Start-Timing for Calcium Chloride Spray Programs Influences Fruit Calcium and Bitter Pit in ‘Braeburn’ and ‘Honeycrisp’ Apples

ABSTRACT

Early-season sprays of calcium chloride (CaCl₂) have been reported to sometimes be more effective than later-season sprays for controlling bitter pit in apple (Malus × domestica Borkh.). We conducted a two-year study examining the influence of start-timing of CaCl₂ spray programs on fruit Ca concentrations and at-harvest bitter pit incidence in ‘Braeburn’ and ‘Honeycrisp’ apples in Washington State, USA. Six biweekly sprays of CaCl₂ were applied starting in mid-May (early-start), mid-June (mid-start, the normal commercial start timing), or mid-July (late-start) 2002 and 2003. Although leaf marginal necrosis occurred in all of the CaCl₂-treated plots, no spray damage on fruit was observed. All of the CaCl₂ spray treatments consistently reduced bitter pit incidence relative to the unsprayed control treatment and usually but not always increased the Ca concentration of subdermal cortical tissue at harvest. The early and mid-start spray programs produced lower bitter pit incidence than the late-start program in only one of four situations. Fruit Ca concentrations in the control and early-start spray program did not differ. Fruit Ca concentrations in the mid- and late-start spray programs were not different, and often were higher than that of the early-start spray program. Bitter pit incidence for each cultivar was inversely related to fruit Ca concentration. Substituting cortical Ca/potassium (K) or Ca/magnesium (Mg) concentration ratios for Ca concentration failed to substantially improve and usually degraded the correlations with bitter pit incidence. The results confirm that starting CaCl₂ spray programs in June sometimes is more effective at controlling bitter pit that delaying the start of the spray program until July. The later season sprays tend to be more effective at increasing fruit Ca concentration. There appears to be no detectable advantage for starting spray programs earlier than June. Applying CaCl₂ sprays throughout the growing season, starting sometime in June, appears to be the most economical and effective spray management practice for maximizing fruit Ca and minimizing risk of bitter pit development.

Keywords::

• Malus × domestica
• tree fruit
• mineral nutrition
• fruit quality
• bitter pit
• calcium

ACKNOWLEDGMENTS

This research was conducted under Project WNP0747, College of Agricultural, Human, and Natural Resource Sciences, Washington State Univ., Pullman. Funding support was provided by the College and by the Washington Tree Fruit Research Commission. I thank Jennifer Moore and Casimir Lorentz for their technical support.

Notes

^z All concentrations are expressed on a dry matter basis.

^y Postbloom Zn sprays were applied.

^x Postbloom Mn spray was applied.

^w Postbloom Na polyborate spray was applied.

^z F-test statistic (4, 12 degrees of freedom except 3, 11 df for ‘Honeycrisp’ bitter pit incidence in 2003).

^y Level of significance for F-test. Boldface indicates significance at P <0.05.

^x Percent of field-run fruit, untransformed. ANOVA for bitter pit, physical damage, and total damage was based on arcsine transformation of percentage data.

^w Within-row mean values followed but different letters are significantly different according to Duncan's multiple range test, P = 0.05.

^z F-test statistic (4, 12 degrees of freedom).

^y Level of significance for F-test. Boldface indicates significance at P <0.05.

^x Within-row mean values followed but different letters are significantly different according to Duncan's multiple range test, P = 0.05.

^z F-test statistic (4, 12 degrees of freedom).

^y Level of significance for F-test. Boldface indicates significance at P <0.05.

^x Within-row mean values followed but different letters are significantly different according to Duncan's multiple range test, P = 0.05.