294 FUNDAMENTALS OF FRUIT PRODUCTION 



Moisture and Temperature Conditions in the Affected Parts. — The 



winter sunscald, however, is much more common. It is not induced by 

 simple insolation but by interacting effects of heat and cold. This is 

 quite evidently the malady described by Downing^'* in 1846 as "frozen 

 sap blight" and rather confused with pear blight by many of the early 

 American pomological writers. The description by Downing clearly 

 indicates this form, as he includes Ailanthus, Spanish chestnut and 

 catalpa among the plants affected. He attributed the trouble to sudden 

 thawing and proposed as a remedy shading the south side of the trunk 

 and whitewashing. Somewhat later he recorded that on Dec. 19, 1846, 

 a bright mild day, with snow on the ground, a naked theremometer regis- 

 tered 97°F. while one with a whitewashed bulb registered 79°F.^^ Various 

 suggestions as to the way sunscald is brought about have been made, in- 

 cluding rapid thawing, increased flow of sap followed by freezing so that 

 the bark is pushed off, breaking of the rest period in the warmed area and 

 alternate freezing and thawing. Miiller-Thurgau''^ found in March a 

 water content of 53.8 per cent, in the bark on the south side of a plum 

 tree and 48.5 per cent, on the north while the bark of a tree wrapped 

 with rushes showed moisture percentages of 51.5 and 51.3 on the south 

 and north sides respectively. He considered these figures to corroborate 

 the suggestion that a localized breaking of the rest period subjected the 

 affected areas to injury from subsequent cold. 



The most extensive investigation on this phase of winter killing is 

 that of Mix. ^3* Particular attention was given to the cambium since 

 this tissue suffers severe injury "and without injury to the cambium and 

 outermost xylem the bark would not separate from the wood." Obser- 

 vations of temperature under the bark on the northeast and southwest 

 sides of apple trunks showed no significant differences on cloudy days but 

 marked variations on bright days, demonstrating the warming effect of 

 the sun's rays. Tables 33 and 34 are selected from data reported by 

 Mix from these observations and are representative of his more extended 

 figures. The temperatures for Mar. 10 are worthy of special attention, 

 being 32°F. on the northeast side and 69°F. on the southwest side at the 

 same time. On Feb. 10, Mix observed on the southwest side of one tree 

 a fall from 59° to 27°F. between 2 o'clock and 9, the air temperature 

 dropping from 28° to 19° F. in the same time, while on the northeast side 

 the temperature fell from 25° to 19°F. The temperature of the southwest 

 side dropped 32°F. while that of the northeast side fell 6°F. On another 

 tree the temperature on the southwest side fell between 5 o'clock and 6 

 (sunset at 5:30) from —0.3° to — 14.4°C. while on the northeast side it 

 dropped from —9.4° to — 18.3°C. This, it should be emphasized, was in 

 1 hour. By morning the temperatures on both sides were frequently 

 observed to be approximately equal. The southwest side of a tree trunk 

 is evidently subject to wider fluctuations in temperature and to more 



