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I've got an apple tree in the garden which has produced a good crop, but most of the apples are covered in black spots:

What is this? It seems to be some sort of fungus, but what's the best way to treat / prevent this and are these apples now inedible?

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to go along with "Mancuniensis" answer you may wish to take a listen to, Gardeners' Question Time podcast "12 Aug 11: Out and about in Blackpool" or direct link to MP3 & start listening @ 26min:10sec in. –  Mike Perry Aug 15 '11 at 21:58
    
i work in produce and was told these spots can be caused by hail. –  user749 Nov 6 '11 at 14:58

1 Answer 1

up vote 9 down vote accepted

This is not a disease, but a disorder called Bitter Pit (see photo here) which, like blossom end rot in cucurbits (pumpkins, zucchini etc), is caused by a calcium deficiency:

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.

Royal Horticultural Society/ Bitter Pit

If the pits are only skin-deep:

  • you can peel and eat the apples, although they may taste a little bitter;

  • cook or freeze them as soon as possible after harvesting, to prevent the disorder from progressing.

You should be able to control the disorder by:

  • watering regularly during dry spells so as to minimize fluctuations in the water supply;

  • mulching around the tree to retain moisture in the soil in summer;

  • applying a general-purpose balanced fertilizer in early spring (such as Grow More) and avoiding over-feeding.;

  • pruning lightly in summer to reduce the leaf area and redirect calcium to the fruit.

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Thanks - It's been fairly well watered, but I've a suspicion that it's had a lot of potassium thrown at it, which may be a cause. –  growse Aug 9 '11 at 12:02
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Yes, as the RHS points out, excessive use of potassium can exacerbate the problem, but the prime cause seems to be fluctuations in the tree's water supply and/or lack of in-depth watering or over-watering during dry spells - see this answer: gardening.stackexchange.com/questions/1264/… and aces.edu/pubs/docs/A/ANR-1059/ANR-1059.pdf. –  Mancuniensis Aug 9 '11 at 12:56
    
Michael Phillips in The Apple Grower says the calcium is taken in while new cells are vigorously dividing, but in bitter pit the available calcium was lost to leaves. Pruning of watersprouts during the first 6 weeks of fruit development will help the fruit get its share of calcium, but later you get extra vegetation that must be pruned. –  Erik Olson Oct 17 '11 at 7:21

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