68 Diagnosing of Diseases 



grafting. Drainage of soil and more careful pruning will 

 reduce the disease. The cankerous parts should be cut 

 out to the living cambium and covered with hot tar, at the 

 V same time the exposed side should be partially protected by 

 brush or boards, until the cambium overgrows the wound. 



Similar results, i.e., the bursting in longitudinal cracks 

 and killing of the bark and of underlying wood portions, 

 are produced by the so-called sun-scald, which is also espe- 

 cially observed on the south and southwest sides of such 

 smooth-barked trees as Beech, Apple, Pear, and also of 

 the soft-wooded Basswood, Horse-chestnut, Chestnut, Ash, 

 Plane tree. Birch, and Willow. It is an open question 

 whether this is not due in part, at least, to frost, in conjunc- 

 tion with the sun. Protection of the exposed side by boards 

 or straw, or still better by a neighboring shady tree, will 

 avoid the trouble. 



When the fall of a winter temperature is very sudden and 

 very low (below zero) frost splits, due to uneven shrinkage 

 of the wood in the interior, are produced in a variety of 

 trees; especially in those with distinct heart-wood, the sap- 

 wood shrinking more than the heart- wood, a longitudinal 

 split results. These cracks usually close up during warm 

 weather, a callus overgrowing the wound forms a ridge 

 and the damage consists only in this malformation, unless 

 fungi and insects gain access. Such frost splits occur very 

 frequently in northern latitudes, even in the forest, and on 

 very cold days with a noise resembling a gunshot. 



Obnoxious Gases. Finally, special conditions of the 

 atmosphere which become more and more potent in our 

 cities, namely, the smoke and other poisonous gases emanat- 

 ing from factories, must be considered as causes of disease. 

 The merely mechanical black coating of coal particles and 

 soot which settle on the foliage will only slightly reduce the 



