WINTER INJURY 293 



merited fluid and the injury is called "sour sap." Later the bark may 

 loosen and fall away leaving an exposed area of dead sap-wood. Many 

 trees pruned to an open center are affected at the crotch or even high on 

 the south side of those scaffold limbs that lean to the north. In this 

 last position the sun's rays are received nearly at right angles and the 

 injury there is in many cases very severe. 



The chief importance of this injury lies in its ultimate effects rather 

 than in its immediate results. It leads at once, obviously, to partial 

 obstruction of conduction of nutrient and food materials, but of greater 

 moment is the exposure to fungi and borers and the resultant mechanical 

 weakening of the tree. 



Distinguished from Summer Sunscald and Injuries Associated with 

 Immaturity. — Distinction between this type and winter killing associated 

 with immaturity on the one hand and between this type and summer 

 sunscald on the other is sometimes difficult. In fact some writers have 

 denied the existence of sunscald and some have maintained that summer 

 heat never kills bark. Evidence showing that bark is sometimes killed 

 by high temperatures is easily gathered. Fisher^* quotes Vonhausen as 

 finding, between the sapwood and bark, a temperature of 120°F. when the 

 air temperature was 91°F., while in Bavaria, Hartig observed a tempera- 

 ture of 131°F. between the bark and sapwood of some isolated 80-year 

 old spruce trees. This is a lethal temperature for leaves and herbaceous 

 shoots and is presumably so for cambium cells. In forests when an open- 

 ing is made, the standing trees on the north side of the clearing in many 

 cases show the sunscald high on the south side of their trunks. Young 

 apple trees set late in the spring in sandy soil and headed back so they 

 had little protecting top, have been observed even in New Hampshire, 

 to show severe sunscald by midsummer. 



Caution should be observed, however, in attributing all injuries on the 

 southwest side of the tree to late winter sunscald. Balmer^'- describing 

 the effects of a November freeze in Washington mentions that trees with 

 high trunks, leaning from the afternoon sun, suffered notably. In several 

 cases the bark on the southwest side of the trunks split open. Investiga- 

 tors seem to have overlooked the possible effects of radiation in this 

 connection. It is shown under Frost Injury that the temperature 

 near the soil on a frosty night may be 10° or more lower than that recorded 

 by a sheltered thermometer near by. An October temperature of 20°F. 

 is not uncommon; with suitable radiation conditions the ternperature 

 near the soil, if 10° lower, would be 10°F., low enough to cause consider- 

 able injury to immature tissues. Since somewhat lower temperatures 

 occur over sod under these conditions than over cultivated ground the 

 occurrence of "sunscald" in sod orchards need not be surprising. Injury 

 of this kind is obviously associated with immaturity. Therefore it is not 

 safe to consider sunscald altogether a late winter injury. 



