Written by Rob Blakey, WSU Extension. April 10, 2017
Cracking is one of the of the primary cull factors in sweet cherry production. There are two causes of cracking: rain-induced and plant internal-water-induced (internal-water) cracking. Rain-induced cracking is more common and is characterized by cracking in the stem-bowl or tip. Rain-induced cracking occurs when water is osmotically absorbed into the fruit from the fruit surface. Internal-water cracking is caused from excessive soil water being transported to, and accumulating in the fruit. Side cracking is often a symptom of internal-water cracking but can also be symptomatic of rain-induced side cracking if a water droplet collects between two adjacent fruit.
Although sprays cannot completely control cracking, they may reduce the severity of cracking and allow for an economically viable harvest. A rain event of more than 0.1″ (2.5mm) is the general threshold for rain-induced cracking. And an event of more than about 0.5” (13mm) in a day generally overwhelms spray interventions (1).
Gibberellins (GA3) have shown a mixed response with respect to cracking and should not be considered for cracking mitigation.
Salts (usually calcium chloride) can be used to reduce and slow the osmotic infiltration of water into the fruit by decreasing the osmotic potential of the water on the fruit surface. To be effective, the salt must be on the fruit surface while there is water on the surface. As these salts are water-soluble, they will need to be re-applied after a rain event.
An application rate of 0.5 – 1.0% CaCl2 has proven effective if the system and automated program are effective in supplying an adequate concentration of CaCl2 at the appropriate time (2). A non-ionic wetter/spreader should improve effectiveness (3). Note that the salt can leave a residue on the fruit that will require postharvest washing.
Coatings, sometimes called biofilms, offer some protection by limiting the movement of water on the fruit surface into the fruit but have no positive effect on internal-water cracking. Coatings require good fruit coverage to be effective. Repeat applications are required to ensure good coverage and effectiveness as the fruit grow. Parka™ and RainGard™ contain natural compounds that supplement the natural fruit cuticle and seal micro-fractures.
Parka contains fatty acids, phospholipids, cellulose, and calcium. It provides an elastic fruit coating that stretches as the fruit grows. Parka uses SureSeal™ technology, which was developed at Oregon State University.
Apply at 0.5 – 1.0% v/v and 100 – 200 gal/A (4,5). Apply at fruit set and at straw color. Another application is recommended if a rain event is anticipated more than 10 days after the second application. Do not apply with surfactants, stickers, or Pinolene®-based materials (e.g. Vapor Gard® and Nu-Film) to prevent phytotoxicity. Parka has been reported to cause phytotoxic burn (see brown rings indicated in photograph) when applied to Bing and Benton about 25 days before harvest when applied at more than the label rate. Growers are advised to consult with warehouse field staff or Extension personnel for more information.
Unlike Parka™, the RainGard™ coating is not elastic and needs to be applied regularly as the fruit grows.
RainGard™ should be applied at 0.8% v/v i.e. 102 fl oz./100 gal at 100-200 gal/A (4,6). The first application should be made about 4 weeks prior to harvest (approximately straw color) with repeat sprays every 7-10 days thereafter, i.e. a total of 3-4 applications. Additional applications before anticipated rain events, with a minimum of 2 hours drying time, have proven beneficial. The re-application interval is a minimum of 24 hours.
A more cost effective two-application regime has also proven to be effective. The first application is done in combination with GA3 and the second application with RainGard® alone once fruit had reached a cracking index reading of 20% (see test method below) and before a rain event has been predicted (1).
- Vapor Gard®
Vapor Gard® is an anti-transpirant film principally composed of Pinolene® (di-1-p-Menthene). It is not recommended for cracking mitigation. It was found to be ineffective, may leave a sticky residue that reduces fruit glossiness, and may cause phytotoxicity (1). An anti-transpirant coating may actually increase the risk of internal-water cracking by limiting evapotranspiration from leaves which would increase the turgor pressure in the fruit to levels sufficient to cause the cells to lyse (7).
Always follow manufacturers’ instructions and read all product labels carefully before use.
Cultivar Susceptibility & Testing
The table below indicates cultivar susceptibility to cracking, but cracking susceptibility is highly variable within blocks of the same cultivar and within the same block in different years (1,6). The protocol for testing an orchard’s susceptibility to cracking, using artificial imbibition with distilled water for 50 fruit (8), is available from the WTFRC here. This protocol can be used as a guideline for the application of cracking mitigation sprays.
Table 1: Susceptibility of cherry cultivars in Washington – adapted from Hanrahan (2014) and Valent Biosciences (2015).
|High||Medium to High||Medium||Low|
Tree Fruit Extension Specialist, WSU Extension
Postharvest Information & Technology Transfer
References and Further Reading
- Hanrahan I. Prediction and mitigation of rain-induced cherry cracking. WTFRC Final Report; 2014. Available at: http://jenny.tfrec.wsu.edu/wtfrc/PDFfinalReports/2014FinalReports/Hanrahan_Final_report_NO_METHODS.pdf
- Lang G, Guimond C, Southwick S, Kappel F, Flore JA, Facteau T, et al. Performance of calcium/sprinkler-based strategies to reduce sweet cherry rain-cracking. In: III International Cherry Symposium 468. 1997. p. 649–656. Available at: http://www.actahort.org/books/468/468_81.htm
- BCFGA. Fruit Tree Nutrition. In: British Columbia Tree Fruit Production Guide. Available at: http://www.bctfpg.ca/horticulture/fruit-tree-nutrition/
- Bush M. Cherry Rain Cracking. In: 2017 Crop Protection Guide for Tree Fruits in Washington (EB0419). Pullman, WA: Washington State University; 2017. Available at: http://www.tfrec.wsu.edu/pages/cpg/Cherry_Rain_Cracking
- Cultiva. Parka Label. Parka Label. 2017. Available at: http://www.cultivaipm.com/wp-content/uploads/2014/06/Parka-SPEC-11.14.16-w_watermark.pdf
- Valent Biosciences. RainGard® – The best defense against cherry cracking. 2015. Available at: https://www.valent.com/agriculture/products/raingard/loader.cfm?csModule=security/getfile&pageid=78692
- Lang G. Stone Fruit: Avoiding cherry fruit cracking is a balancing act. Growing Produce. 2014. Available at: http://www.hrt.msu.edu/uploads/535/78639/AFG-Cherry-Fruit-Cracking-Lang.pdf
- Hanrahan I. Bench-top test to determine cracking susceptibility of sweet cherry. WTFRC; 2013. Available at: http://www.treefruitresearch.com/images/stories/2013_Cracking_Susceptibility_Grower_Test_Kit.pdf