Cork Spot and Bitter Pit Fruit Disorders 

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Cork Spot and Bitter Pit Fruit Disorders

Cork Spot and Bitter Pit Fruit Disorders Cork Spot and Bitter Pit Fruit Disorders

Cork spot and bitter pit in York Imperial, Delicious, and Golden Delicious apples, along with other calcium-deficiency physiological disorders, continue to cause apple producers economic losses. These disorders appear to be specifically related to low levels of calcium and sometimes high levels of nitrogen in the fruit flesh. Fruit flesh calcium content is influenced by many factors. Good horticultural management techniques that improve soil conditions, encourage uniform annual cropping, and encourage moderate tree vigor will decrease calcium-related fruit disorders.

Corking, characterized by spherical dead areas in the flesh, is an orchard disorder, while a bitter pit is primarily a storage disorder and usually attacks the skin and adjacent cells. However, symptoms vary according to area, variety, and environmental conditions, making this distinction less than clearcut.

While corking is worse under conditions of low calcium, lack of calcium does not appear to be the sole cause. Corking is worse under conditions of excessive tree vigor or moisture stress and has been most severe on Delicious and York Imperial. Bitter pit is worse on Golden Delicious and is aggravated by early harvest.

During the past 20 years, much research has been conducted on calcium nutrition of apples in South Africa, Australia, New Zealand, England, and the United States. From this research, as well as research conducted in Pennsylvania, a comprehensive program is recommended to reduce corking and bitter pit.

A program to control bitter pit and corking should involve almost all cultural practices conducted in apple production, since no one practice guarantees control of the disorder below the economic injury level. An effective program should be based on the consideration of all five factors explained below, since in any specific orchard block one factor could be primarily responsible for the problem. Growers are urged to use these recommendations when the cost of control practices is less than losses from the disorders.

The five points in the program are listed as a person should think of them in the life of an orchard and not in order of effectiveness. Calcium chloride sprays, though listed last, offer many advantages over other parts of the program mainly because they can be started in June the year of harvest, while some of the other practices take years to accomplish.

Soil conditions

Poor soil conditions can contribute to low-calcium fruit; factors to consider are explained in Table 1-11. Water stress caused by either excessive or deficient soil moisture can contribute to increases in corking and bitter pit. Tiling to remove excessive moisture and irrigation to supply supplemental water should be practiced as appropriate.

Correcting low soil pH with agricultural limestone is recommended to reduce the availability of soil aluminum and manganese, thereby maximizing the size of the root system. The magnesium content of the lime should be regulated by the tree’s requirement for magnesium and the total amount of lime needed. High-magnesium (dolomitic) lime should not be used for routine soil pH correction. High-magnesium lime should be used when a soil test indicates the need for lime and a leaf analysis indicates the need for a large amount of magnesium.

Table 1-11. Poor soil conditions that may result in low-calcium fruit disorders, their modes of action, and corrective measures

Modes of actionCorrective measures
Excessive soil moisture
Water stress in trees owing to lack of air for rootsPlant orchards on deep, well-drained soils with good water-holding capacity.
Restricted root system caused by poor root growthInstall drainage tile
Deficient soil moisture
Water stress in trees caused by deficient soil moisturePlant orchards on deep, well-drained soils with good water-holding capacity
Restricted root system owing to poor root growthIrrigate as needed to reduce water stress
Low soil pH
Restricted root growth caused by aluminum and manganese toxicityMaintain soil pH between 6.0 and 6.5 with high lime. Regulate the quantity of magnesium applied in lime with a leaf analysis. Use high-magnesium (dolomitic) lime only in proven cases where leaf analysis indicates magnesium is very low. Dolomitic lime should probably be thought of as a potent magnesium fertilizer and also as a liming agent.

Balanced nutrition

Soil testing to check soil pH and leaf analysis to determine the plant’s uptake of essential nutrients are necessary in managing an orchard fertilization program. Listed in Table 1-12 are the nutritional elements that need to be managed to produce high-calcium fruit, their modes of action, and grower actions. Avoid excessive levels of nitrogen, potassium, and magnesium and deficient levels of calcium, boron, and zinc, since these conditions may contribute to deficient fruit-calcium levels.

Although soil testing and leaf analysis are not practiced by all growers, they are universally recognized as the best methods on which to base a sensible nutritional program. These two tools will minimize wasted money on unnecessary fertilizer, prevent the application of nutrient elements already present inadequate or excessive amounts, and recommend the application of only those fertilizer elements necessary to ensure a profit for the grower. Sharp orchard managers use these tools.

Table 1-12. Nutritional imbalances that may interfere with the production of high-calcium apples, their modes of action, and corrective measures

Modes of actionCorrective measures
Excessive nitrogen (N) 
1. The flesh of fruit from high N trees is more likely to have corking (direct effect)
2. High N trees normally are overly vigorous (indirect effect).
Regulate the N status of trees with the aid of leaf analysis and field observations. Keep other nutrients in balance so the desired vigor level can be attained with minimal N levels.
Excessive potassium (K) 
1. Some calcium deficiency disorders appear to be related to high levels of K as well as low calcium.
2. Direct cation competition between K and calcium in soil and at the root surface.
1. Regulate the K status of trees with the aid of leaf analysis.
2. Do not apply K unless it’s definitely needed
Excessive magnesium (Mg) 
1. Some calcium deficiency disorders appear to be related to high levels of Mg as well as low calcium.
2. Direct cation competition between Mg and calcium in soil and at the root surface.
1. Regulate the Mg status of trees with the aid of leaf analysis.
2. Do not apply Mg unless it’s definitely needed.
3. Do not correct low soil pH with high magnesium (dolomitic) lime.
Deficient calcium (Ca) 
Many physiological disorders of apples are directly related to low fruit flesh Ca levels although low Ca may not be the direct cause.1. Maintain a soil pH of 6.0 to 6.5 with high-calcium lime.
2. Use high-magnesium (dolomitic) lime only in cases with a proven need for large quantities of magnesium.
3. Apply Ca sprays.
4. Use all other parts of the program to increase fruit Ca levels.
Deficient boron (B) 
1. B deficiency can directly cause fruit flesh deformities.
2. Some B deficiencies appear to increase corking.
3. Some B deficiencies appear to interfere with normal translocation of calcium
1. Regulate the B status of trees with the aid of leaf analysis. Maintain 35-60 ppm of leaf B.
2. Make ground applications of borax or tree sprays of boron when needed.

Calcium sulfate

The use of gypsum (calcium sulfate), also called land plaster, to correct poor soil structure is a reasonably old practice. Gypsum can improve soil structure by increasing the aggregation of several small soil particles into larger particles. This can result in faster rates of water infiltration. Gypsum also is used as a source of calcium for soil applications on some crops. Peanuts in the southeastern U.S. are routinely side dressed with about 1,000 pounds of gypsum annually. The use of gypsum to improve the calcium status of apple trees and fruits has been investigated in only two North American sites.

In Massachusetts, a series of studies have been conducted on Delicious and Cortland apple trees. In general, it appears that it takes 2–3 years for results to appear in the fruit. The treatments raised leaf and fruit calcium levels (20% and 10%, respectively), depressed leaf and fruit magnesium levels (20% and 5%, respectively), but had no effect on leaf and fruit potassium levels. Little or no effect was seen on fruit firmness at harvest or after storage, but bitter pit (50% reduction) and senescent breakdown after storage were reduced. Although high rates were used in early studies, in later studies it appeared that annual rates of as low as 3–4 tons per treated acre per year were effective. In a Nova Scotia study, annual applications of 5 tons per treated acre, for 5 years, raised leaf and fruit calcium levels in years 2–7 of the study but lowered magnesium levels.

For growers who have persistent calcium problems and who feel adventurous, the following treatment regime is suggested on an experimental basis. Apply 3 tons per treated acre, banded under the trees. This likely will be between 1.5 and 2.0 tons per acre of orchard. Follow a strict soil and leaf analysis program on an every-other-year basis to avoid nutritional problems. Special attention should be paid to magnesium, as gypsum may depress magnesium uptake.

Gypsum comes in various grades based largely on its color, with white grades being more expensive. For this use, the only relevant characteristic is the percentage of calcium in the product. The use of gypsum will not reduce the grower’s need for lime.

Moderate tree vigor

Since the vegetative portions of a tree have relatively high concentrations of calcium and are seldom deficient in it, excessive tree vigor can use calcium that otherwise might be available for the fruit. Table 1-13 contains a list of factors that need to be considered to promote moderate vegetative vigor in apple trees. Excessive pruning and nitrogen fertilization, coupled with overcrowding of trees, are often interrelated and can result in overly vigorous trees. Excessive tree vigor can also result from an inadequate fruit load. Growth regulators should be used to obtain a uniform fruit load in order to promote uniform, moderate tree vigor.

Table 1-13. Causes of excessive vegetative growth that may compete for available calcium, their modes of action, and corrective measures

Mode of actionCorrective measures
Excessive pruning
Severe pruning can overinvigorate an apple tree.1. Reduce tree vigor so that moderate pruning can be used to maintain tree size.
2. Maintain an annual, moderate pruning program.
Excessive nitrogen (N)
Excessive N fertilization often results in overly vigorous trees.Maintain a nutritionally healthy tree so that a minimum level of N can be used to maintain moderate tree vigor.
Inadequate spacing
Planting trees too close together can result in a vicious cycle of excessive pruning followed by excessive vigor.Integrate variety, rootstock, soil type, and your management intentions into plant spacing considerations.
Low fruit load
Trees bearing a light crop normally divert growth into excessive vegetation.Maintain a system of annual cropping to avoid excessive tree vigor.

Moderate fruit density

High levels of corking and bitter pit may be found on trees with a light crop. When trees bear a light crop of apples, the fruits are normally very large and low in calcium. They are prone to low-calcium physiological disorders. Apples on trees with an excessively large crop usually have little corking and bitter pit but seldom reach optimum size to maximize profitability. Table 1-14 contains a list of factors that need to be managed to produce annual crops of moderately sized fruit. Some factors to be managed for uniformity of cropping are frost protection, pollen source, bee population, and pollinating weather.

A prerequisite for achieving moderate annual fruit density is the annual production of high-vigor fruit buds. An essential ingredient in this program is the effective use of growth regulators to thin excessive crops and to encourage the production of high-vigor flower buds for the following year’s crop. Many registered growth regulators are available for this purpose, including NAA, NAAm, Ethrel, and Sevin. See other sections of this guide for current recommendations in the proper use of these products.

Table 1-14. Factors that may result in a small crop of large fruit, their modes of action, and corrective measures
Mode of actionCorrective measures
Poor fruit bud formation
Production of annual crops must integrate the production of a fruit crop and a bud crop each and every year.1. Maintain moderate tree vigor and tree health to encourage production of good numbers of fruit buds each year.
2. Use growth regulators to encourage fruit bud formation.
3. Thin excessive fruits to encourage annual production.
Insufficient desirable pollen
Without adequate pollen for cross-pollination, a crop cannot be produced.1. Plant two desirable pollen-source varieties for the main varieties in all blocks. Can be crabapples.
2. Add bouquets at full bloom.
3. Plant additional pollinizers in mature orchards.
4. Graft additional pollinizers onto trees of the main variety.
5. Use pollen inserts in beehives.
Poor weather conditions
Cross-pollination by insects is most easily accomplished in warm sunny weather.
Not enough bees
1. Cross-pollination by insects is considered essential for fruit set.
2. Additional bees should normally be placed in an orchard if other factors are not optimal.
Under normal conditions, use one hive for each acre.
Frost damage
1. Eliminating or reducing the crop severely upsets any management system for annual production.
2. Crop reduction results in stimulation of vegetative growth and excessive fruit size. frost damage.
1. Select frost-free sites for orchards.
2. Use techniques such as wind machines or overhead irrigation to reduce.
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Calcium sprays

Sprays of calcium chloride have been successful in reducing or commercially controlling corking and bitter pit, but seldom have these sprays completely eliminated the problem. Extensive research has been conducted around the world to define the products, rates, and timings that will minimize the incidence of low-calcium physiological disorders in apples. The major portion of the research has been conducted on Golden Delicious and York Imperials. However, recommendations developed from research in Pennsylvania have effectively controlled corking and bitter pitting in nearly all varieties.

The effective use of calcium chloride tree sprays may be the most cost-effective, quickest cultural practice for reducing low-calcium physiological disorders in apples. We recommend applying 15 to 50 pounds of calcium chloride per acre per season in six to eight cover sprays. Calcium in the form of calcium chloride is recommended because of its proven effectiveness and lower cost.

Other products that supply calcium are available. Many are recommended at rates that supply lower amounts. These products may be beneficial when only small amounts of calcium are needed to correct the deficiency. To evaluate other materials effectively, growers should compare the cost per pound of actual calcium and the amount of formulation needed to achieve an equivalent rate to the 15 to 50 pounds of calcium chloride per acre per season needed to control problems.

15–20 pounds per acre per year: This is the lowest rate that should be used. It will give some control of bitter pitting and corking, will cause no leaf burning, and will probably not enhance the storage life of the fruit.

20–30 pounds per acre per year: This rate should give good control of preharvest physiological disorders and probably should be the standard rate where these disorders are chronic problems. It will not cause any significant leaf injury and will probably not enhance the storage life of the fruit.

30–40 pounds per acre per year: This rate should give excellent control of corking and bitter pitting and should be the intermediate rate for Pennsylvania. It may somewhat enhance the storage potential of apples and should result in almost no leaf injury.

40–50 pounds per acre per year: This is probably the highest rate that should be used in Pennsylvania and should give outstanding control of corking and bitter pit. This rate may result in some slight burning on the edges of the leaves, but it usually does not appear until mid-September or October. This rate may enhance the storage life of the fruit.

Applying calcium chloride sprays

Time of application: Include in all cover sprays. Do not premix calcium chloride with Solubor in a small volume of water before adding to the tank, when both materials are to be applied together.

Gallons per acre: No restrictions; sprays with as little as 20 gallons per acre have been effective.

Compatibility: At the rates recommended, calcium chloride and/or Solubor may be mixed with spray oil (Superior 70 Sec.), with WP formulations, or with EC formulations of the more common fruit pesticides. Compatibility of materials other than calcium chloride is uncertain, and growers should either check the label for information or conduct a compatibility test in a small jar.

Leaf injury: Some leaf injury may occur from calcium chloride sprays following wet, cool springs or hot, dry summers. When an injury is noticed, reduce calcium chloride to one-half the rate in the next spray or delete calcium chloride from the cover sprays until a one-half inch of rain has fallen.

Equipment: Calcium chloride can corrode some types of spray equipment. Few problems have occurred if sprayers and tractors are rinsed after use. The newer sprayers made of stainless steel, fiberglass, or various plastics that are rust-resistant are desirable.

Special considerations: If early-maturing cultivars continue to exhibit bitter pitting and storage breakdown after the standard rate of calcium chloride has been used, a higher rate should be used. Only calcium that hits the skin of the fruit can increase fruit quality. Therefore, in the standard program, 8 pounds of calcium per acre per year may be applied to Delicious, Rome Beauty, and Golden Delicious, but early-maturing cultivars such as Summer Rambo may be receiving only 4 pounds of calcium per acre per year prior to harvest.

In summary, many factors influence fruit calcium concentration, and since it is difficult to raise fruit calcium level, growers should use all methods possible to gain the upper hand against corking, bitter pitting, and other low-calcium-related disorders. Cultural practices involve soil and nutritional factors as well as tree vigor and fruit density.

Determining the amount of elemental calcium in a commercially formulated product

1. Look for, or determine, the percentage of elemental calcium in the product. This should be listed somewhere on the label.

2. For a liquid formulation multiply the percentage by the weight of the material per gallon. For a solid multiply the percentage by the weight of the material you will add to the tank. The result equals the pounds of calcium per gallon or pound of formulated product.

3. Determine the rate of formulated material you intend to apply per acre per application. For a specific calcium product, this is usually listed on the label.

4. Multiply the amount of material per acre by the number of applications to be made during the season. The result equals the amount of total product per acre per season.

5. Multiply the amount of total product per acre per season (from Step 4) by the pounds of calcium per gallon or pound of the formulated product (from Step 2). The result equals the total amount of elemental calcium per acre per season.

6. Compare the result from Step 5 with our recommendation of 4 to 14 pounds of elemental calcium per acre per season.


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