INSECTS, DISEASES, AND INJURIES 145 



may appear normal in the spring and until the soil begins to 

 dry out; then the death of the tree follows rapidly, usually 

 commencing at the tip of the leader. In the case of a partial 

 killing of the roots, severe sun-scorch may develop with the 

 appearance of the first hot, dry weather. 



This injury may be prevented by the use of a mulch when 

 such weather conditions exist. 



Frost-injury to the bark. 



This is a type of winter-injury which occurs on the sun- 

 exposed side of the trees. Among the conifers, pine and spruce 

 are particularly susceptible. It is thought that repeated 

 freezing and thawing of the tissues of this side of the tree 

 causes the death of patches of the bark which often peel off 

 and expose the sapwood. In some cases, the bark adheres 

 firmly but becomes somewhat sunken so that canker-like 

 areas are produced. These dead spots form suitable places 

 for the activities of various fungi. 



In order to prevent the entrance of fungi, the injured bark 

 should be removed by proper tree-surgical methods and the 

 exposed wood protected by a suitable dressing. For this 

 purpose a coating of good shellac should be applied to prevent 

 drying out and when this is dry it should be covered with a coat 

 of tar or thick bark-colored paint. (See under tree surgery.) 



Frost-cracks. 



Frost-cracks are much more common on deciduous trees 

 than on conifers, although they occur fairly frequently in the 

 case of the spruce and fir. The cracks appear as longitudinal 

 slits running for some distance up and down the trunk on the 

 sunny side, and are caused by sudden falls in temperature. 

 They open wider during cold weather, while during warm 

 periods they may completely close up. They are chiefly of 



