Bitter pit in apples

Introduction

Bitter pit is a nutritional disorder in apples caused by calcium (Ca) deficiency in the fruit as it matures. It results from a complex relationship between climate, nutrition and tree vigour. Bitter pit symptoms often appear before harvest or after long-term storage.

Bitter pit is usually seen in fruit that receives an irregular water supply, which prevents calcium from being taken up by the tree. Bitter pit is generally worse in seasons when rainfall and temperature fluctuate, combined with water shortage to trees at critical times during fruit development.

Excessive use of nitrogen, potassium and magnesium fertilisers can also cause or worsen the problem.

Symptoms

Bitter pit will start internally, but as it progresses, small bruise-like spots develop on the skin. When the cells underneath dry out and shrink, sunken ‘pits’ about 3 to 6 mm in diameter form. These spongy areas can look cone-shaped when the fruit is cut in half. Brown areas about the same diameter as surface spots might also develop further into the flesh in severe cases, though symptoms most commonly occur close to the skin.

Spots usually develop first at the calyx end of the fruit, but highly susceptible varieties can be damaged right up to the shoulder. Often, the damage is more obvious on the shaded side of the fruit, where the skin is paler.

External bitter pit symptoms often worsen when the fruit is in cold storage. Fruit with no obvious symptoms at harvest can develop skin pitting in storage. Badly affected flesh often has a bitter taste, which gives the disorder its name.

Factors favouring development

Bitter pit can occur even in soil with good calcium levels. Calcium moves from the roots into the xylem and is carried to areas where transpiration occurs. It moves slowly into other parts of the plant. Bitter pit results when there is competition between leaves and fruit for calcium.

Water availability

Low soil moisture reduces the ability of the tree to take up calcium from the soil and transport it to the fruit. This is why the trees must get adequate water during cell division and cell expansion.

Climate

Hot, dry weather during fruit growth can cause the calcium to concentrate in the leaves if soil moisture levels are low. Excessively hot days will cause trees to shut down by closing the leaf stomata to reduce water loss through transpiration. When transpiration stops, calcium transport in the plant is reduced.

Nutrition

Excess nitrogen favours vegetative growth. Fruit does not receive enough calcium when it has to compete with shoot growth. Nitrogen should only be applied at levels that maintain balanced fruit and leaf growth.

Excessive amounts of potassium (K) and magnesium (Mg) can directly compete with calcium within the fruit cells. Potassium, Mg and Ca are all positively charged ions, but due to their chemistry, K and Mg are often taken up in preference to Ca.

Soil and tissue testing results should be used to guide fertiliser decisions and to ensure the plant has balanced nutrition. Check the soil pH, as calcium uptake can be limited if it is less than 6.0.

Tree vigour

Bitter pit occurs more commonly on young trees and mature trees with light crops. Excessively vigorous trees direct their energy into vegetative growth at the expense of fruit development. Heavy pruning and excess nitrogen will promote vigorous leafy growth and fewer fruit. Summer pruning can often achieve less regrowth than severe winter pruning.

Crop regulation

Thinning techniques decrease the number of fruit on the tree to optimise fruit size and quality, also reducing biennial bearing. Bitter pit is more likely to occur in an 'off’ year when crop load is light and the fruit are large. Thinning on susceptible trees should be modified to achieve a slightly heavier crop than usual.

Variety

Some varieties, such as Granny Smith, are more susceptible to bitter pit than others.

Control

Foliar calcium sprays can be applied to apples from fruit set until just before harvest. Good spray coverage is essential because calcium does not move quickly in the plant. A wetting agent might be required with some calcium sprays, so always check the label.

Summary

Bitter pit is a complex condition and good management is critical to avoid it, because applying calcium will not solve the problem if there are physical factors that limit calcium uptake and transport.

• Maintain optimum vigour by careful timing of irrigation, fertilisers and pruning.
• Use crop regulation to maintain a moderate crop.
• Use nutrient testing to apply fertilisers according to the trees’ needs.
• In varieties to be stored, ensure fruit is picked at the correct level of maturity, as immature fruit is more susceptible to developing bitter pit in storage.