APPLE DISEASES 



455 



new wood, followed by further 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. 



Frost Canker 



Much of the disease of apple trees 

 which orchardists have been calling 

 canker has its origin in severe winters. 

 Some injuries so resulting might be 

 properly classified under the term "frost 

 canker." On the other hand, when whole 

 trees are so badly injured that they die 

 either that year or the year following, 

 the injury is too widespread and acts 

 too quickly to be regarded as canker. 

 There are 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 temperature 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 are, 

 in a large measure, wound parasites. 

 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-killed trees. In the case 

 of young trees the fungus may in some 

 cases girdle the tree in a few weeks and 

 thereby cause its death. The same holds 

 true of small branches of old trees, but 

 in the case of large branches the fungus 

 usually spreads but a few inches each 

 year forming true cankers, and the 

 rough, blackened areas that are fre- 

 quently seen on large branches often 

 represent a development of several years. 

 The living tissues attempt to heal over 

 the wound by the formation of callus 

 and in some cases with considerable 

 success. Often the parasite ceases to 

 spread in the bark when the dry season 

 of summer comes on and a crack forms 

 between the healthy and diseased bark. 

 The following year the diseased area 

 may continue to spread or the callus 

 may check it considerably. Often other 

 fungi, some of them saprophytes, grow 

 upon the dead bark. 



There are many other wounds than 

 those caused by freezing through which 

 parasitic fungi may enter. By this it is 

 not meant that every wound that is 

 made in the bark will necessarily be- 

 come infected and develop into a dis- 

 eased area. In many cases, however, 

 the spores of parasitic fungi are carried 

 to wounds. This is especially liable to 

 be the case when diseased branches are 

 allowed to remain on the trees, or old 

 neglected trees in the neighborhood pro- 

 duce abundant crops of fungus spores 

 from year to year. Some of the ways 

 in which wounds are made are: Bark- 

 ing of trunk and branches by machinery 

 in cultivating and caring for the or- 

 chard: injuries by ladders and by men 

 in picking fruit; branches are some- 

 times injured by props used to support 

 a heavy load of fruit especially when 

 they are carelessly placed in position; in 

 some cases hailstones split the bark of 

 small branches. Care should be taken 

 to avoid any injury which is within the 

 control of the orchardist. Wounds are 

 sometimes kept from healing over by 

 the woolly aphis which forms little cot- 



