412 AGRICUI.TURE; OF MAINE. 



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

 ii! 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 cavised by fungi do not spread so 

 rapidly as to kill trees in the short time which has been ob- 

 served in the case of winter-killed trees in this State. 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 

 frequently seen on large branches often represent a develop- 

 ment 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 sap- 

 rophytes, grow upon the dead bark. Thus it may be seen that 

 while fungi which enter through winter injuries may spread 

 and ultimately do great damage to the tree, there is no reason 

 to believe that the death of large trees which was observed in 

 Maine orchards immediately following the winter of 1906-7 

 was due to a parasitic organism because the trees died in many 

 cases in too short a time for their death to have been caused 

 by such organisms. On the other hand, there were many places 

 injured by that winter, which became infected by fungi. Where 

 these have been neglected the diseased areas have spread from 

 year to year and have done much damage, often developing into 

 true cankers. 



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 diseased area. Frequently 

 small wounds in the bark of young branches heal over quickly, 

 without infection and no serious damage is done. In many 

 cases, however, the spores of parasitic fungi are carried to 

 wounds. This is especially liable to be the case when diseased 



