MAINE APPLE DISEASES. 369 



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 killing 

 of the living tissue. In early stages of development, cankers show 

 a region of sunken discolored bark and it is only in later stages 

 that the bark breaks away. Cankers have been described 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 occurs rarely or not at all in 

 another region. 



The injury of apple trees through winter-killing is discussed 

 on pages 341-344. 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 wide-spread and acted too quickly to be regarded as canker. 

 There arc a number of forms of winter injury and the frost 

 canker is only one of them. The frost canker is a local injury 

 which tends to heal over under favorable conditions for growth 

 unless the new growth is killed by another period of low tem- 

 perature before it has become hardened. In this way the frost 

 canker may spread, or in other cases the injured 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 

 the organisms which cause canker of fruit trees in Maine are, in 

 a large measure, wound paratites. They are unable, as a rule, to 

 penetrate the uninjured bark but must enter through wounds. In 

 this sense, the places injured by freezing serve the same end as 

 wounds of any other kind. However, it may be pointed out that 

 cankers caused by fungi do not spread so rapidly as to kill trees 

 in the short time which has been observed in the case of winter- 



