Maximizing Calcium Uptake In Apples

Janet Caprile
Farm Advisor
UC Cooperative Extension
Contra Costa County

Bitter pit is a common problem that shows up in apples just before harvest or, more frequently, after a period of cold storage. It appears as small, dark, bruise-like spots or depressions which tend to be concentrated towards the calyx end of the fruit. The tissue beneath these spots typically becomes hard and corky, and, needless to say, the apples can't be sold for fresh market. While the disorder doesn't show up until late in the season, you should be taking measures now to prevent its occurrence later.

This is a complex disorder that is thought to be related to a localized calcium deficiency in the fruit. I say the problem is complex because our California soils usually contain high levels of calcium. Also trees showing bitter pit in the fruit usually have sufficient calcium in the leaves. The problem seems to be induced by factors that tend to limit the timely uptake or distribution of calcium within the fruit. There are a number of factors that can contribute to this condition.

A low soil pH or a high magnesium content can contribute to bitter pit. Soil pH below 6.0 can reduce calcium availability and should be adjusted by adding a liming material to bring the pH up to 6.0 to 6.5. Avoid using a liming material that contains magnesium (like dolomite) as a high level of magnesium can also interfere with calcium uptake. If your soil has an exchangeable calcium to magnesium ratio of lower than about 5:11 it may be of some benefit to add calcium even if you don't need to adjust your pH. Gypsum (calcium sulfate) is high in calcium but won't affect the soil pH.

If your soil does not have either a low pH or a high magnesium content, bitter pit is more likely to be triggered by excessive tree vigor, over-fertilizing with potassium or nitrogen, uneven irrigation, cooling delays after harvest or varietal susceptibility. All these factors can influence the amount and distribution of calcium in the fruit.

Granny Smith, Golden Delicious, Mutsu, Gravenstein, Yellow Newtown, Jonathan, and Red Delicious are among the more susceptible varieties, although almost any young, extremely vigorous tree may exhibit symptoms.

Adjust your growing practices to minimize excessively vigorous, vegetative growth, particularly on the more susceptible varieties. This means being prudent with your fertilizer. Too much nitrogen can lead to rank growth and too much potassium can interfere with calcium uptake. It also means providing proper conditions to assure good fruit set (bees and pollenizer varieties) and judicious thinning, if needed, to regulate the crop load. Too light a set and overthinning will encourage larger fruit and more bitter pit. In addition, minimizing winter pruning in favor of delayed dormant or summer pruning can also help to reduce orchard vigor.

Providing trees with a steady and sufficient quantity of water throughout the growing season is also important in reducing bitter pit. Calcium, like other nutrients, can only be absorbed by the roots if it is dissolved in water. And being a nutrient that is fairly immobile in the plant, it won't readily move from an area that has enough calcium (like the leaves) to an area that doesn't have enough (like the fruit). Alternating periods of drought and moisture prevent the fruit from obtaining the calcium it needs.

Rapid cooling and avoiding the harvest of immature fruit can significantly reduce the incidence of bitter pit. In a UC research trial in Sonoma County, apples that were held in bins for 24 hours before being placed in a cold storage room developed 44% bitter pit after 2 months of storage. Apples that were placed in a cold storage room within a few hours of picking developed 37% bitter pit over the same period. However, apples that were cooled with a forced air cooler down to 35°(F within a few hours of picking and then in a cold storage room only developed 17% bitter pit.

Using a combination of the above corrective measures may sufficiently control bitter pit. However, if the problem persists, then calcium applications to the fruit are most likely needed. Calcium sprays applied during the summer months or calcium dips applied postharvest have both been shown to be effective.

There are a variety of calcium materials available on the market. The most common types contain calcium nitrate, calcium chloride or calcium acetate. The concentration of calcium in the product can vary with manufacturer; the materials and rates listed in the table below have proven effective as summer sprays in UC tests. Keep in mind that calcium nitrate sprays will contribute nitrogen to your crop so adjust your fertilizer program accordingly to avoid excessively vigorous growth. Also, keep in mind that the repetitive use of calcium chloride may lead to some degree of foliar burn toward the end of the season. Apply sprays under conditions favorable for rapid drying to prevent russetting.

 * meets organic standards

A minimum of 3 sprays applied at monthly intervals beginning in May or June are usually required to achieve control. Dilute sprays (300-400 gal/acre) are most effective as good coverage of the fruit is essential ( calcium doesn't move rapidly within the fruit or from leaf to fruit. For more severe cases, shorter treatment intervals (every 2 weeks) over the same 3-month period are recommended. There is also evidence to indicate that the earlier treatment, beginning in May or even at one-inch fruit size may be more effective in very susceptible varieties.

A single post harvest dip of calcium chloride, in place of in-season sprays, has been effective on some varieties, including Granny Smith and Yellow Newtown, but not on Mutsu and others. A 60 second dip in a 2% (for Yellow Newtown) or 3% (for Granny Smith) solution has typically been used. Time the dip and mix the solution carefully as longer exposures or a higher percent solution can lead to fruit bum. Also, don't use a steel or iron tank for dipping as that can contribute to fruit damage.