LAUSR

Abstract Honeycrisp apple trees are highly prone to biennial bearing and bitter pit (BP). This 3-y study tested the hypothesis that tank mix sprays of ethephon (ETH), naphthaleneacetic acid (NAA), and 1-aminocyclopropane carboxylic acid (ACC) combined with calcium chloride (CaCl2) can mitigate these production problems. Mature ‘Honeycrisp’ were treated with either three of six applications of 150 mg.L-1 ETH, 5 mg.L-1 NAA or two of five applications of 150 mg.L-1 ACC, all tank-mixed with and without CaCl2 and applied at 10-d intervals starting 21–26 June over three growing seasons. Fruit cortical tissue nutrient concentrations were influenced by applications of CaCl2, ETH, NAA, and ACC. In one year, Ca concentrations were significantly higher in fruit from trees treated with six sprays of ETH or NAA with CaCl2 compared with the untreated control. Compared with untreated trees, fruit Ca concentrations were 115% and 148% higher with three and six applications, respectively. Applications of CaCl2 significantly reduced the incidence and severity of BP post-harvest in two of three years when fruit tissue Ca levels were low. ETH alone also increased the incidence and severity of BP two-fold compared with the control, while early season sprays of NAA reduced BP incidence and severity compared with the untreated control. High and similar levels of BP incidence and severity were observed when six sprays of NAA were applied with or without Ca. Fruit tissue K levels were generally reduced with the addition of CaCl2, and within 1 year, K levels were higher in fruit treated with ACC. Fruit Mg concentrations generally decreased with the addition of Ca or NAA. Six applications of ACC resulted in a 30-fold increase in fruit Mg concentrations compared to the untreated control. Tank mixing CaCl2 with the flower-promoting bio-regulators ETH and NAA proved to effectively reduce BP and increase fruit tissue Ca levels. This approach would offer cost savings when repeat sprays of each are required on a bi-weekly basis.