208 MASS. EXPERIMENT STATION BULLETIN 170. 



being more commonly on the southwestern side, where the maximum 

 temperature was usually found. 



Sun scald does not usually involve the whole trunk of large trees, but 

 in many cases, particularly small maples, the whole tree will suffer. A 

 few years ago, in a comparatively short distance on one street, 16 maples 

 had died from sun scald, and at one time our wild cornel {Cornus circinata) 

 suffered severely from this trouble, manj'^ of them never recovering. 



Quite often young rock maples will show only small spots affected by 

 sun scald, proving that the injury may be only local, as in the case of the 

 apple, on which tree sun scald often takes the form of collar rot. Sun 

 scald on apples is often confined to the shaded branches, and sometimes 

 occurs on severely pruned or dehorned trees. 



In some cases sun scald will be found on tree roots 

 and root buttresses exposed by regrading. Instances of 

 this class of injury have been noted, particularly in the 

 case of hickories. Any regrading necessitating the ex- 

 posure of roots should be done in the spring rather than 

 in the fall. Piling soil too high around the base of young 

 apple trees produces injury, and frequently results in 

 girdling the trunk and the death of the tree. 



Most cases of sun scald "are followed by an outbreak 

 of Nectria cinnabarina, as is often the case with winter- 

 killing. The treatment of sun-scalded areas should 

 consist in scraping the wood, after removing the bark, 

 and treating with some such antiseptic or preservative 

 material as creosote and coal tar, or thick paint. 



Fig. 81. — Fungi 

 followingaltacks 

 of borers on rock 

 maples, resulting 

 from extreme 

 drought. 



Drought. 



The unprecedently long period of drought of the past 

 five or six years has been an unusually severe drain on 

 vegetation in general. While the rainfall records for 

 this period show quite a marked falling off from 

 normal, it should be borne in mind that rainfall is only one factor in 

 producing drought, and the amount of rainfall seldom gives a correct 

 idea of the severity of drought. So far as crops are concerned, the amount 

 of water contained in the soil is a most essential factor. This is determined 

 not only by rainfall but by the amount of water withdrawn from the soil 

 by surface evaporation and the transpiration of plants. Enormous quan- 

 tities of water are removed from the soil by these processes, which are 

 much influenced by sunshine and wind. The amount of water transpired 

 by the foliage of trees varies greatly from day to day. When the meteor- 

 ological conditions are favorable for this function, as they usually are 

 during hot, dry seasons, enormous quantities of water are taken from the 

 soil into the air; consequently the soil may contain much less water than 

 rainfall records would indicate. 



