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Addressing low winter chill in apples

Technology & Data

How can dormancy breaking sprays be used to assist with orchard management during mild winters? Camilla Humphries crunches the numbers.

During milder winters, dormancy-breaking sprays are used to help with the metabolism of carbohydrates when adequate winter chill (< 600 Richardson Chill Units for apples) has not been met. Inadequate winter chill leads to delayed and prolonged uneven flowering. As green tip date is variable between regions from season to season and difficult to determine, timing of the dormancy breaking sprays were based on accumulated RCU calculated from 1 July.

This trial, which was conducted at Launching Place in the Yarra Valley, July 2018, is an extension of the work previously done in Queensland, Tasmania and Western Australia, to gain more of an understanding on winter chill accumulation in the southern Victorian apple growing region.

Previous work by Heidi Parkes, of Department of Agriculture and Fisheries (DAF), Queensland, Susie Murphy-White, of Pomewest, WA, and Sally Bound, of University of Tasmania (UTAS) has examined the effects of dormancy-breaking sprays: Waiken™, Dormex® and Erger® on green tip dates, flowering window and harvest timing in Gala apples. In these trials Dormex® showed the greatest advancement on flowering and harvest in Western Australia and Queensland when compared with other dormancy breaking products. However, the active constituent of Dormex® is hydrogen cyanimide, which has known risks for plant and human health, and therefore alternative products are needed and were tested in this trial.

low winter chill camilla humphries treatment list

Figure 1: Treatment list.

The aim of this trial was to compact the flowering period of Scifresh (marketed as Jazz™), and thereby bring forward the harvest date. The trial also investigated how these changes impacted on orchard management and fruit quality.

Two commercially available dormancy breaker products were used, Erger® and Waiken™.

The trial was able to effectively demonstrate the relationship between chill unit accumulation and relative green tip dates. The early Erger® application was the only treatment that had a significant effect.

low winter chill apples camilla humphries

Figure 2: Photo taken 1 October: Erger® treated (top) at 80 per cent bloom and Control (bottom) at 40 per cent bloom.

Flowering impacts

The earlier Erger® spray advanced full bloom date; flowering starting five days early with full bloom six days earlier. Flowering was also more uniform throughout the tree, whilst the later Erger® spray applied delayed full bloom.

Fruitlet sizing

Fruitlet size was advanced and more even in the earlier Erger® treatment indicating a more advanced fruit set compared to the untreated control.

Fruit quality

Dormancy-breaking sprays’ effect on fruit quality remains unclear, however the high starch index measured for the earlier Erger® treated fruit indicate that the fruit were more mature than the fruit not treated with a dormancy-breaking spray.

low winter chill camilla humphries flowering early application

Figure 3: Percentage flowering for the early application.

Implications for orchard management and harvest timing

The earlier Erger® treatment brought forward the harvest date by four days and was more condensed finishing within a 13-day period compared to the control which was over a 17-day harvest window. The other treatments had no effect over the control (Figure 4).

low winter chill camilla humphries results

Figure 4: Results summary.

Key Findings

Advantages for earlier Erger® applied 40 days before green tip date:

  • Flowering was advanced and more compacted
  • Fruit set was earlier and the fruitlets were more even in size. Secondary thinning sprays were consequently more effective
  • Harvest date was advanced and more compact. This allowed harvest to be completed within the ideal maturity window. This is likely to improve fruit quality and storability

Final comments

The trial highlighted the importance of timing of dormancy-breaker sprays. Every season is different and will impact on bud burst and flowering dates. Growers are encouraged to monitor and record:

  • Localised chilling data. Use the Chill Calculator website to access and accurate local winter chill data
  • Green-tip (budburst) dates

Collecting this data over time will provide more reliable guidance as to the need for dormancy-breaker sprays and then the best timing of application for optimal effect.



A special thank you to Heidi Parkes, Senior Horticulturalist from the Department of Agriculture and Fisheries, Queensland who assisted with providing the Richardson Chill Unit Model adapted from DAF and for sharing her research findings to assist with the methodology and background information for the trial. Thank you to David and Sue Finger for agreeing to host the trial at their orchard and for the help from his workers who conducted the spray application and assisted with fruit monitoring. Thank you to Valagro for providing the trial product Erger® and to Susie Murphy-White Project Manager for Pomewest, Western Australia for her advice and guidance. Finally, thank you to Paul Selleck, Senior Pome Fruit Agronomist at E.E. Muir and Sons for his expertise in apple nutrition and for assisting with field data collection.

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