Mobility within tree:  Low

Uptake Form: Ca²⁺

Function: cell division, cell elongation, cell structure

Deficiency/Toxicity Symptoms:  Calcium is very important for plant tissues particularly for fruit since it is needed in cell walls.  Calcium can affect the supply of other nutrients so calcium deficiency in leaf tissue is difficult to identify.   Since Ca is not mobile in plants, Ca deficiency can develop in fruit which affects its quality.  Calcium plays an important role in apple, pear and cherry fruit quality.  Many studies have examined calcium deficiencies particularly in apple fruit.  The role of calcium in apple and pear, and cherry fruit will be examined separately.

Excess Ca is usually not an issue for plants. High levels of Ca however can lead to competition in the uptake of other mineral cations such as ammonium (NH₄⁺), Mg, K, Na and Fe.

Calcium in Pome Fruit

Bramlage et al. (1980) reviewed the role of Ca and its interaction with N, K, Mg, P and B on the quality of apples and pears in North America.  They reported bitter pit in apples was recognized in 1936 as involving low Ca. Cork spot and other problems such as storage rots, breakdown, early ripening, poor visual quality and damage susceptibility have since been linked to low Ca as well.  Calcium deficiency may also make fruit more susceptible to sunburn.  In apples, bark can develop internal bark necrosis or “measles” with low Ca, high B and sometimes low Mn. 

Bitter pit in apples shows up as small pits in the skin usually near the calyx end. The pit turns brown and the flesh beneath is dry and spongy. Bitter pit is most common on vigorous trees bearing light crops and is especially common on large fruit in the upper part of a tree. Honeycrisp apples are particularly sensitive to bitter pit development.  The symptoms may manifest in the orchard, but more frequently they become apparent in storage. Cork spot appears as small blushed areas on the skin over hard brown spots in the flesh that become corky underneath. 

In Anjou pears only, cork spot (Anjou pit) and alfalfa greening (green stain) can occur. Pears with cork spot develop a bumpy, uneven surface as they approach maturity. Affected areas turn yellow and soften prematurely. The flesh beneath the depressions is soft and turns brown or grey.   Anjou pit occurs more severely in seasons when hot, dry weather precedes picking.  Alfalfa greening (green stain) causes Anjou pears to develop dark green, slightly sunken areas on the skin shortly before picking.  Both Anjou pit and alfalfa greening are physiological disorders, most likely caused by a nutritional imbalance between Ca and N, and excessive irrigation.

Calcium in Cherry Fruit

Winkler and Knoche (2019) reviewed the effects of calcium on sweet cherry physiology.  They could only find limited studies on the uptake of calcium into cherry fruit.  They found many papers reporting the benefits of Ca treatments on preventing  preharvest cracking or splitting from rain and increasing postharvest cherry firmness.  There were inconsistencies on the effects of calcium on cracking among various studies.  Balbontín et al. (2013) reviewed the physiological, molecular and genomic aspects of cherry cracking.  Besides Ca, they concluded that osmotic potential, cultivar, cutin, cuticular waxes, expansin and B-galactosidase may also play a role in cracking.   Further research to understand the role of calcium in cherries is still needed.

Calcium Sources

Calcium is usually applied through foliar sprays every year.  There are several foliar calcium products to choose from.  Calcium chloride and Briner’s Choice are often used.  Briner’s Choice is highly caustic so precautions should also be taken with its use.  It should be noted that the use of lime to correct pH can add calcium to the soil. Some calcium products such as calcium chloride should not be tank mixed with other nutrients or pesticides.  For example, considerations for using calcium chloride are:

1)  It is compatible with wettable powder formulations of Imidan.

2)  Do not mix with spray oils or emulsifiable concentrate formulations of pesticides.

3)  Do not add spreaders, stickers or emulsifiers unless specific instructions for their use are given.

4)  Sprays should be applied promptly and not left in the spray tank longer than necessary.

5)  It is not compatible with Mg, Zn or other plant nutrient minerals.

6)  It is not compatible with captan, carbaryl (Sevin), thiophanate-methyl (Senator).

7)  Application of unbuffered calcium chloride almost always causes non-serious burn of leaf edges, but severe leaf and fruit injury is possible under some conditions.

Calcium Application

Calcium sprays are most effective when applications start during the period of fruit cell division (petal-fall) when demand and interplant competition is high.  The next good timing is later in the season when the fruit is larger and acts as a bigger target.  To reduce the chance of severe injury with foliar sprays, be careful not to exceed recommended rates.  Do not apply the spray under wet, humid, or slow drying conditions.  Spraying when temperatures are high may also produce injury.  Although the safe upper temperature limit is not known, it is suggested that damage may occur at about 27°C.  In hot weather, growers should apply the spray during the coolest part of the day.   

The choice foliar spray product should be applied at 10 day intervals beginning at petal-fall.  For specific recommendations, consult your field representative or horticultural consultant.  In general, the following information can be used as guide. 

1)  Apples – Most orchards should aim for a total application rate of 10 kg/ha (4 kg/acre) actual calcium per orchard per year, however, more may be required in bitter pit prone varieties or cold wet springs. 

2)  Pears – Do not apply at the apple rate as this may result in leaf and/or fruit burn. Apply three or more sprays at 2-week intervals beginning mid-May.  Apply Briner’s Choice (34.5% calcium). For dilute applications use 0.4 kg/450L of water.  Apply Briner’s Choice at up to 4.5 kg/ha (1.8 kg/acre).

3)  Stone Fruit – Some calcium products, such as Caltrac, can cause visible residue issues on many stone fruit that don’t go through a vigorous washing process before market.  Early applications at the petal-fall or husk-fall stages are also beneficial for fruit quality in all stone fruit.  Consult charts for application rates of specific products.

4)  Preventing Cherry Cracking in Rain - This application may not always work and is based on research using overhead irrigation application.  Calcium sprays to prevent rain splitting go on during the rain event. 

First spray:  3.2 kg CaCl₂/100 gal + 200 mL Agral 90

Subsequent sprays:  1.6 kg CaCl₂ /100 gal + 200 mL Agral 90

Can be applied every two hours during continuous rain showers.  Potential problems are reduced fruit size if applied too often and some leaf burning due to the high concentration of salts.

Calcium sprays alone cannot be expected to eliminate all calcium related fruit disorders. There are several cultural factors that impact the amount of calcium in the fruit as well.  These include:

1)  Reduce N application to moderate tree vigour.

2)  Avoid dormant pruning practices that stimulate excessive growing points and growth in tree.

3)  Do not over or under irrigate in the spring.

4)  Lime when soil pH is low.

5)  Use bees to ensure good pollination for crop load and full seed set for drawing calcium to fruit.

6)  Practice summer pruning in vigorous trees mid-July to expose fruit to calcium sprays.

7)  Do not apply excessive amounts of potassium in the spring.