SureSeal, a new protective coating, at right, patented by Oregon State University that stretches as the fruit grows, needs complete fruit coverage—to the point of dripping  off fruit—to be effective.

SureSeal, a new protective coating, at right, patented by Oregon State University that stretches as the fruit grows, needs complete fruit coverage—to the point of dripping off fruit—to be effective.

Photos Courtesy of Washington Tree Fruit Research Commission

Several years of field trials have yet to find a sure-fire material that completely prevents cherries from cracking after rain. But a few antisplitting materials do help reduce the amount of cracking and are worth applying, especially if the blocks are high-yielding with expected high returns, says a Washington Tree Fruit Research Commission scientist.

As cherry harvest nears, nothing gets the adrenaline going for a cherry grower quite like a forecast of rain. A year’s worth of effort can be quickly diminished if rain comes at the wrong time, causing fruit to split and crack. In some years, cracking incidence can be high in an orchard, while other years, fruit in the same orchard escapes major problems.

Rain-induced cherry cracking is a complex combination of factors that are constantly interacting with each other and changing within the orchard, said Dr. Ines Hanrahan, project manager for the Research Commission. Genetic factors involve the cracking susceptibility of cherry cultivars; physiological factors, like tree age, use of plant growth regulators, and frequency and amount of irrigation; morphological factors include stage of fruit development, skin thickness, and chemical composition of wax; and environmental factors include frequency and amount of rain, temperature, humidity, and such.

Causes of cracking

“We know that cherries have a double sigmoidal growth curve and that most growth occurs in the last few weeks before harvest,” Hanrahan said during a session of the Northwest Cherry Institute’s annual meeting held in Yakima, Washington. “That’s also when cracking susceptibility develops—in the last growth stage, and it peaks at harvest.” However, her research shows that there are several different patterns present in cherry varieties grown in the Pacific Northwest, such as: a) fruit of the same variety can become highly susceptible four weeks before harvest, or b) they may never reach high cracking potentials.

“The incidence or amount of cracking varies tremendously and depends on hundreds of factors,” she said. Her research shows that cracking susceptibility within an orchard doesn’t necessarily increase in a linear fashion as the fruit matures. Cracking ­susceptibility varies from block to block, by variety, year to year, and within the season.

Scientists around the world have studied how water enters the cherry to cause the splitting, with inconclusive results. Her work has looked at water uptake throughout the fruit development process, and she hasn’t observed changes in water uptake rates per unit of surface area as the fruit matures. Instead, she’s found a relatively consistent rate of uptake through fruit development. “Most believe that the majority of water uptake happens by diffusion of osmosis through the cherry fruit skin,” she noted.

“However, based on data obtained last year, we have reason to believe that the main route of uptake is indeed the pedicel/fruit juncture,” she said. Lesser factors in cracking are the stage of closure of stomates or lenticels, which can let water into the fruit, and the loss of the fruit’s natural protective coating. In the last stage of cherry growth, the ­triterpene coating diminishes as the fruit matures.

Cherry cracking is also related to fruit turgor, which typically peaks in the early morning hours and is influenced by irrigation amounts and frequency. “That’s why it’s best not to let the orchard dry out because that can encourage more cracking,” Hanrahan said. “Soil moisture should be kept at a more constant level with frequent ­irrigations to prevent drying out.”

Antisplitting materials

Hanrahan has tested a dozen antisplitting materials in seven varieties, four replicates each, in 40 field trials the last four years. Orchards must have at least 10 percent cracking to yield data included in the trial, which has made the trial slow going as rain doesn’t occur in every location every year. “I’ve set up a lot of trials, but not a lot of sites get to the 10 percent cracking level, so I only have a limited amount of data and cannot tell you about every material I’ve tried.”

Thus far, the two protective coatings of SureSeal and RainGard have consistently reduced rain-induced cherry cracking in Washington State trials, she said, adding that some of the other materials tested—BlueStim (glycine betaine), calcium nitrate, and VaporGard (di-1-p-menthene)—have not had enough rain in their trials to provide meaningful data. RainGard is made from natural fatty acids and lipophilic substances, developed and patented by ­Washington State University’s Dr. Larry Schrader.

SureSeal, developed and patented by Clive Kaiser of Oregon State University, is an elastic, organic biofilm made of a hydrophobic copolymer of stearic acid, cellulose, and calcium. The coating stretches as the fruit grows, but Hanrahan stresses that good coverage, to the point of dripping, is needed. She also found during trials last year that the material is very sensitive to the spreader-sticker or adjuvant used, and was only compatible with one particular product. However, the company is working to improve the spreader-sticker issue. SureSeal, at 1 percent solution, was applied twice—once at four weeks before harvest and again at two weeks before harvest.

In one of the RainGard trials of Tieton cherries on Gisela 6 rootstock in Pasco, rain has occurred three years in a row. RainGard reduced cracking in all three years compared with the untreated ­control.

While she often sees some reduction in cracking from the anti-splitting materials, they have never reduced cracking by more than 50 percent compared with the untreated control.

“So you need to consider the susceptibility to cracking of the orchard when considering if it’s cost effective to apply these materials,” Hanrahan said. A high-yielding block with ten tons per acre is probably worth it, but maybe not for an orchard yielding three or four tons per acre.