Dr Xolani Siboza of Hortgro has been leading a research project on rest-breaking agents in apple trees. Dian Craven is the master’s student involved in this project, and he is co-supervised by Prof Karen Theron, Hortgro Chair in Applied Preharvest Deciduous Fruit Research at Stellenbosch University. The project is funded by Hortgro. By Anna Mouton
Siboza is seconded to the Department of Horticultural Science at Stellenbosch University, where we caught up with him to hear about their preliminary results.
The cyanamide conundrum
“In South Africa, growers can’t farm without hydrogen cyanamide,” states Siboza. Cyanamide is the active ingredient in several products that are registered for promoting bud-break and foliation in deciduous fruit. These so-called rest-breaking agents help growers to overcome the effects of insufficient winter chill on their trees. “Hydrogen cyanamide is a very successful rest-breaking agent,” explains Siboza. “The problem that we’re facing is that hydrogen cyanamide has been banned in many countries.”Read More
Cyanamide has lost approval for use as a plant growth regulator in the European Union and the United Kingdom. It is toxic to humans and other animals. But, interestingly, a cyanamide salt is used to treat alcoholism, as people who take it cannot consume alcohol without feeling seriously ill.
Cyanamide works by increasing oxidative stress, inhibiting certain enzymes, and reducing respiration in the plant. Oxidative stress occurs when reactive oxygen molecules overwhelm the antioxidant capacity of tissues, leading to cell damage. Cyanamide can have phytotoxic effects when more is applied than is called for in a specific season.
“If you miss the phenological window, it becomes dangerous to the tree to spray.”
Oils, especially mineral oils, are also used as rest-breaking agents, often in combination with cyanamide. The oils reduce respiration by limiting gas exchange. The aim of this project is to test potential alternative rest-breaking agents, including cytokinins, gibberellins, potassium nitrate, sodium nitrite, and mineral oil, in the hope that these agents can either reduce the concentration of cyanamide required, or replace it altogether.
Some of the products tested were identified as potential rest-breaking agents by Dr Esmé Louw of the Department of Horticultural Science at Stellenbosch University during an earlier laboratory-based study. Louw and her team assessed bud-break on excised shoots under controlled temperatures.
Testing the alternatives
Trials have been conducted in a commercial Fuji orchard, in Vyeboom, and a commercial Rosy Glow orchard, in Elgin, over more than one season. Both areas experience insufficient winter chill for apple trees to complete a normal dormancy cycle.
Table 1 summarises the rest-breaking treatments that were assessed in 2017 and 2019. Not all treatments were applied in all seasons. The researchers tested each treatment on several trees. On each tree, they tagged branches that had both oneand two-year-old shoots, and counted the dormant buds on those branches.
Siboza describes some of the challenges they faced when applying the treatments: “We need at least three days of no rain after spraying. But the buds don’t say, there’s rain, I’m not going to sprout, because they need to spray me now. If you miss the phenological window, it becomes dangerous to the tree to spray.”
After spraying the rest-breaking treatments, the researchers monitored the trees weekly, and recorded bud-break of reproductive and vegetative buds separately. At the end of the season, they also collected fruit yield and maturity data.
Lastly, they gathered data on the return bloom in the following spring.
Treatment performance in two seasons
In 2017, total bud-break in Fuji was similar for all treatments, except 3% cyanamide, which induced less bud-break, both initially and after three weeks. The results showed that 0.5% cyanamide in combination with other rest-breaking agents gave better results than 3% cyanamide on its own, and that treatments without cyanamide were equally effective.
The poor total bud-break with the 3%-cyanamide treatment was due to reduced reproductive bud-break. Vegetative bud-break percentage was similar for all treatments. Cyanamide had a phytotoxic ffect on reproductive buds, according to Siboza. He explains that over-application of cyanamide may reduce yields if reproductive buds are damaged, but with more return bloom the following season.
Yields were reduced and fruit size was increased in the 3%-cyanamide treatment relative to the other treatments.
In 2017, total bud-break in Rosy Glow was similar across treatments – there was no high-concentration cyanamide treatment in 2017. Fruit size was also similar for the various treatments, except for the 1% cyanamide + 4% mineral oil treatment, which showed significantly larger fruit, but not an increased total yield.
In 2019, total bud-break in Fuji was similar across treatments, with no significant differences in final bud-break percentages, or in fruit yield or quality. There were also no significant differences in final bud-break percentages and fruit yield for Rosy Glow.
Testing the timing
In 2019, the effect of treatment application at different intervals — five, six, and seven weeks before full-bloom — was also evaluated. The treatments are listed in table 2.
The experiments were done on Fuji. “We saw that with 3% hydrogen cyanamide, it doesn’t matter whether you apply it five, six, or seven weeks before full bloom, you break the dormancy effectively,” says Siboza. However, this treatment had the lowest final reproductive bud-break percentage.
For the other treatments, application at seven weeks before full-bloom stimulated significantly less total bud-break than later applications, except for 0.5% cyanamide + 3% mineral oil + 100 mg/ℓ [6-benzyladenine + gibberellins], which had the same effect whether applied at seven or at six weeks prior to full-bloom.
Total percentage bud-break also increased when applications were done five weeks before full-bloom, compared to six weeks before full bloom, for treatments with 0.5% cyanamide + 3% mineral oil, and 0.5% cyanamide + 3% mineral oil + 150 mg/ℓ [6-benzyladenine + gibberellins].
Conditions immediately after application can also have a significant effect on bud-break performance.
In general, the researchers conclude that the application of rest-breaking agents five weeks before full-bloom resulted in more reproductive bud-break than applications six or seven weeks before full-bloom. However, more research is needed to confirm these findings.
Conditions immediately after application can also have a significant effect on budbreak performance. If cold and wet weather is forecast for the most optimal timing of restbreaking treatment, it might be better to treat trees earlier. Treatment with strong products closer to bud-break could cause phytotoxicity.
So far, it appears that 0.5% cyanamide + 3% mineral oil + 100 mg/ℓ [6-benzyladenine + gibberellins] is the most consistent alternative to 3% cyanamide for Fuji. The better option for Rosy Glow seems to be 1% cyanamide + 4% mineral oil + 100 mg/ℓ [6-benzyladenine + gibberellins]. But more data is required to establish whether adding 6-benzyladenine and gibberellins confers a real benefit over using only a combination of cyanamide and oil.
“It would be good to use treatments without hydrogen cyanamide,” concludes Siboza, “but we can at least reduce the concentration of hydrogen cyanamide.” Fruit growers are encouraged to consult a technical adviser each year when deciding on their rest-breaking programme for the upcoming season.