352 MANUAL OF TREE DISEASES 



care must be taken to cut back into the healthy bark beyond 

 the advance of any parasite which may be present. The bark 

 should be cut at right angles to the surface and the edges should 

 be clean and smooth. The wood beneath the cankered area 

 should in most cases be removed to the depth of a half inch or 

 more. Then the ends of the wound must be shaped to a point. 

 All of this work can be done with a draw-shave, chisel, gouge, 

 mallet and heavy sharp knife. The diseased tissue which is 

 removed should be burned, since it may serve as a source of 

 infectious material. The Endothia canker of chestnut (see 

 page 140) affords a good example of the care necessary to remove 

 all affected tissues. The early failures to prevent the extension 

 of the cankers by incision of the bark only was later explained 

 by the discovery that the mycelium penetrated into the wood, 

 even to the fifth annual ring at times. With this fact in mind, 

 successful treatment of the cankers is effected by the removal 

 of all diseased wood beneath the cankered area. The tendency 

 of certain parasites to spread up and down the limb or trunk in 

 the cambium region or sapwood, without causing apparent in- 

 jury to the overlying bark, presents another difficulty in canker 

 excision. This habit of growth is common for many parasitic 

 fungi and bacteria. The mycelium of a fungus may spread in 

 this way for many inches. above and below the cankered area. 

 Its presence is usually evident as dark colored streaks in the 

 sapwood. 



When the lesions extend deeply and extensively into the wood 

 as in the case of heartwood- and sapwood-rots, the complete 

 removal of all the affected tissue is very diflBcult. Large cavi- 

 ties must be excavated, which is a tedious and an expensive 

 operation. Tissues in the advanced stages of decay are easily 

 detected and removed. Such wood is usually soft and criunbly. 

 But if the excavating stops with the removal of the punky wood 

 only, no benefit will result from the work, for many of the fungi 

 which cause wood-rots advance a considerable distance into the 



