Fruit size

The principles and practices affecting pome- and stone-fruit development throughout the year. By Anna Mouton.

“There will never be an oversupply of big stone fruit. All our markets want big fruit,” says Charl Stander, technical adviser at Freshness First. “Big fruit sell themselves and earn better prices than small fruit.”

Although fruit size is less critical for apples and pears, it still matters. Packham’s Triumph aside, growers rarely complain about cultivars bearing excessively large fruit. Additionally, practices that boost fruit size tend to promote uniformity and quality.

“The principles are the same whether you’re discussing size in stone or pome fruit,” says Prof. Karen Theron, emeritus professor in the Department of Horticultural Science at Stellenbosch University.

According to Theron, good fruit size starts with high-quality flowers. “But having good flowers isn’t enough,” she notes. “Other things can still go wrong after flowering.”

Begin with quality buds

Shoot meristems (undifferentiated cells) undergo intracellular processes (induction) followed by tissue-level changes (initiation) to transition from vegetative to reproductive development. Reproductive meristems differentiate (give rise to primordial flowers) inside reproductive (stone fruit) or mixed buds (pome fruit).

“The timing of initiation influences flower quality at bloom,” says Theron. “Remember that flower development for next season starts while the current season’s crop is still on the tree.”

In apples, flower induction occurs about six weeks after full bloom, and initiation follows at the start of January. High irradiation levels and mature leaves promote flower induction. It is suppressed by high crop load and strong shoot growth (seeds and shoots increase gibberellin levels).

“When you overcrop in one season, you have smaller fruit in that season,” says Theron, “and you can have fewer and weaker flowers in the next season because the tree has fewer reserves while it is forming its flowers.”

Comparable research is lacking for stone fruit. However, Theron suspects a similar relationship between crop load and flower differentation.

Bud quality is also affected by autumn and winter temperatures. For example, hot autumns and winters may lead to aborted buds or infertile flowers in stone-fruit trees. Work in related plants has shown that high autumn temperatures raise gibberellin levels.

In her own research, Theron has demonstrated that applying anti-gibberellins to pears and plums in autumn improved flower quality.

Winter should preferably start in May. “In an ideal world, we want cold, wet winters in the Western Cape,” says Christo Strydom, general manager at Wolfpack. “We know good winter chill gives us good fruit size in pears.”

A cold winter should then transition rapidly to a warm spring. “We want moderate temperatures of about 28 °C for cell division and enlargement to kick in,” says Strydom. “But we don’t want extreme heat.”

Pruning for bud quality

“Optimal bud quality is linked to the position of the bud on the tree,” says Theron. “Many of your pruning actions aim to improve the bearing positions for a particular cultivar. One way of thinning is by pruning away weak spurs.”

In apples, the best flower buds form on upright spurs in dominant positions on thick, two-year-old shoots. Old spur systems have weaker flowers because their vascular connections deteriorate over time.

“It is very clear that old branches with old spurs give you smaller pears,” says Strydom. “We must ensure that we renew our bearing wood to get two- or three-year-old bearing units.”

He points out that many pear cultivars set parthenocarpically (without fertilisation). In Forelle, parthenocarpic set is assisted by gibberellin applications, but Forelle that set on old spurs are small.

Furthermore, gibberellins aren’t effective on all cultivars and may cause misshapen fruit in some. In these cultivars, younger bearing wood is key to obtaining sufficient parthenocarpic set.

Young bearing wood is equally as important for the size of stone fruit, especially plums, confirms Stander. “Plums mostly bear on secondary spurs on two-year-old wood,” he says. “Remember that spurs only live for four years, and from the second year onward they bear smaller fruit.”

Besides renewal, winter pruning supports bud quality and fruit size in both pome- and stone-fruit trees by improving light penetration and distribution in the coming season.

More cells equal bigger fruit

Cell numbers are important for final fruit size. In apples, 19–21 cell divisions occur between flower initiation and full bloom, mostly during bud swell and break. After bloom, cell division slows for the first week, while fertilisation takes place. In the second week, cell division ramps up and peaks. Cell division ends by 4–5 weeks after bloom.

Warm winters and artificial rest-breaking may disrupt flower development. “When you walk in a South African orchard, our flowers are smaller than those of the same cultivar in the rest of the world,” observes Theron.

During warm weather, the time between rest-breaking treatments and full bloom can be short, and Theron speculates that this could result in fewer cell divisions. “We don’t have data for this,” she says, “but I think it could also be why our flowers and fruit sometimes have short stems.”

On top of this, high spring temperatures drive up respiration at a time when trees are reliant on their reserves, thereby reducing the carbohydrates available for flowering and fruit set.

In stone fruit, pit hardening follows cell division, occurring 42–56 days after full bloom. During pit hardening, fruit growth plateaus, generally resuming about three weeks before harvest. Any stress leading up to pit hardening can compromise cell division, leading to smaller fruit at harvest.

Pollination and fruit size

Pollination is essential for fruit set, but doesn’t directly affect the size of stone fruit. In pome fruit, seed numbers correlate positively with fruit size, so effective pollination makes a difference. In pears, pollination is mainly a concern in Abate Fetel.

“Abate Fetel is a long pear. To get the shape and mass the market wants, you need it to have a thick base,” explains Strydom. “Seeds and shape definitely correlate, so cross-pollination is important in Abate Fetel.”

Indirectly, however, pollination matters for all fruit types because a good set gives growers the option of leaving fewer fruit per inflorescence. Less competition between fruit in a bunch leads to bigger fruit, better colour, reduced rub marks, and fewer push-offs.

In stone fruit, fruit that set from first-generation flowers (the first wave of flowers to open) are much larger than those that set from second-generation flowers. “It’s seldom that both generations set,” says Stander. “The second generation sets when too few of the first generation have set.”

The weather during pollination frequently determines which generation sets. Still, growers can improve the odds of a first-generation set through sound pollination practices and by ensuring their trees have sufficient reserves.

Stander also emphasises the importance of sufficient soil moisture from bud break to fruit set. “Growers often overestimate their soil moisture, especially after a dry winter,” he says. “They may need to start irrigating before bloom to prevent stress in their trees.”

Thinning

“Crop load management is surely the single biggest factor influencing fruit size,” says Strydom. “Not in Packham’s, but in cultivars like Forelle, we’ve seen many examples of too many fruit on the tree leading to too many small fruit.”

Of course, crop load management isn’t restricted to fruit thinning. “Good fruit size already starts with winter pruning so that you don’t have an oversupply of flowers at bloom,” says Johan Kotze, Head: Dutoit Agri Eastern Cape. “More flowers result in more competition between fruit.”

The flowers themselves also compete, so having fewer buds as early as possible allows flowers to maximise their cell divisions. Flower thinning before full bloom helps as well, albeit less than winter pruning.

Once fruit have set, the earlier they are thinned, the greater the benefit for fruit size. “You must get rid of surplus fruit as quickly as possible with a good chemical thinning programme,” says Kotze. “And you must reduce fruit numbers manually as soon as possible thereafter.”

Pit hardening is the cut-off for thinning in stone fruit, but Stander encourages growers not to wait until then. “The secret is to do it earlier,” he says. “With some plum cultivars, we’ve seen that you need to finish thinning by 30 days after full bloom.”

When thinning manually, focus on removing smaller fruitlets. “A big fruit will always remain big,” says Stander. “If it doesn’t have an injury or isn’t in a position where you know it won’t make it into a carton, leave it.”

Strydom agrees, but admits that implementation can be challenging. “We must try to teach our workers the difference between big and small fruit at the stage when they’re thinning, so they can thin to size, not just to numbers.”

Additionally, for applicable cultivars, workers should aggressively thin pears on one-year-old shoots, perhaps only leaving a single fruit per shoot.

“I’ve never known a pear cultivar that bears large fruit on one-year-old shoots,” says Strydom. “The purpose of those shoots is to build future bearing units, not to bear fruit in the current season.”

Moreover, overcropping one-year-old shoots can hinder spur development for the following season. And, in the windy Western Cape, fruit on one-year-old shoots are at greater risk of rub marks.

Weather and water

Pome and stone fruit grow mainly through cell enlargement, which requires efficient photosynthesis and sufficient moisture. Fruit trees close their stomata when temperatures are high or soil moisture is low. Stomatal closure limits photosynthesis and transpiration, so developing fruit suffer a lack of carbohydrates and water.

“When temperatures are high, you can lose about a millimetre of fruit growth per week,” says Kotze. “If you then also irrigate less, you can easily lose 1–2 millimetres per week, and the growth is unlikely to catch up again before harvest.”

In pome fruit, the impact of insufficient irrigation on fruit size is greatest from 4–5 weeks after full bloom. Furthermore, dry conditions in this period are detrimental to flower induction.

For stone fruit, adequate soil moisture is critical from bud break to pit hardening, and then again from three weeks before harvest. “You can’t save water during those periods,” says Stander. “If your water availability is limited, then share it among your cultivars according to their developmental stage.”

Cold weather at the wrong time can likewise lead to smaller fruit. Rapid accumulation of heat units promotes fruit size in early peaches and nectarines, which is one reason why these fruit tend to be smaller in the Western Cape than in our Northern growing regions.

On the other hand, a cool, wet spell in January can stimulate new shoot growth in some apple cultivars, such as Fuji. Not only does this impair fruit growth and colour development, but it comes at the cost of the terminal reproductive buds that were supposed to set fruit in the following season.

Other factors affecting fruit size

Within a fruit type, cultivars have different size potentials, depending on their time from bloom to harvest (longer growth periods generally mean bigger fruit) and other genetic factors (for example, African Delight and Ruby Star plums are bigger than Angeleno, despite all three being harvested in the same window).

For stone fruit, Stander cautions against spacing trees less than 1.5 metres apart in the tree row. “When trees are too close together, the root competition is enormous, and you sacrifice fruit size,” he says.

High tree densities also increase shading in peaches and nectarines, compromising bud development. Summer pruning to alleviate shading creates more problems, as peaches and nectarines bear on one-year-old shoots, and the best buds end up being cut away during late summer pruning.

In pears, injudicious summer pruning can remove shoots needed for future renewal of bearing units, as well as possibly stimulating regrowth. “In the wrong hands, summer pruning can do more harm than good,” comments Strydom.

Appropriate irrigation and nutrition contribute directly to fruit size and support bud quality, bud break, flowering, and set. Stander advocates applying postharvest fertilisation in a few instalments, starting about two weeks after harvest, so that the tree can build up reserves.

“In the Western Cape, with our cold, wet springs, the trees don’t take up fertilisers in spring,” he says. “In the Northern parts of the country, spring fertilisation is more effective, because the weather warms faster and they have different soil types.”

When it comes to fruit size, it can seem as though every environmental and management factor plays a part. But some have a bigger role than others.

“The quality of flower buds and bearing wood is important,” says Kotze. “Then, the winning recipe for good fruit size is early reduction of flower numbers and removal of surplus fruit, and maintaining a balance between fruit and shoot growth throughout the season to ensure the tree puts its energy into the fruit.”