Calcium sprays applied to fruit during the growing season may reduce the incidence of certain fruit disorders and may improve fruit quality. Responses to calcium sprays are not predictable from calcium levels in soil or leaves. Physiological disorders such as bitter pit of apples, cork spot and alfalfa greening of Anjou pears, and cracking and firmness of cherries are often related to calcium content of the fruit; however, the relationships are not precise.
The most commonly used calcium spray material is calcium chloride. Construction-grade calcium chloride contains impurities that can severely damage fruit. Calcium chloride can cause leaf burn and fruit injury, especially at high temperatures and has limited compatibility with pesticides (see sections on Plant Injury-Chemical Combinations, and Limited Compatibility Materials). Calcium nitrate also has been successfully used to reduce bitter pit of apple; however, it is more likely to cause fruit injury than calcium chloride. Calcium nitrate sprays applied at the rates and frequencies used for bitter pit control will not improve green color of green apple varieties and may produce a duller red color in red apple varieties. The practice of using calcium nitrate during the first half of the season then switching to calcium chloride for the latter half has not been tested in Washington but has theoretical merit. Foliar sprays of calcium sulfate may actually increase bitter pit and should not be used. Calcium-containing chelates and organic complexes have not been more effective than calcium chloride. Use only chelates and organic complexes that are specifically labeled for foliar application to tree fruits.
Caution: The risk of calcium chloride or calcium nitrate causing fruit russet rises with increasing number of applications, high rates, and when applied in less than 100 gallons of water per acre. The possibility of fruit injury is highest at gallonages where droplets coalesce and pool on the lower part of the fruit. The hazard is reduced by using low rates and dilute sprays. Avoid spraying calcium chloride or calcium nitrate under slow drying conditions or at temperatures above 80°F to 85°F. Fruit size of cherries may be reduced by calcium chloride sprays.
Bitter pit of apple is a physiological disorder often related to low fruit calcium levels. 6-12 applications of calcium chloride or calcium nitrate applied at periodic intervals from early June to late August will significantly reduce the risk of bitter pit development. Effectiveness varies with variety, orchard location, and growing season. If severe bitter pit is common, more frequent applications may be required. Calcium sprays are not required in orchards that historically have not produced fruit with bitter pit.
Cork spot and alfalfa greening of Anjou pears are physiological disorders which are often reduced by foliar calcium chloride sprays. Pears are more susceptible to calcium spray injury than are apples. Foliar sprays of calcium nitrate should not be used on pears. Apply no more than 4 pounds calcium chloride dissolved in 400 to 800 gallons per acre (the 800 gallon rate is for larger trees). Make four to five applications at 3-week intervals from June to August.
Fruit firmness and rain cracking of cherries are influenced by calcium chloride sprays. Research suggests that three or more sprays applied at weekly intervals before anticipated harvest are likely to reduce fruit softening, postharvest injury, and minor rain cracking. Severe cracking will not be prevented. Fruit size may be reduced.
For apples: 2-4lb/A per application. Make 6 to 12 applications from early June to Late August. 5 to 15 lbs of actual Ca per season is recommended which equals 15 to 50 pounds of calcium chloride per acre per season. Calcium in the form of calcium chloride is recommended because of its proven effectiveness and lower cost. See Penn State Extension’s useful calculator for comparing calcium chloride to other sources of calcium, as it is important to make sure you develop a season-long program for applying sufficient total amounts of elemental calcium.
Rate Recomendations Actual Ca lb/A per season
|Actual Ca lb/A||Expected Results|
|4-5||This is the lowest rate that should be used. It will give some control of bitter pit and corking, will cause no leaf burning and is not likely to enhance storage.|
|6-8||Should give good control of preharvest physiological disorders. It should not cause any significant leaf injury and will probably not enhance fruit storage life.|
|9-11||Should give excellent control of corking and bitter pitting and should be the intermediate rate. It may enhance fruit storage life and shoudl result in almost no leaf injury.|
|12-14||The highest rate that should be used. Should give outstanding control of corking and bitter pit. May result in some enhanced storage life.|
Courtesy Dr. Rob Crassweller, Penn State Extension.
For cherries: Six weekly sprays of Ca(NO3)2 or chelated Ca sources (Ca2+ at 0.1-0.15%) between pit hardening and harvest has been shown to increase calcium quantity in fruit and post harvest quality (Wang, 2016). Greater than 0.2% Ca2+ increases risk of leaf burning and reducing fruit size. Fruit applications do not replace, but only augment, good management of soil Ca, irrigation and root health.