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Apple growers in the eastern United States should start calcium sprays at petal fall and continue them through the season to reduce the incidence of bitter pit on susceptible varieties.

Dr. Steve Hoying, Cornell University pomologist in New York’s Hudson Valley, laid out his recommendations during fruit growers’ schools this spring.

Large-fruited cultivars—Fuji, Jonagold, Cortland, Red Delicious, Golden Delicious, and, especially, Honeycrisp—need repeated applications of calcium, which growers usually apply as calcium chloride. Most calcium is taken into the fruit by mid-July, he said, so sprays should begin soon after petal fall to be most effective.

Bitter pit-sensitive cultivars should be treated with three to four cover sprays containing one to two pounds of calcium chloride (78 percent calcium) or its equivalent per 100 gallons at 14-day intervals beginning seven to ten days after petal fall, he said. 

Two additional sprays of three to four pounds per 100 gallons should be made four and two weeks prior to harvest. 

This recipe provides 7.5-13.4 pounds of actual calcium per acre for an orchard that requires 300 gallons of dilute spray per acre, he said.

Bitter pit can be more prevalent when there is excess tree vigor, winter injury, large fruit, low crop load, excessive pruning, variable water availability and supply, and nutritional imbalances, he said. Nutritional imbalances include high nitrogen levels and low calcium to potassium and calcium to magnesium ratios.

Control of bitter pit on susceptible cultivars requires foliar application of calcium because calcium is relatively immobile in the tree and may be deficient in fruit even when it is found in adequate amounts in the soil or other tree parts, Hoying said.

Most calcium is taken into the fruit by mid-July. From then to harvest, calcium within the fruit is diluted as the fruit increases in size. Thus, sprays must begin after petal fall to be most effective, he said.

The source of the calcium is not as important as getting sufficient amount on the fruit and at the correct time. 

If nitrogen levels are low, Hoying recommends using calcium nitrate for the first two sprays and then calcium chloride for the remaining sprays. In orchards well supplied with nitrogen, applications of calcium nitrate may result in poor-colored fruit that do not store well.

Calcium sprays must contact the fruit for uptake to be effective.  Therefore, water volumes capable of wetting the entire tree are required. 

Calcium products should be compatible with most wettable powders. Calcium chloride and calcium nitrate are not compatible with magnesium sulfate (Epsom salt), Polyram, certain wettable sulfur formulations, and may inactivate Apogee, he said.

If there is doubt concerning compatibility of calcium with pesticides in foliar sprays, consult the manufacturer of the pesticide or use other sources of calcium known to be compatible, Hoying advised.