LEAF DISEASES AND INJURIES 19 



much alarm and is usually called blight. The true cause is 

 often not known and a rapid spread of the trouble is expected. 

 As explained under seedling diseases (see page 11), winter- 

 drying is similar to sun-scorch or drought-injury, both in symp- 

 toms and cause. Usually the leaves on the exposed side of the 

 tree are first affected. Small trees with shallow root systems 

 are often killed outright. In the case of older trees, only the 

 leaves on scattered twigs or on the branches on the exposed 

 side of the tree are affected. 



Symptoms. 



The affected leaves wither and die in late winter or early 

 spring. Later the dead needles assume a red-brown color and 

 the affected trees appear as if scorched by fire. Later in the 

 summer the dead leaves and affected twigs may be shed. 

 Adventitious buds are produced, from which a new crop of 

 leaves develops in the autumn. 



Cattse. 



Winter-drying is caused by the rapid loss of water from the 

 leaves at a time when the water in the soil is frozen and is not 

 available to the plant. Strong winds, when the air temperature 

 is either high or low, cause the leaves on the exposed side of the 

 tree to lose water to such an extent that the tissues wilt and 

 die. The factors determining the extent of damage by winter- 

 drying are : age of the tree and depth of roots in the soil, 

 exposure to prevailing winds, temperature of the air, depth to 

 which the soil-water is frozen, duration of strong winds blowing 

 from a given direction and the frequency of reoccurrence of a 

 combination of conditions favorable to winter-drying. Numer- 

 ous semi-saprophytic fungi develop on the dead leaves, and 

 thus give the impression, later in the season, that a destructive 

 leaf-blight fungus has caused the damage. An easy way to 

 determine whether injury to conifers noticed in the spring is 



