An example of surface pitting on Sweetheart cherries.

An example of surface pitting on Sweetheart cherries.

Surface pitting in sweet cherries is a common, yet complex problem that researchers are trying to better understand.

For years, the small sunken areas, or pits, on the fruit surface were assumed to be a primarily a postharvest phenomenon that occurred during packing. Pitting is often caused by a mechanical impact or compression, according to information posted on the Web site of the University of California’s Postharvest Technology Research and Information Center, and is associated with damage to cells near the epidermis, which collapse over time.

But some varieties are more prone to pitting than others.

Regional differences

Growers in The Dalles district in Oregon have had pitting and storage problems with Lapins and Sweetheart cherries since they first came into production in the region in the late 1990s. Although they struggle with pitting problems, their fellow growers in Canada, where the two varieties were developed, successfully ship Lapins and Sweetheart cherries around the world.

Oregon State University scientists have been studying pitting the last few years to identify causes and treatments. While most of the research has focused on the packing line, as it has the biggest impact on pitting, scientists are also looking at preharvest factors as some research has shown a relationship between pitting and things like fruit maturity and leaf-to-fruit ratios.

In a research article published in Good Fruit Grower (May 15, 2008), Lynn Long, OSU Extension educator, discussed research from Chile indicating that earlier picking of Lapins and Sweetheart—at the light mahogany stage—and balanced crop loads could reduce pitting. However, other reports have not corroborated these results.

A joint project is now under way by Long, along with OSU horticulturist Dr. Todd Einhorn, and Washington State University horticulturist Dr. Matthew Whiting, to study the effects of harvest timing on fruit quality and surface pitting in a range of sweet cherry cultivars.

"Even before some fruit comes to the packing line, we’ve seen pitting on cherries," said Kristi Deschuytter, postharvest physiology research assistant at OSU’s Mid-Columbia Agricultural Research and Extension Center.

In a recent OSU project that investigated the influence of calcium and nitrogen of surface pitting of Sweetheart cherries, Dr. Frank Yin found that applications of calcium to the fruit did increase fruit firmness, but did not reduce pitting. No effects from nitrogen applications were observed.

In Yin’s study, directly after harvest there was approximately 12 percent pitting in fruit that was not run over the packing line, reported Deschuytter. "After three weeks of storage, the same fruit had a pitting incidence of 50 percent. Therefore, it is clear that there is more to the pitting problem than previously thought," she stated.

Potassium connection

Last year, Deschuytter looked at the roles that gibberellic acid and potassium may play in the physiological disorder. She saw a slight decrease in pitting in cherries treated with GA.

Potassium is a macronutrient required in large amounts by most crops. "In addition, potassium has been linked to increased fruit quality, regulation of several enzymes, and is central to water relations and stomatal conductance," Deschuytter said.

She provided weekly fertigation with potassium on Lapins cherries during the growing season in 2008, starting in May. The first fertigation coincided with pit hardening, the stage at which cells are starting to expand. Timing of the nutrient application and nutrient uptake by the trees is critical, she said.

Deschuytter saw no pitting differences in the potassium study in the first year, but points out that in nutrient studies it can take multiple years to see results. She plans to continue the potassium work for another year.

She will be working with Einhorn in the coming year to search for other factors that may affect preharvest pitting, such as crop load, irrigation, and nutrient management at different stages of fruit development.

Deschuytter and Einhorn have teamed up with Dr. Paul Wiersma, geneticist from the Pacific Agri-Food Research Centre in Summerland, British Columbia, Canada, to explore "outside the box" postharvest ideas as they work to better understand what causes the disorder and search for ways to beef up cherries so the fruit will better handle packing and storing. One concept includes forcing cherries to loosen their epidermal cell walls by dipping fruit in an acidic solution to promote activity of the enzyme expansin, a protein that has a "loosening" effect on cell walls. They want to learn if more elastic cells can recover better from damage during packing.

The OSU scientists are collaborating with industry members on the pitting research, and will continue to look at different aspects in the packing house, from evaluating equipment, such as cluster cutters, to the influence of different storage atmospheres.