Control of bitter pit in 'Honeycrisp' apples (Malus xdomestica) from trees treated during the growing season with foliar sprays of trifloxystrobin fungicide and calcium was evaluated in four replicated trials over 2 years. All trials were in commercial orchards of 'Honeycrisp' trees that were 3 to 6 years old. The effectiveness of combining boron with foliar applications of calcium chloride (CaCl2) was evaluated in two trials, and effectiveness of harpin protein, used either alone or in alternating sprays with CaCl2 was assessed in one trial. Trifloxystrobin applied twice during the 30 days before harvest reduced bitter pit incidence at harvest in one of the four trials, but the reduction was transitory, no longer being evident when fruit were re-evaluated after 63 days of cold storage. Harpin protein did not affect disorder incidence. Calcium sprays failed to control bitter pit in treatments where the total elemental calcium applied was less than 2.7 lb/acre (3.03 kg·ha-1) per year for tree canopies that were sprayed to drip using 100 gal/acre (935.4 L·ha-1) of spray solution. In the two trials where some treatments involved application of at least 2.9 lb/acre (3.25 kg·ha-1) of elemental calcium per season, the incidence of fruit with bitter pit at harvest was reduced by 76% to 90%. Effectiveness of calcium sprays for bitter pit control was not enhanced by superimposing trifloxystrobin, boron, or harpin protein treatments. Flesh firmness at harvest was lower in calcium-treated than in non-treated fruit, and fruit maturity was more advanced on trees receiving boron sprays than on trees receiving no boron. In one trial, where the first calcium application was made approximately 2 weeks after petal fall and 4 days prior to a fruit thinning spray, crop load of trees that received calcium sprays, measured as number of fruit per cm2 trunk cross-sectional area, was 38% greater than on trees that received no calcium sprays. CaCl2 provided better control of bitter pit in 'Honeycrisp' than any of the other materials tested.
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