Apples: Dark spots are signs of bitter pit
Question: We have had a good crop of Bramley apples. The windfalls we have cooked and frozen and were fine. The picked ones we have wrapped in paper and stored in boxes in the dark in the garage. Now we find most of these stored ones are already full of dark brown spots, not fit to eat. Can you please say (a) what causes this and (b) what can we do to prevent it next year? (D Dennis, Ramsey)
This problem is quite common and seems to be worst in Bramley apples in storage. The problem here is called 'bitter pit', which is a disorder that causes dark spots on apples late in the season or in storage. This condition is related to lack of calcium in fruit and is often as a result of dry soil conditions.
Not that 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.
Bitter pit is caused by low levels of calcium in the fruit and poor distribution of calcium within the tree during fruit development.
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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 worsen the problem.
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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').
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, as is often the case, do not develop until the fruits have been stored.
It is more common on young, vigorouslygrowing 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. These include the likes of Bramley's Seedling, Cox's Orange Pippin, Egremont Russet, Hamling's Seedling, Meridian, Merton Worcester, Newton Wonder and Warner's King.
Some cultivars show some resistance to bitter pit. Jonagold and Gala, for example, appear unaffected.
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. Follow the manufacturer's advice on rates and spraying intervals and never mix with fungicides or insecticides. Add a wetting agent or a few drops of detergent to help products adhere to leaves. 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, notably Bramley's Seedling, are sensitive to calcium nitrate so spray with a half-strength solution.
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.
Try to 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.
•This article was first published on November 26, 2011.