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 
same time the eaposed side should be partially protected by 
brush or boards, until the cambium overgrows the wound. 
Similar results, z.e., the bursting in longitudinal cracks 
and killing of the bark and of underlying wood portions, 
are produced by the so-called sum-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 
