Although ‘Honeycrisp’ apples are very popular with consumers, they are prone to a number of production difficulties, which in turn make them challenging to grow and store. One of these challenges is bitter pit, a physiological disorder that has long been associated with low fruit Ca content (Schupp et al., 2001a, 2001b), though greater Ca flesh mineral content is not always associated with lower bitter pit incidence (Watkins et al., 2004). Calcium is not very mobile within trees, so apple growers must apply foliar Ca on multiple occasions through the growing season. Bitter pit symptomatically appears as dark sunken lesions on the peel and brown, oxidized flesh that may extend up to 1 cm below the fruit surface. The disorder may be visible at harvest, but more commonly, it develops after several weeks in storage. Bitter pit is reportedly exacerbated under hot, dry growing conditions, in young orchards (less than 5 years old), when fruit size is large, when boron is deficient, and/or when soil pH is low (Fallahi et al., 2010; Rosenberger et al., 2001; Telias et al., 2006; Watkins et al., 2004).
Apple growers mitigate bitter pit by applying Ca to apple trees, most often in the form of CaCl2 (Neilsen et al., 2005; van Goor, 1971). However, CaCl2 is difficult to mix into solution and can cause salt burn on the leaves after repeated applications (especially in hot, dry seasons). In addition, CaCl2 is corrosive to tractors and sprayers. Calcium products that do not have these negative side effects have long been sought, but it has been difficult to match the efficacy of CaCl2.
Several new Ca products have been brought to the market recently, and several more are being tested for potential commercialization. For example, Miller Chemical and Fertilizer Corp. of Hanover, PA (Miller Chemical) has a product that contains the proprietary CaT (calcium technology), a patented “calcium absorption aid” produced by Plant Impact PLC (Preston, UK) that is reported to chemically bond Ca to create molecules that, in the absence of auxin, can operate the auxin–calcium counter-transport pump. By overriding this mechanism, it should theoretically allow Ca to be taken in by plant organs that would otherwise be poor Ca sinks; i.e., apple flesh tissue. Fertilizer products with CaT, such as InCa™ (Miller Chemical) and similar experimental products reported in this article, are advertised to improve plant growth and fruit quality by reducing the negative effects of abiotic stresses, but there are few published reports about these products in the scientific literature. Sweet cherry (Prunus avium) trees treated with InCa™ increased Ca concentration and dry matter in fruits and leaves relative to an untreated control; however, this experiment did not compare InCa™ against CaCl2 or other Ca products (Mikiciuk et al., 2015). In addition, there are several Agro-K (Minneapolis, MN) Ca products (i.e., Sysstem-Cal™ and Vigor-Cal™), as well as the antitranspirant Vapor Gard® (Miller Chemical), which is thought to reduce transpiration rates and thus mitigate bitter pit development. Similar to InCa™, peer-reviewed reports on Sysstem-Cal™, Vigor-Cal™, and antitranspirant are lacking.
The overall goal of our experiments was to develop useful recommendations for reducing bitter pit in mid-Atlantic U.S. ‘Honeycrisp’ orchards. Our specific objectives were to 1) test the efficacy of several proprietary liquid Ca formulations, 2) test the hypothesis that an antitranspirant can reduce bitter pit incidence, 3) determine CaCl2 application rates for control of bitter pit in ‘Honeycrisp’ apples, and 4) compare total Ca rates per year among the different product formulations.
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