WINTER TRUNK INJURY TO APPLES 



D. A, Kollas, Extension Pomologist 

 University of Connecticut 

 Storrs, Connecticut 



In the spring and summer of 1976, many orchardists became aware 

 of extensive winter injury to the trunks of apple trees. Winter cold 

 injury has not appeared on such a large scale in Southern New England 

 for many years. This article is written to review some of the cur- 

 rent knowledge about cold injury and to relate it to the winter 

 injury of 1975-76. In reviewing the literature it is apparent that 

 we still lack a good understanding of cold injury and cold hardiness. 



For many years, the standard way to study cold hardiness has 

 been to collect samples of shoot, bark, or bud tissue at various 

 times during the year, and expose it under controlled laboratory 

 conditions to freezing temperatures. The resulting damage is then 

 related to conditions that might influence cold hardiness. The 

 researcher has only limited control over conditions under which the 

 tree stood in the orchard. Every season is unique in its sunlight, 

 temperature, rainfall, wind and snow conditions. Consequently, pro- 

 gress in relating cold hardiness to any single one of these, and 

 other factors, is very slow. 



Many studies of cold injury have led researchers to conclude 

 that when low temperature causes direct injury to woody plant tissues 

 it is either because ice has formed within the tissue cells, or 

 because the tissue has dehydrated due to ice formation. Cells of 

 living tissues contain protoplasm, the stuff that carries on basic 

 life processes. A major constituent of protoplasm is water. If 

 this water freezes to ice in the protoplasm, the protoplasm is 

 destroyed and death is assured. 



Woody plants that survive New England winters are able to avoid 

 ice formation within the protoplasm as a result of a process known 

 as acclimation. The acclimation process can be initiated by the 

 shortening day length of August or September, and by temperatures 

 below about 28°F. Only nongrowing (dormant) plants can acclimate, 

 and become hardy to sub-freezing temperatures. 



Just what changes occur during acclimation that make survival 

 to freezing temperatures possible are still not known. It is known 

 that the acclimation process takes time. Exposure of the plant to 

 temperatures below about 28°F can, over a period of several days, 

 result in hardiness to temperatures of near zero °F if other factors 

 are favorable. Exposure, for a couple of weeks can result in maximum 

 cold hardiness. But all tissues in a plant do not develop hardiness 

 in the same way, or to the same degree. Dormant apple flower buds, 

 for example, are hardy to 0°F long before trunk bark develops much 

 cold resistance . 



