Bitter pit is a disorder, not a disease. It is caused by low levels of calcium in the fruit and it is more common after hot, dry summers. It can usually be reduced or, sometimes, prevented with good cultivation practices.

Small sunken pits develop on the surface of the fruit and the flesh beneath the pits is discoloured and dry. In severe cases, brown areas of tissue are scattered throughout the flesh of an infected apple and it takes on an unpleasant, bitter taste.

Symptoms can appear from when the fruits are about half developed until they are harvested or, often, do not develop until the fruits have been stored.

It is more common on young, vigorously-growing trees, especially those fed heavily with nitrogenous fertilisers; but it can also develop on fairly old trees, especially culinary cultivars with large fruit.

Some cultivars are particularly susceptible: 'Bramley's Seedling', 'Cox's Orange Pippin', 'Egremont Russet', 'Hamling's Seedling', 'Meridian', 'Merton Worcester', 'Newton Wonder' and 'Warner's King'.

Resistance: 'Jonagold' and 'Gala' appear unaffected by bitter pit.

Correct feeding and watering to maintain steady growth throughout the growing season is the key to reducing problems with bitter pit.

• Use a general-purpose, balanced fertiliser and avoid excessive feeding with nitrogenous (such as sulphate of ammonia) or potassium-rich (such as sulphate of potash) fertilisers
• Install irrigation to maintain a uniform supply of water throughout dry periods and mulch to retain moisture in the soil around the tree
• Summer pruning of apple trees reduces the leaf area, which helps to control the vigour of trees and redirects calcium to fruits as well as foliage. However, avoid heavy pruning

Foliar sprays of calcium nitrate can be applied from mid-June to mid-September to increase the concentration of calcium within the developing apples.

• The more sprays the better the fruit quality, but it is unlikely that most gardeners could apply more than six per year. In most cases this would greatly reduce damage
• Recommendations are 110g of calcium nitrate per hectare or 11g per square metre per season. For home gardeners the RHS suggests dissolving 10g of calcium nitrate per litre of water and applying this as a fine spray to trees to the point where the solution wets the leaves but does not run off
• Add a wetting agent or a few drops of detergent to help products wet the leaves
• How much solution is required to be made up will depend on the number and size of trees. For example two small trees occupying 12 square metres will require 132g of calcium nitrate per season, which at five sprays would require 26g per session, or about 2.5l of solution
•  It is unwise to mix calcium nitrate solution with other fertilisers or with fungicides or insecticides
• Spray only when temperatures are below 21°C (70°F) and only in the evening to avoid 'russeting' (where apple skins develop rough, brownish patches)
• A very few cultivars are sensitive to calcium nitrate so spray with a half-strength solution. If in doubt treat a branch and wait five days. If no damage is seen, the tree can be treated

Harvesting affected fruit

• Fruit should be left on the tree until it is fully ripe. Unripe fruit are more likely to have low calcium levels
• Where the brown patches only appear near the skin, they can be removed by peeling the fruit. Bad cases may make the flesh inedible
• Cook up affected fruit immediately after harvest before the disorder progresses (sugar should disguise any bitterness)
• Fruit from trees with a recurrent bitter pit problem can be frozen, rather than stored, to prevent the disorder developing

Bitter pit is caused by low levels of calcium in the fruit and poor distribution of calcium within the tree during fruit development. However, it is rarely due to a deficiency of calcium in the soil and can even occur in trees growing on chalk.

Bitter pit is more usually connected with an irregular supply of water, which prevents calcium being taken up and circulated around the tree. Problems are generally worse in seasons when there are wide fluctuations in rainfall and temperature and a shortage of water to trees at critical times during fruit development.

It is also worth noting that excessive use of nitrogen, potassium and magnesium fertilisers can cause or exacerbate the problem.

Low levels of calcium are also thought to be a cause of discolouration of pear and quince flesh and a susceptibility to translucent, water-soaked areas in the flesh of apples (a problem known as water core).