Bitterpit on apples
Sorry to hear your apples are suffering from Bitter pit.
Bitter pit is a disorder usually associated with an irregular supply of water, which prevents calcium being taken up and circulated around the tree. The problem is generally worse in seasons when there are fluctuations in rainfall and temperature and a shortage of water to trees at critical times during fruit development.
It is often more common on young, vigorously-growing trees, especially those fed heavily with nitrogen based fertilisers; but it can also develop on fairly old trees, especially culinary cultivars with large fruit. It is worth noting that excessive use of nitrogen, potassium and magnesium fertilisers can cause or exacerbate the problem. Some cultivars are more susceptible than others.
So what can you do about it?
Correct feeding and watering to maintain steady growth throughout the growing season is the key to reducing problems with bitter pit. Although to some extent this is out of your control in wet summers like we’ve just had.
Use a general-purpose, balanced fertiliser and avoid excessive feeding with nitrogen (such as sulphate of ammonia) or potassium-rich (such as sulphate of potash) fertilisers. Harrod Horticultural supplies Maxicrop Organic Plant Cal-Sea-Feed which would be ideal for your trees as it is rich in calcium and magnesium.
Install irrigation to maintain a uniform supply of water throughout dry periods and mulch to retain moisture in the soil around the tree. Strulch Garden mulch is an ideal product to use here.
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. 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.
I hope you find the information helpful and good luck with eradicating the bitter pit.