Update on plum-pollination project

The latest results from a three-year Hortgro-funded investigation of effective pollination period in Japanese plums. By Anna Mouton.

Export cartons per hectare drive the profitability of Japanese plum orchards. The first step in filling those cartons is to have a sufficient fruit set, but South African growers often struggle to achieve this. One reason may be deficient data on the effective pollination period of common plum cultivars.

The effective pollination period is the window for fertilisation. It’s determined by the ovule’s longevity and the pollen tube’s growth rate. Cultivar is one of the many factors that influence the duration of the effective pollination period.

A Hortgro-funded project led by senior researcher Louisa Blomerus of the ARC Infruitec-Nietvoorbij assessed the effective pollination period of nine Japanese plum cultivars for three seasons.

The selected plums — African Delight, Angeleno, Fortune, Laetitia, Ruby Crisp, Ruby Star, Ruby Sun, Sapphire, and Songold — are those open cultivars that contribute the highest export volumes. Blomerus conducted trials in commercial orchards in Bonnievale, Robertson, and Worcester. Each cultivar was only evaluated in one area.

“Ideally, we would want to test all the cultivars in all three areas,” says Blomerus, “but it wasn’t possible to handle more replicates.” She and her team hand-pollinated nearly 10 000 plum blossoms each season from 2021–2023.

The details of her methodology are described in a previous Fresh Quarterly article. To recap, Blomerus’ team chose 100 flower clusters per cultivar. Each cluster had to contain at least ten flowers that would open over the next ten days. They bagged the clusters to exclude pollinators.

The team returned when the flowers opened and hand-pollinated ten clusters per cultivar every day for ten days. They measured fruit set 12, 21 and 42 days after completing pollination.

“Pit hardening occurs by day 42, and Japanese plums usually don’t abort fruit after that stage,” explains Blomerus. “But the growers sometimes thinned the fruit just before we could do the final evaluation. So we rely more on the 21-day data.”

Seasonal shifts

“This research project started due to an abnormal season that disrupted bloom,” recalls Blomerus. “So we’ve also collected weather data to see if there are correlations with the effective pollination period.”

Trees’ exit from dormancy is one factor affecting flowering time. Better winter chill generally supports earlier and stronger bud break in spring.

Growers will undoubtedly remember the excellent winter chill of 2023, which probably explains the early bud break in many regions. “The 2023 season was one to two weeks earlier than the other trial years, and the flowering season was shorter,” says Blomerus.

All nine cultivars flowered earlier in 2023 — see Figure 1 — but not consistently earlier or later in the other years. For example, Laetitia flowered a few days earlier in 2021 than in 2022, but Ruby Star flowered a few days later in 2021 than in 2022.

Figure 1: Flowering periods for each cultivar in 2021–2023.

“We performed the pollinations in a more or less consistent flowering phase,” says Blomerus. “We started at 20%–30% of full bloom.”

Blomerus compared the average fruit set in each of the nine cultivars for 2021–2023 and found no pattern. For example, the set of African Delight was best in 2021 and worst in 2022. Set in Angeleno was markedly superior in 2022 to either 2021 or 2023. Ruby Star was the only cultivar where fruit set in 2023 exceeded that of 2021 and 2022.

“I can’t make a global statement that plums have a better or worse fruit set when they flower early,” concludes Blomerus. “I need to analyse the weather data further.”

For example, her data shows that in Worcester, the initial pollination period in 2023 was hotter for Ruby Sun than for Ruby Star, which could have contributed to a relatively better set in Ruby Star compared with Ruby Sun.

However, temperatures were also generally lower after pollination of the earlier Ruby Sun than the later Ruby Star — temperatures began trending up within a day or two after Ruby Star’s pollination period.

Cultivar comparisons

The literature suggests that only 5%–14% of pollinated plum blossoms develop into fruit. Blomerus used a 5% fruit set as the cut-off when estimating the effective pollination period. Table 1 shows the days after pollination where the fruit set was at least 5%.

The Sapphire trees were too young to fruit in 2021 and failed to set in 2023. “The grower did produce fruit on Sapphire, so it’s possible that we picked a poor pollination window,” speculates Blomerus.

Most cultivars appeared to have effective pollination periods of approximately five days. Of those with shorter periods, Songold didn’t achieve a 5% fruit set at 12 days in 2022 or 22 days in 2023, and Ruby Crisp’s flowers were aborted early in 2023.

“The Ruby Crisp orchard is in Bonnievale, in an unfavourable environment,” comments Blomerus. “The trees are struggling to fill their space.”

Table 1: Estimated effective pollination period based on the number of days after bloom with fruit set above 5%.

She still needs to complete her data analysis and plans to dig deeper to see whether there are any associations between weather and fruit set. However, the consistency of each cultivar’s effective pollination period despite large differences in seasonal weather is interesting.

The preliminary results show that, of all the cultivars tested, Laetitia came out tops with a repeated effective pollination period of 10 days.

“Laetitia seems to be fairly tenacious and to have the potential for good fruit set in more challenging environments,” says Blomerus.

Ruby Crisp and Songold didn’t do as well in these trials. Whereas the Songold data seems to agree with the general industry experience, less information is available for Ruby Crisp, and more research on its performance in different environments is needed.