593 



the same standpoint for the canker of pear trees'. Tf the frost wounds are 

 flat surfaces such as will be found later under "Blight," the Nectria will 

 infest the tree without its formation of luxuriant overgrowth edges. Tf, 

 however, narrow frost tears, extending into the cambium, are produced 

 into which the Nectria find entrance, the tree responds with the formation 

 of canker excrescences in case climate, habitat or specific characteristics 

 make it capable of so doing. 



Accordingly, the fungous canker also appears to be essentially depend- 

 ent upon frost injury and its combatting or avoidance will have to be 

 carried on with due consideration of the danger from frost. 



b. Crotch Cankkr in Fruit and Forest Trees. 



"Crotch canker," which is of frequent occurrence in forest and fruit 

 trees, should be mentioned as an especial form. It 

 consists of frost wounds found at the bases of the 

 branches, or twigs, which belong to the group of 

 open cankers and are formed from black, dead 

 surfaces differing in size with luxuriant, irregular 

 overgrowth edges. The angle where the branch 

 joins the main trunk is separately attacked in 

 many varieties. In the so-called ''bifitrcations." or 

 forkings, where the difference between the main 

 and the lateral branch disappears so that two ecfually 

 strong branches grow out from one point, the ex- 

 posed and blackened place in the wood is usually 

 elevated at both sides and, accordingly, the over- 

 growtli edge is formed from the material of both 

 branches (cf. Fig. 138). Aside from the more 

 sensitive, imj^orted trees, our indigenous forest 

 trees, according to Nordlinger-, are also exposed to 

 injuries at the crotch, especially when young; thus, 

 for example, beeches in shady positions and on poor 



soil, in which the intemodes at some distance from the crotches are also 

 often covered with frosted surfaces. The annual growth of the oak also 

 suff'ers on poor soils and the ash is found to be injured if the tree stands in 

 depressions with a heavy clay soil. In such damp places I found the over- 

 growth unusually luxuriant but so covered with thick, split bark, overgrown 

 with lichens, that it had become irrecognizable. 



Opposed to the theory, which Hartig represents, that crotch canker is 

 conditioned by spring frosts, Nordlinger thinks the cause is frost at the 

 beginning of winter. He bases his opinion on the investigation of the wood 

 ring and on the fact that, in thousands of cases, crotch canker is very 



Crotch canker 



1 Paparozzi. G., II cancio del pero. Roma, Offizina poligraflca; cit. Bot. Cen- 

 tralbl. 1904, v. XXVIII, p. 94. 



- Die Septemberfroste 1877 unci der Astwurzelschaden (A.st\vurzelkrebs) an 

 Biiumen. Centralbl. f. das ges. Forstwesen. Wien 1878, Part 10. 



