Rest-breaking in pome fruit

Technical advisers from the main production regions share their approaches to rest-breaking in apples and pears. By Anna Mouton.

“I think there are very few places in the Western Cape where you don’t need to spray rest-breaking agents,” says Christo Strydom, general manager of Wolfpack. “We recommend rest-breaking as a standard practice.”

Rest-breaking is also routine on all pome fruit in the Langkloof. “We are a medium-chill area,” says Johan Kotze, Head: Dutoit Agri Eastern Cape. “We seldom accumulate more than 1 000 Richardson chill units, and our chill accumulation often only starts in mid- or late May.”

Even orchards in the Koue Bokkeveld, which experiences relatively good winter chill, benefit from rest-breaking programmes, whereas in the EGVV, rest-breaking in apples and pears is non-negotiable.

“The Bokkeveld is becoming more like Grabouw used to be,” says plant breeder and horticulturist Dr Nigel Cook. “And Grabouw is becoming warmer.”

Graeme Krige, general manager of Fruitmax Agri, agrees that rest-breaking practices have evolved. “We are spraying a little more aggressively than 20 years ago,” he says. “And we have more sites where we no longer spray a single application of a single product.”

With the transformation of rest-breaking sprays into rest-breaking programmes, what factors should growers consider to optimise their chances for success?

Reasons for rest-breaking

“There are three reasons to spray rest-breaking agents,” says Willie Kotze, Technical Manager: Dutoit Innovation Hub. “The first is to stimulate breaks and spur development in young trees. The other two are to achieve uniform flowering and facilitate renewal in full-bearing orchards.”

Protracted bloom leads to a wide range of fruit sizes on the tree, complicating management practices such as russet control and chemical thinning, and yielding mixed maturities at harvest.

Willie Kotze points out that some cultivars, such as Cripps Pink and Cripps Red, tend to flower from bottom to top, especially when the bearing wood varies in age. “The total flowering period in Ceres can be as long as a month,” he says. “We apply rest-breaking to homogenise flowering within the tree.”

Although addressing protracted bloom is important, all the technical advisers emphasised the need to stimulate vegetative bud break, which is crucial for medium- and long-term development of bearing positions.

In warmer regions, such as the EGVV and the Langkloof, vegetative buds often fail to break naturally, resulting in blind wood, especially toward the top of the tree. On the other hand, vegetative buds usually break naturally in Ceres, but they break late, so they don’t finish their extension growth in time to initiate flower buds for the following season.

“People mustn’t overlook the importance of rest-breaking for the next season’s harvest,” says Johan Kotze. “When your one-year-old shoots don’t develop spurs, or their spurs are weak, you end up with insufficient flower buds, which can lead to biennial bearing.”

Cultivar effects

“Insufficient winter chill is one reason why a bud may not break,” says Krige. “But the other reason is correlative inhibition, and this is strongly influenced by cultivar.”

Correlative inhibition occurs when the growth of a plant organ is inhibited by other plant organs. It is promoted by auxins and counteracted by cytokinins. The dynamics of these hormones differ between cultivars and rootstocks (roots export cytokinins).

Apical dominance is a kind of correlative inhibition in which the bud at the tip of a shoot (the apical bud) breaks first and prevents buds further down the shoot from breaking.

It is promoted by auxins exported by distal buds and counteracted by root-derived cytokinins. The combination of scion cultivar and rootstock may influence these hormone dynamics.

“I’ve felt for a long time that the rest-breaking agents mainly impact the apical dominance part of the dormancy problem,” says Cook, who has researched dormancy extensively. He speculates that rest-breaking agents disrupt auxin transport, temporarily removing correlative inhibition and placing all buds on an equal footing.

Chill requirement and correlative inhibition are different but intertwined. Therefore, the tendency to develop blind wood can differ for cultivars with similar chill requirements. For example, Granny Smith isn’t the apple with the highest chill requirement, but it is highly prone to developing blind wood.

The same holds for pears. “Oftentimes, growers won’t spray rest-breaking agents on Early Bon Chrétien, because it has a low chill requirement,” remarks Strydom. “But then too few buds break to grow shoots for new bearing units, or to form spurs.”

Primers and programmes

When planning a rest-breaking programme, growers can spray products alone or in combination, at different concentrations, and once or more than once. The first application in a programme is often called a primer.

Many factors influence a rest-breaking programme, including the goal, fruit type and cultivar, tree age, site, winter chill, and weather. There is no one-size-fits-all recipe, so this section focuses on principles rather than details, and on full-bearing orchards (young trees are covered later).

Four active ingredients are used for rest-breaking on pome fruit: hydrogen cyanamide, oil (mineral or vegetable), thidiazuron, and nitrates (potassium or ammonium nitrate). In general, nitrates are considered the mildest.

Cultivars differ in their susceptibility to phytotoxicity caused by rest-breaking agents. “For example, Forelle is highly sensitive to hydrogen cyanamide and mineral oil combinations, especially on wet soils,” says Strydom. “Cape Rose has a similar tendency.”

On average, pears have a lower chill requirement and a more acrotonic growth habit than apples, so they usually receive only one rest-breaking application. Many apple cultivars are sprayed twice.

For example, multiple applications on apples are a standard practice in the Langkloof. “Depending on the orchard, we often start with a nitrate plus oil primer spray,” says Johan Kotze, “followed by our main rest-breaking application of either hydrogen cyanamide and oil or thidiazuron and oil 10–14 days later.”

In orchards where the tops lag, the primer can be applied to the tops only. Alternatively, the primer is applied to the whole tree, a rest-breaking spray is applied to the tops only, and a separate rest-breaking spray is applied to the whole tree.

“Here in Ceres, the issue is mostly that the top and bottom of the tree flower separately, especially in Cripps Pink, Cripps Red, and Golden Delicious,” says Willie Kotze. “I will often spray the tops first. It’s a low-volume application, so quick to spray.”

Besides galvanising straggling parts of the tree, two applications are less risky than one, according to Kotze.

“With all rest-breaking agents, the more you apply, the greater the effect, but the closer you get to the tipping point where you cause phytotoxicity,” he says. “Splitting the applications seems less problematic than spraying high doses, especially in cold, wet years or under extremely hot conditions.”

Krige thinks another advantage of rest-breaking programmes is that the modes of action of the various agents can be synergistic. “But you should know the goal of your programme,” he says. “Don’t spray a primer just because your neighbour does it.”

Timing

“I think people often get caught up in trying to revise their rest-breaking programme according to that season’s chill accumulation,” says Willie Kotze. “But we don’t fully understand chill units and the answers we get from chill models.”

The difficulty, as Cook explains, is that trees enter dormancy very slowly in a warm climate, and chill only accumulates once they are fully dormant. To make matters worse, hot days while the trees are still entering dormancy reverse their progress.

“Our data show that more than half the chill units can pass before the trees are fully dormant,” he says. “In Grabouw, they only begin accumulating chill in June or July. And we have long periods in winter when it’s not cold enough for chill accumulation.”

Consequently, growers should be guided by their trees rather than by chill units when deciding what to spray and when to start. In general, the earlier they spray, the less the risk of phytotoxicity but the greater the chances of a weak or no response.

Ideally, rest-breaking should commence when the trees show signs of waking (bud swell, green tip, and white root development).

“You need to see bud movement, particularly when applying oil or oil and hydrogen cyanamide combinations,” says Strydom. “But, in my opinion, the temperatures after you spray matter more than the tree’s phenological stage.”

Temperatures above 18 °C, preferably accompanied by sunny, dry weather, increase the trees’ respiration rate and help jolt them into growth. In practice, the right conditions are often in short supply in late winter, and growers can struggle to get through all their orchards with the required high-volume applications.

“We try to handle our cultivars in three groups: pears, earlier flowering apples, and later flowering apples. But there are times when we are under pressure,” says Willie Kotze. “So, if the plant is 90% ready, the buds are swollen, and the weather looks right, we take the opportunity to spray.”

If growers aren’t able to initiate rest-breaking before trees reach green tip, they shouldn’t give up. “You can adapt your recipe,” says Krige. “For example, you could take hydrogen cyanamide out of your mix.”

Young trees

“Our strategy with young trees is to put all apple trees in a cold room for four weeks before planting,” says Johan Kotze. “After planting, we will also apply a standard rest-breaking programme. We want the maximum vegetative buds to break from top to bottom.”

In the case of pears, he plants them directly from the nursery and cuts back the leader, so they don’t receive rest-breaking sprays in their first leaf. “From the second leaf, pears are on a rest-breaking programme to force breaks on the leader and one-year-old shoots,” he says. “We want to fill in the leader with the right branches in the right places.”

Strydom says he prefers hydrogen cyanamide without mineral oil rather than a combination or only mineral oil on young pear trees. He uses low-volume sprays to reduce the risk of phytotoxicity.

In warm regions such as Elgin, getting good breaks on young apple trees is becoming harder, observes Cook. “We apply hydrogen cyanamide to the whole tree, followed two weeks later with a mix of oil and nitrates. In high-chill cultivars, such as Golden Delicious and Fuji, we add hydrogen cyanamide to the second application,” he says.

After the rest-breaking programme, he also employs other practices to encourage breaks. “We do this up to the fourth leaf, when we’re trying to build branches,” he says. “But we still have a problem with blind wood.”

This illustrates that, while rest-breaking is invaluable for pome-fruit production in warm-winter climates, it isn’t a miracle cure.

Hidden dangers

When asked whether they would ever skip rest-breaking entirely, the technical advisers have to think hard to come up with a scenario where they wouldn’t spray.

“Even if we accumulated more than 1 000 Richardson chill units, we would still proceed with a rest-breaking programme,” says Johan Kotze. “But I am very careful about spraying when soils are waterlogged, because you can do more harm than good.”

Rest-breaking agents increase the trees’ susceptibility to suffocation, so applications when trees have wet feet can lead to phytotoxicity, limb die-back, and even death.

“In some cultivars, the flowers are more sensitive to rest-breaking agents than people realise,” says Willie Kotze. “The hidden danger is that trees flower profusely after hydrogen cyanamide applications, but the flowers are damaged and don’t set. This is why I’m wary of high doses, especially on cultivars that set poorly.”

Incorrect spray calibration is a common error with rest-breaking applications. “We are used to applications in orchards with 4.0-metre or 4.5-metre row widths,” says Krige. “In high-density plantings with 3.0-metre or 3.2-metre rows, you end up spraying through the trees, and each tree is sprayed multiple times from both sides.”

At the other extreme, Strydom notes that he too often sees trees with the tops not sufficiently sprayed. “I think people misjudge the water volumes they’re delivering,” he says. “And I think there are still too many mistakes.”

The future of rest-breaking

Cook cautions that the industry must look beyond rest-breaking. “If you are a third away from your chilling requirement, you can get away with rest-breaking,” he says. “But in Grabouw, we have years where we’re trying to fix a 50%–60% deficit in chilling, and that doesn’t work.”

As a plant breeder, he advocates the wider adoption of low-chill cultivars. “The tree you see reflects the interaction between its genetics and the environment. Our environment is warm and getting warmer,” he says. “There is a threshold beyond which the genetics of existing cultivars can’t adapt.”

Apple growers, in particular, would welcome lower-chill cultivars with solid commercial potential. “Given that the climate problems we already have are predicted to worsen, I think we need to stay informed of and support breeding programmes focused on cultivars adapted to our conditions,” comments Krige. “A low chill requirement is part of that.”

Meanwhile, rest-breaking remains critically important for the South African pome-fruit industry. “We can’t farm without it,” says Willie Kotze. He adds that this is becoming true in many other parts of the world, with visiting foreign growers keenly interested in South African rest-breaking practices.

However, with hydrogen cyanamide under pressure from local activists and export markets, rest-breaking practices will need to continue evolving. “This is why it’s so important to keep researching alternatives,” says Krige. “I know Hortgro Science is doing this, and we need to endorse that effort strongly.”