When researching rain-induced cracking of sweet cherries, it’s obvious that rain is a requirement. Two years of research, though yielding little rain-induced cracking, have shown potential for reduced cracking from some materials, as well as a varietal difference in the timing of susceptibility to cracking.
The Washington Tree Fruit Research Commission is funding internal research to develop management strategies to mitigate rain-induced cherry cracking. During the last two years, field-testing of five new and improved materials was conducted in 25 sites in the state, along with tests in the laboratory.
“Unfortunately, we have been chasing the rain a little bit and only two sites in the trial had sufficient rainfall,” said Dr. Ines Hanrahan, project manager for the Research Commission. “Sufficient rainfall for our study is when we have 10 percent or more of natural cracking.”
Hanrahan shared the preliminary cherry cracking results on January 15 during the annual meeting of the Cherry Institute in Yakima, Washington.
Materials to prevent or protect from cherry cracking fall into three types of categories: calcium salts, coatings, and those with hormonal action. The difficulty with coatings is that applications must be made often, she said. And while hormone-type materials are not applied often, she said they can be difficult to get the timing right.
The five materials tested in the trial and compared to the industry standard of using calcium nitrate (1 percent solution) include:
Bluestim: a Swedish product described as an osmoregulator that is derived from sugar beets (glycine betaine).
Platina: a precursor to a naturally occurring plant auxin (L-Tryptophan) that works by enhancing the plasticity of cells.
RainGard: an osmoprotectant made from natural fatty acids and lipophilic substances.
SureSeal: new semipermanent coating made from edible organic components (stearic acid, cellulose, and calcium) that stretches with fruit growth, developed at Oregon State University.
VaporGard: an organic concentrate (di-1-p-menthene) that reduces water transpiration by plants.
In one of the two sites that had .10 inch of rain during the first of the two years of the trial, the control had an average of 27 percent cracking, while the Bluestim had 24 percent cracking. The Bluestim treatment on the Sweetheart cherries grown on Mazzard rootstock slightly reduced cracking of the top and side splits, she reported. Data collected at harvest showed that the Bluestim Sweetheart cherries had slightly lower malic acid levels compared to the control, but statistical differences were not detected in sugars or firmness.
After being held in storage for 14 days, the Bluestim-treated cherries were about 50 percent cleaner in terms of pitting when compared to the control. However, Hanrahan noted that such results were not observed in all of the trials.
In the other site that provided rain-induced field cracking were Tieton cherries grown on Gisela 6 rootstock. Rain occurred five days after an application of RainGard, which reduced cracking on the tree by half when compared to the control without any treatment.
Additional testing is needed to test all of the materials side by side, she noted. “We have yet to determine if most spray materials reduce rain-induced cherry cracking,” she concluded. “Product efficacy can be tested only if sufficient field cracking is present.”
Hanrahan also looked at the cracking susceptibility for Tieton, Bing, and Rainier cultivars during maturation. To induce cracking in the laboratory, cherry samples were submerged in water and then checked every hour for a five-hour period, with the cracking results and timing recorded.
“The cracking looks totally different than the cracking you see in the field,” she said, adding that the locations of the cracks are different than rain-induced cracking.
She found that Bing and Rainer cultivars were susceptible to cracking three weeks before harvest, while Tieton was susceptible only ten days before harvest. Also, Rainier seems to plateau in its severity of cracking, while Bing and Tieton continued to increase in cracking severity as harvest neared.
“Bing, at the time fruit reaches about 75 percent of its final weight, it begins to show cracking susceptibility,” she said. “Curiously enough, the same is true for Tieton, if you look at it from the fruit weight standpoint.”
In future trials, Hanrahan will continue both field and laboratory testing. More field testing and rain are needed to test material efficacy. Bench top testing will help develop variety-specific information that can then be used to fine-tune current spray recommendations based on variety and fruit developmental stage.