some sapwood-rotting fungus. Wounds of all sorts may be 

 caused in the forest by browsing animals, woodpeckers, boring 

 insects, wind, snow and ice breakage of limbs, &e scars at the 

 base, and natural pruning of limbs, while out of the forest many 

 additional agencies may cause wounds. Some wood-destroy- 

 ing fungi attack commonly the wood of the roots. Wounds 

 in the roots afford places of infection, and from the roots the 

 mycelium may spread upward into the lower part of the trunk. 

 But the most common mode of entrance for fungi is by way of 

 the wood exposed when branches are broken or pruned-off, either 

 naturally or artificially. The proper method of cutting the 

 limb so as to leave no projecting stub cannot be too greatly 

 emphasized in the case of shade and ornamental trees (see page 

 346). The wound must be flush with the parent limb in order 

 that the callus may cover it. In the meantime, while the 

 callusing is taking place, the exposed wood should be covered 

 with a wound-dressing (see page 348). Total disregard for 

 these two procedures in pruning leaves the way open for most 

 of the damage to valuable trees by wood-rotting fungi. In 

 the forest a certain amount of wounding may be avoided, but 

 the main method of control is by the removal of the sources 

 of infection. 



The spores of a fungus causing wood-rot of a certain kind of 

 tree, after lodging upon exposed sapwood or heartwood of that 

 tree, will germinate in the presence of moisture and develop 

 a mycelium which grows into the wood. The sapwood-rotting 

 fungi immediately spread their mycelium in all directions and 

 soon large dead areas result. The species which attack the 

 heartwood preferably, develop mycelium which reaches down 

 through the wood of the branch stub into the heartwood of the 



