Overcoming poor fruit set

Researchers are trying to unlock the secrets of fruit set in Regina.

The Regina sweet cherry cultivar has been widely planted by growers in Oregon’s Willamette Valley and eastern cherry-producing regions. Regina has a lot of good qualities going for it, but growers have struggled with poor fruit set.

Dr. Anita Azarenko, Oregon State University horticulturist, describes the German variety as an exceptional cherry, possessing superb fruit quality, good storability, and grower-friendly tree structure. It is well suited for precocious rootstocks, and most importantly, is late blooming, which helps eliminate chances of bloom damage from spring frosts.

"But she is a lazy girl who doesn’t want to produce," says Azarenko.

Azarenko has spent the last three years trying to unlock the fruit set secrets of Regina. "She serves as a very bad female parent with self-incompatibility. Regina has lots of flowers and is late blooming (a positive benefit for Willamette Valley growers), but she has inadequate fruit set and production."

Azarenko noted that research has shown Bing and Lambert cherries in Washington State to have an effective pollination period lasting between four to seven days, while the effective pollination period for Royal Ann cherries in Oregon lasts four to five days. But researchers don’t know what the effective pollination period for Regina is in Oregon, although a research literature search showed a range in other locations of one to seven days.

While there are still many unknowns when it comes to Regina pollination, Azarenko’s work has answered some questions. "We know that Regina is self-incompatible and needs pollinizers. We don’t know how long the pollen takes to arrive at the ovule—what the rate is of germination, penetration, and growth of the pollen tube. We don’t know if the population of Regina flowers has mature embryo sacs or viable ovules. Do they complete double fertilization?"

Compatible pollinizers

With S1 and S3 alleles, Regina is self-incompatible. OSU researchers have studied cherry varieties for bloom overlap and germination viability to develop a list of potential pollinizers for Regina. The list of those with late-blooming characteristics that could be good pollinizers includes:

• Stark’s Gold

• Sam

• Schneider’s Späte Knorpel

• Kordia

• Sandra Rose

• Skeena

• Sylvia

• Hudson

Ovule longevity

Azarenko has determined ovule longevity of Regina by excising ovules from flowers, flattening the ovules, and staining them for viewing under a microscope. Ovules that show fluorescence under the microscope are dead. Ovule longevity is affected by temperature and time, she said, adding that there is a rapid decline in ovule viability under higher temperatures.

The OSU horticulturist learned from a three-year field study that by 1,500 growing degree hours—at orchard locations in The Dalles and Corvallis—all of the Regina ovules were dead. At 800 to 900 growing degree hours, about half of the ovules were viable.

Translating the growing degree hours into days after bloom, about 50 percent of the ovules were still viable by day four through six, while all of the ovules were dead as early as day seven in The Dalles and in days nine and ten in Corvallis.

Azarenko also studied how long it took the pollen grain to penetrate and move down through the pollen tubes to the ovary. In 2006, at the OSU Corvallis location, it took only two days for the effective pollination period, but generally, it was a two- to four-day pollination period.

"We can predict ovule longevity, regardless of geographic location, more readily across multiple environments, but we’re not able to predict as well how long it takes for germination, penetration, and pollen tube growth," she said.

With Regina, it does matter how close the variety is to the pollinizer, she said. A trial using Sam as the pollinizer planted within rows showed that yield was reduced the further away Regina was from the pollinizer.

She believes that choosing a precocious rootstock can help increase Regina’s floral density.

Another practice that has worked in pears and may have application to cherries is a fall application of urea to enhance ovule longevity. Additionally, Azarenko thinks that a boron application, known to increase fruit set in plums, could have a similar response in Regina cherries.

"Using multiple pollinizers is crucial to ensure that there is bloom overlap," she advised.

For Regina, she recommends using a precocious rootstock like Gisela 5 or G.3 for the pollinizer to see if the amount of available pollen can be increased within the space without losing productivity. Gisela 3 produces a small tree size that could fit between the main variety without taking up too much space.

An example of her pollinizing strategy: going down an orchard row, she suggested a grower plant a pollinizer, then five trees of Regina, a pollinizer, then five trees of Regina instead of a solid row of pollinizers, followed by several solid rows of Regina.