Comparing apples with apples

Why is there so much variability? By Anna Mouton.

Inconsistent fruit quality and ripening is the bane of apple growers everywhere. How is one supposed to harvest at optimum maturity when there is so much variation on the same tree? And the emphasis on export markets has raised the stakes — a few overripe fruit and you could end up with storage disorders that sink an entire consignment.

Insufficient winter chill is one possible cause of variability. Insufficient winter chill results in prolonged bud-break and an extended flowering period that ultimately lead to uneven ripening and mixed maturities. Many growers have come to rely on rest-breaking agents to promote more uniform bud-break and to condense the flowering period. But are rest-breaking agents enough?

“How much is lack of winter chill costing the producer?” asks Dr Esmé Louw of the Department of Horticultural Sciences at Stellenbosch University. “You might think it’s simply the money spent on rest-breaking agents, but there’s this year-on-year accumulation factor because it changes the architecture of the tree. And you might have increased variability and effects on fruit size and quality.”

Louw is leading a project funded by Hortgro Pome to quantify the impact of insufficient winter chill on apples. Fresh Quarterly recently spoke with her and with Andrew van Lingen, horticultural adviser at Two-a-Day. Van Lingen has been working on the project for his master’s degree.

In search of the source

“Initially the focus was on comparing two different climates and how the climates affect the total variation in fruit quality and maturity on the tree,” says Van Lingen. “We considered orchards within two regions, trees within an orchard, spatial positions in a tree, as well as the different types of bearing wood.”

Elgin and the Koue Bokkeveld were selected on the assumption that the Koue Bokkeveld receives sufficient winter chill whereas Elgin does not. Representative full-bearing orchards of Golden Delicious were sampled for two seasons in each region. All trees were grafted on M793 rootstocks and had similar management practices and yields. Similar rest-breaking applications were used in both seasons.

Apples were harvested at optimum maturity at the time of commercial harvest. Fruit were grouped according to whether they were on a top or bottom branch, were sun-exposed or shaded, and were born on spurs or shoots. Van Lingen recounts that they analysed 22 000 apples over the course of two seasons.

All the fruit were weighed and measured, and external characteristics were recorded. One set was immediately subjected to destructive analysis while another set was stored at -0.5 ˚C in regular atmosphere for twelve weeks before analysis. Measurements included firmness, percentage starch conversion, total soluble sugars, and internal defects.

Almost a quarter of the variation in fruit mass was attributable to region in 2017 but region did not affect fruit mass in 2018. “There was possibly a lingering effect of the drought in Ceres in 2018 because we saw smaller fruit,” says Van Lingen. “Typically Ceres has bigger fruit than Grabouw.”

Differences between orchards made the largest contribution to variability in maturity parameters such as percentage starch conversion, total soluble sugars, and firmness. Fruit harvested from top branches tended to have lower starch conversion and greater firmness and total soluble sugar levels than fruit harvested from bottom branches. Sun-exposed fruit generally had lower starch breakdown and greater firmness and total soluble sugars than shaded fruit.

“There isn’t much difference in total variability between the two regions, but orchard-to-orchard and tree-to-tree variability is surprisingly high,” concludes Van Lingen. “The most surprising to me was that the top and the bottom of the tree ripens separately.”

From blossoms to rootstocks

Louw and Van Lingen also examined the effect of flowering period on fruit quality for the 2018 season.

Trees were again sampled from representative orchards in both Elgin and the Koue Bokkeveld. Buds were monitored on selected branches and the resulting flowers classified as early, middle, or late relative to full bloom. Flowers were tagged so that fruit that developed from different flowering times could be compared. The fruit were analysed for various maturity parameters after harvest.

Elgin fruit that developed from early blossoms had the greatest starch breakdown and the lowest firmness whereas fruit that developed from late blossoms had the least starch breakdown and greatest firmness. Fruit that developed from full-bloom blossoms were intermediate. This seems logical considering that apples that set first have had a longer time to develop until harvest.

The same trend was not observed for starch breakdown in the Koue Bokkeveld apples, but fruit that developed from late blossoms were the firmest after storage.

Orchards in both regions experienced an extended bloom period during the trial. Almost half of the fruit developed from full-bloom blossoms in Elgin compared to only about a third in the Koue Bokkeveld. “One would think that your bloom period would be more condensed in a cold area than in a warm area,” observes Louw. “This shows you that climate change has already started to happen. Ceres is no longer as different as we thought a couple of years ago.”

Louw has also been testing whether rootstocks can tone down the impact of insufficient winter chill on dormancy and fruit variability. The trial orchard is in Villiersdorp and consists of mature Golden Delicious grafted onto a selection of seven Malling and Cornell-Geneva rootstocks. Shoots were collected from the sample trees every two weeks during dormancy and forced into growth in the laboratory. The number of days that a shoot takes to break its rest is an indication of the depth of dormancy.

Fruit were harvested from the trees on the different rootstocks. Fruit quality and maturity were assessed at harvest and after storage.

The trials started in 2018 and fruit were harvested for analysis in 2019 and 2020. Data analysis has not been completed. Preliminary results from 2018 indicate that the Malling rootstocks have a lower dormancy level than the Cornell-Geneva rootstocks. This suggests that the Cornell-Geneva rootstocks used in these trials may be more suited to colder areas. The results have yet to be confirmed by the 2019 data.

Aiming for identical apples

How can producers achieve uniform fruit when variability in quality and maturity is caused by so many different factors? Van Lingen has some recommendations. “A lot of those factors can be addressed with modern planting systems. If you have a high-density planting, you’re simplifying the entire canopy.” A two-dimensional canopy ensures that fruit have equal light exposure and reduces variation due to top and bottom branches.

High-density plantings are also easier to prune and have fewer spurs to renew. This reduces variability in fruit quality resulting from the age and type of bearing wood.

Van Lingen stresses the importance of planting trees correctly. “Historically it wasn’t emphasised that your tree should be planted with the graft union above the ground. I go to orchards planted up to five years ago and I see trees planted with the graft union covered in soil.” The problem is that the scion of some of these trees has rooted. This contributes significantly to variability between trees.

The dataset from the rootstock trials is still being analysed but Van Lingen is expecting interesting results. “I’m quite fascinated with what the Genevas are going to give us. Genevas were bred to be replant tolerant and disease resistant. They do appear to alternate less on certain cultivars and are more willing to set fruit. It’s a far more robust rootstock.”

Louw adds that rootstock choice together with high-density planting could overcome issues with tree architecture that result from insufficient winter chill. Apple trees in warm areas tend to develop a shrub-like habit due to increased basal dominance. “We struggle to get our trees to fill their space in the warm areas,” explains Louw. Dwarfing rootstocks and high-density planting would help by reducing the space that the tree needs to fill.

“When I started researching dormancy it was a difficult topic to sell, because some people would fail to see the problem,” recalls Louw. “Especially in the Koue Bokkeveld where they didn’t have a lack of winter chill at the time. They didn’t use rest-breaking agents to wake up their trees. Over the past eight years it’s amazing how that mindset has changed. The Koue Bokkeveld are now also using rest-breaking agents.”

Insufficient winter chill is one of the many factors driving variability in fruit quality and maturity. “It was surprising to see the amount of variability from orchard to orchard and from tree to tree,” says Louw. “When you think you’re harvesting at 15%–20% starch breakdown, you’re actually not.” The average might be 15%–20% but some apples will be at zero whereas others are at 100%.

Van Lingen is currently completing his analysis of early, full-bloom and late flowers. He hopes to understand how differences in these flowers impact fruit quality. This research, together with the findings of the rootstock trials, will help growers better manage sources of variability in their apples.

What are people saying about this research?

Anton Müller | Horticultural adviser

“This work is important because the impact of our poor winters on fruit quality has never been quantified before. South African fruit has much more variation than we see in fruit from countries with more winter chill. Storing fruit with variable maturity presents an additional challenge that our growers need to overcome. Those of us that have visited colder areas have subjectively observed the difference, but we need to quantify the effect on maturity and quality. Growers need empirical data before they will be motivated to invest in countermeasures to reduce the economic impact of poor winter chilling.”

Prof. Wiehann Steyn | Hortgro Science

“Variation in fruit quality and maturity is one of the biggest problems in our industry. Variation on the tree and between trees makes it difficult for growers to harvest fruit of uniform maturity. The grower samples the fruit, and the maturity is correct on average, so you’d think that the fruit are suitable for long-term storage and export to a high-value market. But then the fruit reaches the market, and it has internal browning or other quality issues.

“This project highlighted the magnitude of the variation in our fruit — every year in every region. The research fits into our larger goal of understanding the sources of variation so we can find ways to reduce or manage it. We want to produce fruit that are as uniform as possible. Research in this field is an increasing priority worldwide. It’s a trend even in places like New Zealand where you would think conditions for apple production are optimal.”