Using the shoot assay to screen combinations and sequential application of existing and possible new rest breaking agents in apples

The use of rest breaking agents (RBAs) is essential for successful budburst in areas with insufficient winter chill. Dormex® and mineral oil is currently the South African apple production industry standard for rest breaking. Dormex® is becoming controversial and under continuous scrutiny for its hazardous effects on man and environment. The first objective of the study was to investigate the use of growth enhancing compounds to improve the vigour of apple shoots after treatment with Dormex® and oil. Secondly, we aimed to determine the rest breaking capabilities of nitric oxide (NO) (produced chemically and enzymatically), in the presence of oil, as a safer more environmental friendly alternative to Dormex®.

We used the laboratory shoot assay (developed in 2014) to test the efficacy of a range of vigour enhancing compounds (hormones and N-containing) through visual assessment (scoring into 4 categories) and dry mass determination after exposing treated shoots to standard forcing conditions. Similar forcing conditions were used to quantify the budbreak ability of alternative RBAs that produce NO by measuring the % budbreak (%BB), onset and duration of budbreak.

Compared to the industry standard (Dormex® and oil) the vigour enhancing compounds tested all failed to show improved growth after 28 days post treatment. In general, all results were equal to that of Dormex® and oil. Promalin® and Maxcel® showed some potential as they always came out as the top performers, especially in 2015, and could be considered for further testing and/or field trails. No conclusive evidence could be found for improved %BB using vigour enhancers. All the NO producing rest breaking alternatives that were tested proved to be effective in delivering a %BB equal or higher to that of Dormex® and oil. However, they all showed a prolonged onset to budburst and an extended time period to completion. KNO₃ was the most promising with positive results at the lowest concentration and NaNO₂ the least effective with some contradicting results and lethal activity at concentrations higher than 69g/L (1M). The use of combinations of vigour enhancing compounds and nitric oxide producing alternatives did not deliver favourable results as all the cocktails tested lower for the rest breaking and vigour assessments.

The vigour enhancers, both hormonal and chemical, did not prove to increase the vigour of the treated shoots. Our results did not support the notion that Dormex® and oil treatments have a lower vigour compared to the fully chilled samples. Nitric oxide acts as a RBA in apple buds and has a rest breaking capability equal to that of Dormex® and oil but is slow acting. The use of combined treatment should be tested on intact plants before disregarded as an ineffective RBA.