MAINE AGRICUI.TURAL EXPERIMENT STATION. 191O. 4II 

 DISEASES OF THE WOOD. 



Canker and Twig-blight. The term canker has become such 

 a general one as not to admit of easy definition. It is commonly 

 used to describe the condition of branches of trees in which an 

 area of bark has been killed and has broken away so that a por- 

 tion of the wood is laid bare or is covered only by cracked and 

 roughened bark which does not protect the wood. In the 

 writers' opinion the term "canker" as applied to diseased areas 

 on trees should be restricted to those characteristic lesions on the 

 trunk and limbs which are the result of alternate attempts to 

 heal, with the formation of new wood, followed by farther kill- 

 ing of the living tissue. In early stages of development, cank- 

 ers show a region of sunken discolored bark and it is only in 

 later stages that the bark breaks away. Cankers have been de- 

 scribed as caused by frost, sun-scald, fungi, and bacteria. A 

 considerable number of different fungi have been reported as 

 causing canker of apple trees in different parts of the United 

 States. These vary greatly in the amount of damage which 

 they do in different regions. In some cases, a fungus which 

 causes a great amount of injury to the trees of one region oc- 

 curs rarely or not at all in another region. 



The injury of apple trees through winter-killing is discussed 

 on pages 384-387. Much of the disease of apple trees which 

 Maine orchardists have been calling canker for the past 3 years 

 had its origin in the severe winter of 1906-7. Some of the in- 

 juries resulting from that winter and the seasons following 

 might possibly be properly classified under the term "frost 

 canker." On the other hand, when whole trees were so badly 

 injured that they died either that year or the year following, 

 the injury was too widespread and acted too quickly to be re- 

 garded as canker. There are a number of forms of w^inter in- 

 jury and the frost canker is only one of them. The frost canker 

 is a local injury which tends to heal over under favorable con- 

 ditions for growth unless the new growth is killed by another 

 period of low temperature before it has become hardened. In 

 this way the frost canker may spread, or in other cases the in- 

 jured bark may serve as a place for the entrance of a parasite 

 which may then spread in the bark and outer layers of wood 

 and kill a rather large area in a single year. 



So far as they have been investigated it has been found that 



