How adaptable are cherries?

What is adaptability?

Every cherry cultivar has a unique set of genes (a genotype) that underlies its observable characteristics (its traits) and potential performance. “The choice of genetics is the single most important factor in setting up an orchard. It makes or breaks an orchard,” says Van Wyk. “If you choose the right genetics, in the long term, you win. If you choose poorly, you’ll know that you’ve lost in the early production phase of an orchard.”

However, genes don’t function in isolation. The trees’ environment influences their expression. Adaptable cultivars can perform reliably in diverse environments and across different seasons, making them a safer option for growers.

Broad adaptability may be due to the trees possessing a spectrum of genes that enables them to cope with different conditions. For example, they may be able to upregulate genes in response to drought or disease that allow them to outperform cultivars that lack those genes. Broad adaptability could also be due to the cultivar’s gene expression being largely independent of external conditions. This is known as stability. Van Wyk illustrates stability with the examples of Royal Hazel and Royal Tioga. “Across all three environments, Hazel branched well, so it’s genetically stable for branching,” he says. “Tioga was the highest spurring across all environments. So, Tioga was genetically stable for spurring, but not for the formation of long shoots.”

A highly adaptable cultivar might not be the top performer in a particular environment, but its behaviour will be more consistent across environments. The highly adaptable cultivar is also more likely to be stable from season to season. Therefore, adaptable cultivars are more predictable and present a lower risk than less adaptable cultivars.

However, there’s no such thing as the perfect cultivar. “There isn’t one all-rounder. There are pros and cons to any cultivar or rootstock,” says Van Wyk. “A lot of people who work with genetics have said this.”

The cultivars and sites

For the adaptability trials, 10 cultivars were selected to represent a range of chill requirements (500–1 000 daily-positive Utah or Infruitec chill units), estimated based on their parentage. Royal Lynn, Royal Tioga, and Royal Hazel were chosen as low-chill cultivars; Royal Dawn as low to medium; Frisco, Sequoia and Lapins as medium; and Royal Helen, Royal Edie, and Sweetheart as high. The rootstock was the semi-vigorous Maxma Delbard 14 Brokforest.

All cultivars were evaluated at three Pro-Hort sites: the high-chill Nooitgedacht in the Koue Bokkeveld, the medium-chill Lushof in the Warm Bokkeveld, and the low-chill Klipboschlaagte in Robertson.

Nursery trees were stored at 4 °C in a cold room before planting in 2019. No rest-breaking treatments were used in their first season. Thereafter, the trials included a rest-breaking treatment (hydrogen cyanamide) and an untreated control.

The trees were trained in a similar style to tall spindles in apple production. Van Wyk and Vahrmeijer collected data on bud break, vegetative growth, bearing habit and complexity, fruit yield and quality, pruning, and temperature. The trees started bearing in the third leaf.

To disentangle the effects of genetics and environment, the researchers employed sophisticated statistical methods, including principal component analysis, stepwise discriminant analysis, and the use of AMMI (Additive Main-effects and Multiplicative Interaction) and GGE (Genotype plus Genotype by Environment) models.

AMMI and GGE have shown success as cultivar selection tools in agronomical crops worldwide.

Four years of findings

As expected, the trees performed best in the high-chill environment at Nooitgedacht, where they showed greater reproductive potential – more condensed budbreak, more bearing positions and flowers, and moderate vigour. Most cultivars returned high yields at the high-chill site.

Most cultivars also showed good reproductive potential in the medium-chill environment at Lushof, but the trees produced a small amount of fruit. “We were fairly aggressive with the number of hives we put in, and there was no competition. But from bee to fruit, something was missing,” recalls Van Wyk. “I have a question mark over the effective pollination period.”

In the warm environment at Klipboschlaag- te, yields were impacted by the extinction of flowering positions and minimal spur formation. “I would say that only one or two cultivars are adapted to Robertson,” says Van Wyk. “It’s a high-risk zone.”

Cherry production based on these cultivars is possible in warmer areas, but sufficient fruiting would require intensive tree manipulation. Interestingly, bud break was good in all environments and rest-breaking with hydrogen cyanamide had little effect on bud break or tree architecture. In general, the trees didn’t struggle to reach their final height, but cultivars differed in their ability to fill their horizontal space. The adaptability study confirmed that different cherry cultivars express their genetic potential differently depending on their environment. “Royal Hazel came closest to being an all-rounder,” says Van Wyk. “But what we’re trying to understand is what makes it adaptable.”

Improving our understanding of adaptability would allow more accurate evaluations of new cultivars, thereby better informing growers’ planting decisions. Unfortunately, although the cherries are now coming into their sixth leaf, the adaptability project has ended due to funding constraints. “The ultimate purpose after the full trial would have been to create an adaptability index. We need to identify the drivers of adaptability,” says Van Wyk. “We will do that in apples, pears, and plums, and it’s still the goal with cherries.”

BOX

The team Project manager: Dr Iwan Labuschagne. Provar. Academic leader: Dr Esmé Louw. Department of Horticultural Science. Stellenbosch University. Project collaborator: Dr Nigel Cook. Prophyta. Project statistician: Dr Mardé Booyse. Agrimetrics Unit. Agricultural Research Council. MSc students: Chad van Wyk and Karel Vahrmeijer.