Pollen delivery and availability appear to be key to improving fruit set of Balaton cherries, a new sweet-tart cherry that holds promise to become a major variety but has had discouraging yields the past few years.
Balaton made its commercial debut in the United States in 1984 when Dr. Amy Iezzoni brought the dark-skinned, Morello-type cherry from Hungary. Iezzoni, horticulturist for Michigan State University, saw potential in the variety because of its sweet-tart taste and dark burgundy color.
Extensive field tests were conducted with commercial growers and processors in several states. Acreage of Balaton has now grown to about 1,100 acres, according to Iezzoni.
Balaton has several advantages over Montmorency tart cherries. It is resistant to cracking from rain, the fruit is very firm and holds up during shipment, and its dark color eliminates the need to add red food coloring in products like pie filling. The tart cherry industry sees potential for the variety to be sold as a fresh market tart cherry, something that hasn’t been offered before to consumers due to Montmorency’s softness and shipping problems.
“Over the past few years, fruit set in Balaton has been disappointing, particularly in years when cool conditions persist during the bloom period,” Iezzoni said during tree fruit grower talks in Grand Rapids, Michigan. Yields have been up and down in different sites, and were particularly variable in 2005. “To fix this problem, we need to understand cherry tree physiology.”
She explained that cherry flowers have two ovules. Under normal situations, one ovule degenerates and aborts, while the second, after successful fertilization, develops into the seed.
“If fertilization is delayed, it causes problems, and the remaining ovule will die,” Iezzoni said, adding that research has shown that nonfertilized ovules lose their viability after about seven days.
“There is a time limit on how long a flower has to turn into fruit. We are under time pressure to get the flower turned into a fruit. We have to get the trees pollinated quickly.”
Temperature during pollination also influences the fertilization process. Under optimal temperatures, the pollen is shed and moves down the style quickly, resulting in fertilization. Cold conditions can dehydrate the anther lobes; wet conditions may keep the pollen from being available.
Iezzoni and other researchers studying ways to improve fruit set in Balaton have set several long-term research goals, such as learning how to accelerate and maximize pollen delivery to the flowers, accelerate pollen tube growth in the style, and identify cultural practices that maximize flower quality and fruit set.
During 2005, Iezzoni evaluated several issues relating to pollination and fruit set of Balaton.
She collected yield data from orchards that had Balaton trees planted adjacent to sweet cherries or Montmorency cherries to see if there are benefits to using pollinizers. Balaton orchards have not traditionally been planted with other pollinizers because the variety is self-fertile. The data showed that other cherry cultivars appeared to influence Balaton yields in four of six orchards studied, but more data is needed to confirm the importance of alternative sources of pollen.
She also observed that supplemental hand-applied sweet cherry pollen consistently improved Balaton fruit set in 12 Michigan sites, suggesting that sweet cherry pollen was more efficient at pollinating Balaton.
Preliminary data suggest that hornfaced bees, known to be very efficient pollinators, may have the ability to pollinate as well or better than traditional honeybees. Further research is needed to learn more about their biology and pollinating capability in Balaton.
They also found that the speed of pollen tube growth appears to be different among cherry varieties, with pollen of certain varieties better at achieving fertilization in Balaton than self-pollen. She believes that pollen delivery of Balaton is inadequate.
“All cherry pollen is not equal,” she said. “Certain pollen can grow faster than other pollen. This may be especially pronounced at lower temperatures.”
The Balaton study did point out the importance of nutrition. The orchard with the lowest leaf potassium level also had the lowest overall yield. Researchers plan to track the nutrient levels of the same orchards next year.
Gibberellic acid at varying rates was applied to a ten-year-old block in 2005 to test the yield response of Balaton following application of GA. Scientists will monitor the orchard to watch for results in the coming years.
Put bees out early
To improve Balaton fruit set, she recommends maximizing early pollen delivery so the pollen has sufficient time to reach the ovule. “Put honeybees in the orchard early, at first bloom, and put hornfaced bees in the orchard two weeks before bloom.”
Orchardists using honeybees should place two to five healthy hives in each acre.
“Make sure that you do nutrient checks on your orchard,” she suggested, as the nutrient status can influence flower quality and, ultimately, crop load.
Flower bud number and density also impact yield. Balaton fruit weights can reach six grams per fruit compared to four grams for Montmorency. Growers need to know what they want their Balaton crop load to look like. Normally, there are four to six flowers per bud, but the goal is two to three fruits per cluster.
“We do have a ways to go,” she said, referring to research to increase Balaton yields. “I think the key is within the tree, but we do need to get pollen there early, and we need to speed it up.”