Rest-breaking in stone fruit

Effective strategies can boost orchard profitability, but rest-breaking isn’t a cure-all for poor adaptability. By Anna Mouton.

“There are very few orchards in our group that don’t receive rest-breaking applications,” says Pierre Rossouw, technical director at Stems Fruit. “I think the increase in rest-breaking is related to climate change.”

Winter chill in most South African production regions no longer satisfies the requirements of many stone-fruit types and cultivars. Without rest-breaking, the majority of growers would be limited to a narrow range of low-chill cultivars.

“Our aim as growers is to produce large fruit because large fruit give you more cartons per hectare, which give you profitability,” says stone-fruit grower and technical adviser Petru du Plessis. “Fruit size depends on many factors, including chill units.

“We don’t control chill, but we can apply good rest-breaking. That gives our trees bigger leaf areas and greater photosynthetic capacity, which means a bigger factory and better fruit size.”

Use cases for rest-breaking

Many growers apply rest-breaking to condense flowering. Protracted bloom results in a spread of fruit sizes on the tree, complicating actions such as thinning and ending in mixed maturities at harvest. This raises production and picking costs. Worse still, protracted bloom can lead to a mix of fruit maturities in a carton.

By stimulating vegetative bud break, rest-breaking encourages even leaf, shoot, and spur development, which promote fruit size and quality in the short-term, and fertility in the medium- to long-term. In young trees, rest-breaking helps them fill their space and develop bearing positions.

“On mature trees, I use rest-breaking to synchronise pollination,” says Andrew Hacking, technical adviser at Ad Lucem Agricultural Services.

When the flowering of the cross-pollinator and the main cultivar don’t overlap, he will spray when the slower cultivar shows signs of waking up. “The intention is to hold the faster cultivar back a bit and pull the slower one forward,” he explains.

Alternatively, Du Plessis treats the lagging cultivar with handheld sprayers about five days before applying the main rest-breaking treatment to the entire block.

Rest-breaking can also advance flowering, bringing the harvest date of low-chill cultivars forward. “You need the right conditions,” cautions Du Plessis, “but being in the market a week earlier oftentimes translates into R10 or R20 higher income per carton.”

In large blocks, growers can split the area and apply rest-breaking to one section about four days before the other section. This mitigates the risk of poor fruit set due to bad weather during flowering.

Additionally, blocks where rest-breaking was applied at different times will ripen at different times. In a big block, this facilitates harvesting the entire crop at optimal maturity.

Fruit types and cultivars

Chill requirement varies by fruit type and cultivar. Peaches and nectarines generally require slightly less winter chill than Japanese plums, but there are low-, medium-, and high-chill cultivars of each. Low-chill cultivars usually flower earlier than high-chill cultivars (early or late flowering is not related to early or late harvest).

Rest-breaking enables growers to access more cultivars for their region. However, Rossouw warns against pushing the envelope too far.

“Consultants mustn’t recommend cultivars that will need every trick just to obtain a harvest,” he says. “For some cultivars, you need rest-breaking even in the Koue Bokkeveld. If you plant those in Bonnievale, for example, you won’t succeed no matter what you do.”

Although all stone-fruit types can potentially benefit from rest-breaking, interspecific hybrids such as pluots and plumcots are very sensitive to combinations of oil and hydrogen cyanamide. They are also prone to bud extinction.

When asked about rootstocks, all three technical advisers mentioned SAPO 778 as having a particularly high chill requirement.

“If somebody’s chosen a high-chill rootstock and they’re in a low-chill area with a low-chill scion, they’re in trouble,” says Hacking, “and rest-breaking is not going to help them.”

Registered products

Four active ingredients are available for rest-breaking in stone fruit in South Africa: hydrogen cyanamide, oil (mineral and vegetable), thidiazuron, and nitrates (potassium and ammonium nitrate). The product registrations differ in their details, so growers must consult their technical advisers and the product labels.

Products containing hydrogen cyanamide are available for specific cultivars of bearing apricots and bearing and non-bearing plums. Hydrogen cyanamide is not registered for peaches and nectarines.

“Some cultivars don’t like hydrogen cyanamide, and neither do pluots,” says Du Plessis. “In young trees, I recommend low hydrogen cyanamide concentrations, or none, because young trees are often poorly hardened off and prone to scorching.”

Hacking, in contrast, prefers hydrogen cyanamide for young trees. “I haven’t had any trouble with it,” he says, “but I only spray the tree once it shows signs of development.”

Mineral oil is registered alone or in combination with hydrogen cyanamide for apricots and plums. Vegetable oil is registered on apricots, peaches, nectarines, and plums.

“We are wary of oils for early-season cultivars,” says Rossouw. “You need to know your cultivars. For some plums, you should avoid oil completely, and for others, you can’t rest-break without them.”

Thidiazuron is only registered for non-bearing plums.

When to spray

“You can’t have a fixed date when you spray,” says Hacking. “Every year is different, and we must be very careful to adjust according to the tree’s physiological development. Once the tree is moving, we help it.”

Hacking and Du Plessis both believe spraying too early is worse than spraying too late. “Applying rest-breaking when trees don’t want to be woken up is risky,” says Du Plessis. “If in doubt, rather wait three or four more days.”

Spraying too early is easy in years with warm autumns and winters. In these scenarios, growers should plan to spray 5–6 days later than usual.

Rossouw thinks it should be safe to spray at green tip in cold regions such as Ceres. But in warmer regions such as, for example, Bonnievale, the grower will have to make a call based on the cultivar, the site, and that season’s chill.

“In low-chill areas, you need to accumulate at least 60% of a cultivar’s chill requirement for rest-breaking to have an effect,” says Rossouw. “In high-chill areas, you need up to 80%.”

Should the trees not accumulate sufficient chill, rest-breaking can be delayed for a week to allow the trees more time for chill accumulation, and the treatments can be applied at a lower dose to avoid scorching emerging flowers and leaves.

Growers who miss the window for rest-breaking, for example, due to bad weather, can still apply nitrates to boost their trees, but they should steer clear of stronger compounds such as hydrogen cyanamide.

Good and bad weather

Rest-breaking works best when accompanied by warm weather. But, as Hacking points out, warm is relative. “In August, if the sun is shining and it’s 14 °C or 15 °C, that’s warm, even though at night it might be well below 10 °C,” he says.

He tries to spray during sunny weather. Nonetheless, he considers the trees’ development to be more important than heat. If the trees show signs of emerging from dormancy, they will respond to rest-breaking. However, growers should never apply rest-breaking under very wet conditions.

“If growers want to spray after rain, I always tell them to dig a couple of holes in the orchard the day before, and check that there’s no water in them tomorrow morning,” says Hacking. “If there is water in the holes, don’t touch the rest-breaking.”

As a plum grower himself, Du Plessis is acutely aware of the interaction between rest-breaking and pollination.

“In our area, in the Boland, we can still get rain in September, and if your rest-breaking is too compact and the trees flower during a cold front, you can have reduced set,” he says. This is one reason why, for plums, blooming for ten days is preferable to blooming for 5–6 days.

On the other hand, protracted bloom, particularly when flowers and leaves emerge together, is less attractive to bees than orchards with masses of uniform flowers.

Young trees

Opinions differ on whether to apply rest-breaking agents to young stone-fruit trees. “I think rest-breaking on young trees is more relevant to apricots and plums than to peaches and nectarines,” says Rossouw. “It also depends on your training system in the first and second leaf.”

In general, young trees often grow for longer during a season, so their shoots are less well hardened off and more vulnerable to phytotoxicity, especially from hydrogen cyanamide. For this reason, Rossouw tends to start rest-breaking from the third leaf.

Du Plessis starts from the second leaf but applies rest-breaking a week to ten days later to young trees than to older trees. He has had good results with thidiazuron in young plums.

“I don’t like spraying rest-breaking on plum trees that have just been planted,” he says, elaborating that rest-breaking forces the above-ground growth while the trees have reduced root volumes after lifting at the nursery.

Hacking is comfortable spraying rest-breaking after planting in trees that haven’t been headed back. “I prune peaches and nectarines back hard to make them branch,” he says. “For plums, if a grower plants a whip and doesn’t want to cut it back, you need breaks to fill the wires. Unless you push it, the trees often do nothing after planting.”

Regardless of when they start rest-breaking, the advisers agree that care is necessary to prevent damage to young trees. Depending on the fruit type and cultivar, hydrogen cyanamide, or hydrogen cyanamide combined with oil, can be phytotoxic, particularly to pluots.

Potential pitfalls

“The trees’ health is very important,” says Rossouw. “If trees are under stress, you are going to hurt them with rest-breaking sprays, and some will die. There is no doubt about it.”

In orchards with diseases such as cankers or gummosis, nitrates are a safer option than hydrogen cyanamide or oil. Among the environmental stressors, wet conditions are the most dangerous, as mentioned above, but other factors also merit consideration.

“Your postharvest period is critical,” says Rossouw. “If your trees are under stress after harvest, it doesn’t matter how much rest-breaking you apply for the following season. All the buds won’t break, and some will become extinct.”

He advises growers to pay attention to postharvest fertilisation and irrigation if a heavy crop is followed by a dry autumn. Furthermore, if a winter is relatively dry and growers fail to irrigate in spring, they can expect smaller fruit.

When trees are under stress, their reserves often suffer, and they rely on reserves for the first 2–3 weeks after flowering. If they run low during this period, they will set fewer fruit, and fruit size and quality at harvest will be poorer. Trees that exhaust their reserves may die.

“If your trees’ reserve status is low, rather don’t apply rest-breaking, or reduce the concentration by 25% or 50%,” suggests Du Plessis.

Coverage is critical

“When you’re spraying a tree without any foliage, you need a bigger droplet to give you coverage, and coverage is critical,” says Hacking. He recommends a droplet size of at least 250 microns.

“Many people feel you need 100% of tree-row volume,” he adds. “I get very good results with 70%, but then I make sure the droplet size is right.”

In some regions with low chill, trees rarely lose their leaves naturally, hindering good spray coverage. “In those cases, you need to strip or spray off the leaves,” says Rossouw, “especially when the orchard is under a net. In the Boland, a net stops the wind from blowing the leaves off.”

As our production regions continue to warm, and autumns become shorter, rest-breaking is doubtless going to assume the same importance in stone as in pome fruit.

“Rest-breaking isn’t only about preventing delayed foliation,” notes Du Plessis. “You can use rest-breaking tools to manage your risks and grow fruit more successfully.”

The key, emphasises Rossouw, is that growers must identify their goals and seek good advice. “Growers have a responsibility to ask questions and do their homework,” he says. “And there are enough experts that can and want to help them.”