Rest-breaking alternatives for apples and pears

The search continues for chemical rest-breaking alternatives to hydrogen cyanamide for South Africa’s pome-fruit industry. By Jenny Underhill.

Rest-breaking agents stimulate and synchronise bud break in fruit crops that do not receive adequate winter chilling to overcome dormancy. Since most South African fruit-growing regions have mild winters, chemical rest-breaking is essential for pome-fruit production in most regions.

Hydrogen cyanamide and mineral oil have been banned for use as rest-breaking agents in several countries, especially in the European Union, and their use by South African growers will likely be restricted in the future.

According to Chad van Wyk, Project Manager: Rootstock and Alternative Crops Research and Development at Provar, research on alternatives must address two questions. “Firstly, if growers don’t use hydrogen cyanamide, what potential losses or risks do they face? Secondly, what is the next best alternative so that we can move forward?”

These questions inform a Hortgro-funded MSc research project currently being completed by Jan Theron. His supervisors are Van Wyk, Dr Mardé Booyse (ARC), Dr Nigel Cook (Prophyta), Dr Iwan Labuschagné (Provar), and Dr Esmé Louw (Stellenbosch University).

Theron’s project is part of a larger effort to find rest-breaking alternatives for the fruit industry, a project that was initiated by Cook and Dr Xolani Siboza in 2022.

“Siboza and Cook wrote the recipes for all the chemicals we are testing and the spraying protocols,” notes Theron. “Does it matter if you only spray once, or should you spray twice? Should you spray early or late? A lot of factors that they considered were built into my experiments.”

Treatment protocols and data collection

Theron conducted trials over two seasons at one orchard each of Golden Delicious and Cripps Red apples and Packham’s Triumph pears in the EGVV.

The study included ten treatment combinations tested in each cultivar (Table 1). Ammonium nitrate (NH₄NO₃), potassium nitrate (KNO₃), vegetable oil, and biostimulants were compared with standard hydrogen cyanamide and mineral oil treatments. The concentrations of the chemicals used in the treatments varied across the cultivars.

Table 1. The ten treatment combinations evaluated. Exact concentrations were adjusted for each cultivar.

Theron has already begun analysing the data collected in the 2023–2024 season but is still completing the final fieldwork for the 2024–2025 season. The same treatment protocols and measurements in the same orchards were replicated in the second season, but with different trees.

Data to quantify the impact of the chemical rest-breaking agents were collected in five categories: bud break, including total bud break and its progression; fruit set and yield; fruit quality and maturity; vigour or vegetative growth; and return bloom.

“By measuring these five categories, we hope to obtain a clear picture of what each treatment does to the whole tree cycle,” says Theron. “For example, research has shown that hydrogen cyanamide enhances vegetative growth, creating more bearing positions for the following year, and that’s one of the reasons why it’s so beneficial for growers.”

Preliminary results

In the 2023–24 season, all the treatments resulted in bud break above 80%, but some differences in total bud break were observed.

The results for Golden Delicious are shown in Figure 1. Total bud break was significantly more with early and late treatments containing hydrogen cyanamide than treatments without hydrogen cyanamide. Treatments with an initial hydrogen cyanamide application followed by alternatives performed better than those with only alternative treatments.

The treatment that included two hydrogen cyanamide applications and the treatment that included a single, later 1% hydrogen cyanamide application gave significantly better bud break than the treatment that included a single, later 0.5% hydrogen cyanamide application.

However, the conventional later 0.5% hydrogen cyanamide and 3% mineral oil application outperformed the alternative-only treatments. This suggests that even lighter hydrogen cyanamide and mineral oil treatments may be more effective than alternative treatments in breaking vegetative dormancy.

For Cripps Red (Figure 2), total bud break was also significantly higher with early and late treatments containing hydrogen cyanamide. Still, the later treatment with a combination of vegetable oil and biostimulants gave comparable results to some of the treatments with hydrogen cyanamide.

The treatment that included an early application of 2% hydrogen cyanamide followed by a later application of vegetable oil and KNO₃ gave significantly better bud break than the treatment with early 2% hydrogen cyanamide applications followed by vegetable oil and biostimulants, or the treatment that only included a later application of 0.5% hydrogen cyanamide and 3% mineral oil.

For Packham’s Triumph (Figure 3), the differences in the treatments were less clear-cut. Late treatments with hydrogen cyanamide and mineral oil gave significantly more bud break than early treatment with hydrogen cyanamide and mineral oil or late treatment with vegetable oil and potassium nitrate. Bud break for the other treatments lies between these extremes.

No study can control environmental conditions, and both of Theron’s study seasons were preceded by unusually cold winters, which could potentially cause his data to be less conclusive than hoped.

“Although the two seasons were not cold enough that rest-breaking treatments were unnecessary, I think the differences between treatments will be much smaller,” says Theron.

Data collection and analysis are ongoing, but Theron has some preliminary observations. “Having looked at the first and some of the second season’s data, you can see statistically significant differences between treatments in bud break, especially vegetative bud break,” he says, “but those differences don’t carry over to the other categories we’re measuring, for example, fruit set, yield and fruit quality.”

What does industry say about this research?

“Chemical rest-breaking agents play a crucial role in pome fruit production in South Africa. These agents help overcome insufficient chilling, which is essential for proper bud break and flowering in apple and pear trees. Without adequate chilling, the trees may experience delayed foliation, uneven flowering and poor fruit set, leading to reduced yield and quality.

“Our industry is highly reliant on hydrogen cyanamide for continued economic viability. With increasing pressure on the usage of this product, it remains a priority to support projects that investigate alternatives.”

Technical adviser Keith Bradley. General Manager: Fruitways Agri Services.