Bitter pit is a physiological disorder associated with low levels of calcium in the apple tissue, which reduces apple quality and marketability.
Symptoms are often seen in storage as deep brown/black lesions or spots varying from 0.08 - 0.4 inch in diameter. Shallow small areas beneath the spots resemble small bruises.
A balanced crop nutrition is essential to ensure calcium does not compete with and is not restricted by other nutrients.
Crop Nutrition and Bitter Pit
Phosphorus
Trials have shown that phosphorus improves long-term storage characteristics, reducing bitter pit. However, care has to be taken as some forms of phosphate may contain cations that compete with calcium, which can increase bitter pit risks.
Potassium and calcium
Potassium is one of the most important nutrients for a number of quality characteristics. Yet, it is important that potassium concentrations are in balance with other nutrients, particularly calcium, as preferential uptake of potassium over calcium will significantly induce to greater incidence of bitter pit. It is generally accepted that K:Ca ratio should be around 15 for fruit at 40 - 60g and no higher than 30 at harvest in order to reduce bitter pit problems.
Soil supply is important, but this should be backed by fruit applied sprays targeted to get more calcium into the apple. The greater the number of sprays the better the fruit quality and it is important to continue spraying right through to harvest.
Boron
Boron spray programs also support calcium, reducing bitter pit and Jonathon spot in apples.
Magnesium
Care has to be taken since overuse of magnesium can restrict calcium uptake and increase the risk of bitter pit, as shown in studies with Jonagold apples in Poland.
