592 



thick and two years old, at times showed, only \en- weakl}- callused. uni- 

 formly closed protuberances, covered with new hark, which l)reak out from 

 rhe cleft in the old bark. 



The studies here cited determine that each canker spot has, as its 

 initial sta^e, a wound which extends as a narrow radial tear into the cam- 

 bium and kills it slij^jhtl}- back from both sides. This wound must be 

 produced shortly before, or at the time w hen the trunk of the tree develops 

 the greatest growth activity, since the wound surface will attemi)t at 

 once to form a covering by means of very luxuriant overgrowth edges. 

 'J1ic luxuriance of these overgrowth rolls manifests itself in the fact that. 

 especially in the closed form of canker, a partition of rhe annual ring 

 usually occurs, the edges of which chiefl}- consist of parenchyma wood. 

 The edges of the wound are very susceptible because of this porous struc- 

 ture, so that they succumb with ease to injurious attacks. 



We must consider frost as the cause of these forms of disease because 

 it has been possible to produce, by the action of artificial frost, the same 

 initial stages as are found in canker wounds. 



How-ever, a number of \ er}' reliable observers have determined that it 

 is possible by the injection of a (capsule) fungus, Nectria ciitissii}ia\ to 

 produce wounds, the forms of which resemble, perfectly those of the open 

 canker of the apple. I can confirm these statements by my own experi- 

 ments. One has indeed a right to speak of a fungous canker but the above 

 named parasite is not able to attack an uninjured axis. It can spread de- 

 structively only if it gets into a bark wound. All inoculation exi)eriments 

 agree in this. ( )n the other hand, the same Xectria is found in apple trees, 

 beeches and other varieties of deciduous trees without causing any canker 

 excrescences whatever. Therefore, it cannot be termed the specific incitor 

 of canker excrescences but will give rise to these only occasionally when 

 very definite secondary conditions co-operate simultaneously. Besides the 

 presence of a fresh wound surface, it depends also upon the specific pecu- 

 liarity of the tree, i. e., the cultural variety, which must possess the ability 

 to respond to the wound stimulus with ciuickly developing, very luxuriant 

 overgrowth. 



This ability is so ty[)ical that in general practice one speaks of 

 "Varieties with a canker tendency." Besides this, experience has shown 

 that the tree easilv becomes cankered in certain places and kinds r)f soil. 

 These are the so-called frost holes, having a marshy soil consistency, an 

 impervious sub-soil, etc. 



These are well-established facts. If we now keep in view the fact 

 that Nectria ditissima must have some wound for infection, we must ask 

 whence came these w-ounds. I-'rom observations made in nature and from 

 the results of experiments with artificial frost, we are convinced of neces- 

 sity that frost injuries are the most easily accessible. Paparozzi holds to 



1 See literature in the second volume of this manual, p. 20'J. 



