10 MANUAL OF TREE DISEASES 



ever, the trees over the affected area die simultaneously and 

 there is no progressive enlargement of the area. When only 

 the tops of the plants or the lower and older leaves die, the 

 plants may recover, though they remain in a weakened con- 

 dition and easily succumb to later sun-scorch conditions. 

 The affected leaves become chlorotic, turn yellowish and then 

 become dried and dark brown. The entire needles are not 

 always involved and it is very common to find only the tips 

 brown and dead. An accompanying symptom of sun-scorch 

 is found in the roots, which die previous to the appearance of 

 the symptoms in the tops. At the time the leaves are turn- 

 ing yellow, the root system already shows considerable injury. 



Caiise. 



Sun-scorch results when the amount of water in the soil is 

 not sufficient to replace rapidly the amount of water lost by 

 excessive transpiration from the leaves. The extreme combina- 

 tion of conditions bringing about sun-scorch are sandy soil 

 and continuous hot, dry winds, with little dew at night, and 

 little rainfall. Under these environmental conditions the 

 crowding of large numbers of trees into a small space and the 

 lack of any shade during a part or the whole of the day predis- 

 pose the plants to sun-scorch injury. Certain species of trees, 

 such as Norway spruce and Douglas fir, are more susceptible 

 than others, especially if they are growing in soil which is 

 rapidly drained, as is the case in the center of arched beds. 

 The injury develops rapidly and conclusive evidence as to the 

 causal conditions should be obtainable by examining the amount 

 of water present in the soil. The soil around the roots of the 

 affected plants will usually be found much drier than that 

 around adjacent healthy plants. There may be a sufficient 

 total water supply above and below the rooting region, but 

 if this supply is not readily available, the plants will suffer 

 when transpiration is excessive. 



