INSECTS, DISEASES, AND INJURIES 143 



secondary fungous attacks which cause a decay and bring 

 about an extension of the original lesion. 



For whitespot, which is apparently due to surface heating 

 of the soil, shading and frequent light watering have given 

 satisfactory^ control. When possible, soils which are porous 

 and of a dark color should be avoided. 



Winter-killing. 



Winter-killing is very similar to "sun-scorch." It occurs, 

 however, when the ground is frozen so that the roots are 

 unable to supply a sufficient amount of water to the tops 

 during warm periods of winter or in the early spring. It differs 

 also from "sun-scorch" in that open stands are more liable to 

 be affected. 



The application of a light straw mulch to the beds and the 

 erection of windbreaks will give control. Care must be taken 

 in the use of a mulch, as injury might accrue from too heavy 

 an application, as described below. 



Frost-injury. 



This differs from winter-killing in the fact that it is not due 

 to a drying-out effect but to the formation of ice crystals 

 within the unripened tissues of the plant. It results from 

 early frosts which occur before the tissues have matured or 

 from late spring frosts. Certain trees, such as jack pine, com- 

 mence growth very early in the spring so that the terminal 

 buds and young shoots are often killed by such late frosts. 



Very late spring sowing and the forcing of growth toward 

 the end of the growing season should be avoided in order to 

 give the tissues a chance to mature before the advent of frosts. 

 Mulching the beds will also help to prevent injury. Growth 

 should be retarded by shading or some other method in the 

 spring until all danger of severe frosts is passed. 



