The efficacy of rest breaking chemicals on apples as a function of cultivar, chill Unit accumulation, time of application and temperature, under laboratory conditions

We studied the effect of rest breaking agents (RBA) on excised shoots under laboratory conditions to better understand the inconsistancies of RBA effectivity under field conditions year on year. This project is the first phase of a 5 phase project which will ultimately lead to increased efficiency of RBA in the field and concentrated on the methodology of the shoot assay. The methodology can also be used to screen new and combinations of chemicals for rest breaking properties.

One–year-old Fuji and Golden Delicious shoots +/- 35cm long were collected from two commercial orchards in the Elgin District prior to commercial rest breaking agent application. Various concentrations of rest breaking agents were applied to the shoots which were then placed in buckets containing a weak bleach solution and forced at different temperature regimes. A total of 196 treatments were screened over the 2 years period. Bud break was assessed by determining the number of shoots on which bud break occurred, the percentage bud break, the time interval until onset of bud break and the bud break period.

Shoots treated with RBA outperformed non treated samples in both years, locations, temperature regimes and cultivars. However, no RBA treatment was able to improve bud break to the extent that is was comparable to excised shoots which had received sufficient cold. The effectivity of the RBA treatments varied according to the year in which the shoots were collected, the location, cultivar and the temperature regime to which the shoots were subjected.

The methodology developed in the first phase of this project can be seen as a good proxy of the field situation. It can therefore be utilised effectively for the screening of numerous new and combinations of existing RBA under different temperature regimes, making it both a cheaper, faster and possibly less destructive option to field trials. The next phase of this project will be the verification of successful laboratory screening under field conditions before industry implementation can be considered.