Cracking susceptibility varies

Harvest is a few weeks away, and rain is in the forecast. Before rushing to apply products to minimize rain cracking, what if you could know whether the block was susceptible to cracking before taking steps to prevent it?

Dr. Ines Hanrahan, project manager for the Washington Tree Fruit Research Commission, has learned much about the rain-cracking susceptibility of cherries as part of her research to find effective cherry antisplitting products. She’s now working to develop an easy test that growers could use to determine the cracking susceptibility of their fruit in real time.

“If you know if you’re susceptible, then you can decide if it’s cost effective to try to prevent the cracking,” she said in an interview with Good Fruit Grower.

She envisions a grower picking 50 to 100 individual fruits to determine the block’s cracking index within a few hours. The test would involve immersing fruit in distilled water for a few hours and then counting the number of fruit with cracks to determine the cracking index. The cracking index would indicate susceptibility of the fruit if rain came in the next few days.

Highly variable

Hanrahan has learned that cracking susceptibility varies dramatically from year to year and within the season. Her work shows that susceptibility fluctuates during fruit development—it doesn’t just increase as fruit matures. “We know that cracking is influenced by a lot of things, like crop load, weather, temperature, relative humidity, moisture level in the soil, rate of growth of the fruit, variety sensitivity, and rootstock, just to name a few,” she said. “That’s why last year was so different than the year before and why this year will be different again.”

For the last two years, she has used a bench-top test in the lab to determine cracking susceptibility of several cultivars. For the artificial cracking test, she brought fruit from four blocks into the lab twice a week during the last month before harvest in 2009 and 2010. Fruit of Bing, Rainier, and Tieton were collected in 2009; Early Robin was added in 2010. Fruit were immersed in distilled water for up to five hours, and the numbers of split fruit were recorded hourly.

In 2009, Bing and Rainier started to crack in bench-top assays 19 days before harvest. With Rainier, susceptibility rapidly increased for a week before plateauing 10 days before harvest, while Bing continued to increase in susceptibility, she reports. Tieton cherries were crack resistant until 10 days before harvest, with susceptibility then steadily increasing until harvest. At harvest, Rainier ­cherries were much less prone to cracking than Tieton or Bing.

The experiment was repeated in the same blocks in 2010, with a new block of Early Robin added. Values for cracking susceptibility were in the same range as the previous year, but the onset of susceptibility was dramatically different, she said. “For example, Tieton were susceptible to cracking during the entire fruit maturation period of 26 days. These results underline the importance of considering year-to-year variation as one of the main factors in cracking susceptibility for cherries.”

Product efficacy

The goal of the Research Commission-funded cherry cracking project is to develop management strategies to mitigate rain-induced cracking. Much of the early work was done in collaboration with Dr. Larry Schrader, Washington State University, who developed RainGard and studied the underlying physiology. Hanrahan is evaluating the efficacy of a handful of antisplitting products, including their influence on fruit quality and storage ­performance.

In three years, she’s studied five materials at 34 sites in Washington. Materials are applied by grower-cooperators with their own equipment or by Research Commission staff with an Accutech sprayer following protocols developed collaboratively with product distributors. Products being tested fall into three types: osmotica; those that work as a coating and have to be reapplied as the fruit grows; and those that work systemically, more like a hormone, and are be applied early, well before a rain event.

“The biggest obstacle in testing product efficacy is the reliance on adequate rainfall events to cause cracking (above 0.1 inch) during the last three to four weeks before harvest,” she said. Sites must have 10 percent field ­cracking for full evaluation of a product.

She got lucky with sufficient rainfall in the same Pasco site for two years in a row, which provided efficacy data on RainGard, a natural fatty acid coating. In 2009, RainGard was applied once to Tieton on Gisela 6 about five days before rain, and cracking was reduced by more than 50 percent, mainly reducing bottom splits. Last year, the grower applied RainGard three times before harvest in weekly intervals. Cracking in the field was observed to be 10 percent before the first application. By harvest, the control showed 51 percent on-tree cracking, while the treated fruit had 38 percent cracking.

Other research highlights involved Sweetheart cherries in the first year of the trial at a Manson location. Bluestim, one of the materials in the trial, was found to reduce pitting by 50 percent on fruit held 14 days in storage. Bluestim is an osmoregulator derived from sugar beets that’s applied at four and two weeks before harvest.

Although no material completely eliminates cracking, in many cases it can make the difference between a crop that can be harvested profitably and one that is a ­complete loss, she noted.

“I really can’t say much about the other products being tested, one way or another, because I don’t have the data,” she said, but added that she hasn’t found any statistically significant negative or positive effects from the materials in terms of fruit color, maturation, pitting, and soluble solids concentrations, either at harvest and 14 days later. She has learned that materials can reduce rain-induced cracking and that timing of applications and good ­coverage are critical.

Hanrahan plans to test several materials again this year. She’s excited about the cracking susceptibility test and believes that real-time data could help cherry producers determine which blocks are more susceptible and help fine-tune spray recommendations with antisplitting products. Some products require weekly applications a month before harvest when fruit may not be susceptible. Four applications are time consuming and can be cost prohibitive.

She will be working to verify the susceptibility test and index formula this season and invites Washington and Oregon cherry growers to contact her if they are ­interested in helping validate the test.

Hanrahan can be reached at (509) 669-0267 or e-mail hanrahan@treefruitresearch.com.