52 MANUAL OP TREE DISEASES 



Sun-Scald 



Caused by high temperatxires and low humidity 



The actual scorching of the bark of certain susceptible trees 

 occurs under circumstances in which previously shaded smooth- 

 barked limbs are suddenly exposed to the full insolation of the 

 afternoon sun. Severe pruning or the removal of neighboring 

 trees make possible such injury. Beech, spruce and pines are 

 subject to sun-scald. This type of injury is due to the direct 

 wilting and drying-out of the bark-tissues caused by excessive 

 heat and the action of the wind. Small twigs may be killed 

 in the same way, especially at times when the leaves sun- 

 scorch (see page 22). Although this is the type of injury 

 to which the term sun-scald should be restricted, it is more 

 probable that the low humidity of the air and the drying action 

 of the wind are more closely connected with the injury than the 

 actual degree of temperature reached by the sun's action. 



References 



Hartig, R. Bark scorching, sun-cracks and deficiency of light. In 

 Text^book of the diseases of trees, pp. 294r-299, fig. 159. 1894. 



Stone, G. E. Sun scald. In Shade trees, characteristics, adapta- 

 tion, diseases and care. Massachusetts Agr. Exp. Sta. Bui. 170 : 

 207-208. 1916. 



Lichen-Injury 

 Caused by species of hchens 



No definite work has been undertaken to determine the amount 

 of damage caused by lichens. Opinions of writers vary, but 

 many thmk that trees are injiu-ed by the severe infesta- 

 tions that sometimes occur, especially in places where the air 

 is continuously humid. Under such atmospheric conditions 

 and where the nature of the soil causes slow growth, the bark 

 of trees may be covered with lichens of different kinds. It is 

 thought that more rapidly growing trees, in good soil, form their 



