296 



FUNDAMENTALS OF FRUIT PRODUCTION 



The effect of snow in relation to sunscald seems to have escaped the 

 attention of writers on this subject. Sunhght striking snow is to a large 

 extent reflected and a late winter snow is bound to have no little influence 

 in intensifying the heating on the southwest side of tree trunks. If, 

 as frequently happens in late winter in northern latitudes, a snowfall dur- 

 ing the night is followed by a clear warm day and a night of considerable 

 cold the change in temperature of the southwest side must be considerable 

 and abrupt. 



Rapid freezing, especially during the first part of the temperature fall 

 has been shown by Chandler^* to cause killing at a relatively high point. 

 These are the very conditions just recorded and seem adequate to explain 

 killing by sunscald without any assumption that growth has started. 

 Artificial freezings accompanied by microscopic examination of tissues 

 made by Mix showed no difference in hardiness on either side when frozen 

 under identical conditions. Rapid freezing killed at — 20°C. while 

 slow freezing caused no injury at — 28°C. As spring advances these 

 tissues become less hardy, but equally on all sides of the trunk. The 

 conclusion seems inevitable, therefore, that it is rapid freezing after sun- 

 down that causes winter sunscald. 



Preventive Measures. — Prevention of the rapid fall is best effected by 

 keeping the day temperature down. Anything that will shade the trunk, 

 as a stake or a bundle of corn stalks, will do this well. Whitewash also, 

 because of its low heat absorption, may be used to advantage. 



Table 35. — Temperatures of Whitewashed, Tarred and Untreated Trees^' 



Table 35, arranged from data reported by Mix, shows the sharp 

 contrast in sunny side temperatures between a whitewashed and an 

 untreated tree, a difference that becomes more marked as the tempera- 

 tures go higher. The difference appears considerable enough to save 

 treated trees from sunscald in many cases. The same table suggests 

 also a reason why gas tar, occasionally applied as a borer repellant, is 

 said frequently to kill trees. The difi"erence between the temperatures 



