CHAPTER II 

 LEAF DISEASES AND INJURIES 



The diseases and injuries of leaves common, in varying 

 degrees, to all kinds of trees are due mainly to three general 

 causes : extremes of moisture and temperature, atmospheric 

 impiu-ities, and an improper or impaired supply of raw materials 

 and water. Of these, the extremes of moistiu-e and temperature 

 are the most important, especially when the trees are under 

 normal conditions of growth. 



Conifers which keep their leaves throughout the winter may 

 be injured by winter-drying as described under seedling diseases 

 (see page 11). Freezing-to-death may cause destruction of the 

 newly formed twigs and unfolding leaves of both conifers and 

 deciduous trees when late frosts occur in the spring. Trees of all 

 kinds often suffer severely from protracted drought periods 

 because of the lack of sufficient soil-water. The scorching of 

 the leaves occurs most commonly when dry hot winds cause 

 excessive transpiration, as in the sun-scorch of seedlings (see 

 page 9). The severity of such injuries depends largely on 

 the natural requirements of the species and how well adapted a 

 given tree is to its surroundings. Certain types of soils, the 

 nature and direction of the slope and other such factors may 

 predispose certain trees to winter-drying, late frost-injury or 

 sun-scorch when other kinds of trees would not suffer. 



The injuries due to temperature and its important accompany- 

 ing effect on transpiration are peculiar to extremes in weather 

 conditions and usually seen after them. The causes of such 

 injuries must be considered with the symptoms for a correct 

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