EVALUATION OF ALAR AND ETHREL ON THE COLD HARDINESS 

 OF 'MCINTOSH' AND 'DELICIOUS' APPLE TREES 



William W. Jenney and Bertie R. Boyce 

 Department of Plant and Soil Science 

 University of Vermont 

 Burlington, Vermont 



It is well known that the development of cold hardiness in 

 plants is primarily a function of the type or variety of plant and 

 the weather conditions, especially temperature and day length, during 



the autumn. The degree of hardiness that a plant develops, however, 

 may also be modified to a limited extent by other factors such as 

 cultural practices or use of various plant growth regulators. 



Although Alar and Ethrel are used as growth regulators in many 

 apple orchards, we have little knowledge of whether or not their use 

 influences the cold hardiness of the trees. The purpose of this 

 investigation was to determine if the hardiness of bearing-age trees 

 is either increased or decreased by their use. 



The work was carried out over 2 seasons, from June, 1975 to 

 March 1977, on 30 Mcintosh and 30 Delicious trees located at the 

 University of Vermont Horticultural Research Center. All trees were 

 on M. 7 rootstocks, approximately 25 years old, and uniformly vigor- 

 ous . 



Five Mcintosh and 5 Delicious trees were used for each of 6 

 treatments. The treatments were: (1) unsprayed controls; (2) Alar 

 at 2#/100 gal applied in early June; (3) Alar at 2#/100 gal applied 

 in early August; (4) Ethrel at 2-1/2 pts/100 gal applied about 10 

 days prior to normal harvest; (5) the June Alar application plus the 

 Ethrel application, and (6) the August Alar application plus the 

 Ethrel application. 



Terminal shoots collected at monthly intervals from August to 

 March 1975-76 and again in 1976-77 were frozen in the laboratory to 

 several different temperatures and the amount of injury occurring 

 was measured by determining the electrolyte release from the injured 

 cells. 



As expected, the Mcintosh shoots were injured less by freezing 

 than were the Delicious shoots; however, very few significant differ- 

 ences in hardiness between the treated trees and controls were found 

 with either variety. Slightly less injury occurred in both varieties 

 at mid-winter of the first year when trees had been treated either in 

 June or August with Alar. The same treatments brought about a slight 

 increase in injury when the shoots were frozen in March. Ethrel 

 appeared to have even less influence than Alar in altering the hardi- 

 ness of apple shoots. Shoots of Ethrel-treated trees had slightly 

 less injury than the controls when frozen in November, 1976, and 

 slightly more injury when frozen in March. 



