Managing bitterpit in apples

Bitterpit is a complex disorder in apples that is thought to be related to a localized calcium deficiency in the fruit and results from the breakdown of cells just under the skin where calcium levels are too low¹. The problem seems to be induced by factors that tend to limit the timely uptake and distribution of calcium within the fruit³.

Apples use calcium for the maintenance of membranes and cell wall structures, which, in turn, maintains fruit firmness and reduces storage disorders. To control bitterpit and other related disorders, the focus will be on soil, leaf and ultimately fruit calcium levels.

Apples are not efficient at obtaining calcium from the soil and are not especially efficient at translocating calcium from the roots to the leaves and the fruit³. 

The small fruitlets have high calcium levels in the spring, but as the fruit grows in size, fruit calcium concentration decreases. Fruit will accumulate calcium over the entire season, but environmental conditions can cause bitterpit problems². 

Fruit crops benefit from balanced levels of calcium, magnesium and potassium, expressed as a percentage of soil bases. As a general guide, calcium levels should be 60-70 %, magnesium levels 20-30 %, and potassium levels 10 to 20 %. If calcium represents less than 60% of the bases, particularly on sandy soils, gypsum may be helpful as it provides a moderately soluble source of calcium for plant uptake¹.

Soil applied lime and gypsum will increase fruit calcium, but will need to be applied annually with results being slow to detect (2 to 3 years). Generally, frequent spray applications of foliar calcium are best to improve bitterpit control, but might not cure the disorder. In some years on some varieties, weather can overwhelm even the best calcium program². 

As a way to manage bitterpit and other related disorders, focus on increasing calcium levels^2,4 by;

• Keeping soil pH at the optimum range for apples, 6.5 to 7.0.
• Maintain calcium soil levels at abundant levels with the use of gypsum, lime and high cal-lime.
• Keep boron, zinc, magnesium, and potassium levels at optimum levels, deficiencies or excesses will increase calcium disorders.
• Excessive amounts of potassium and magnesium in the soil will compete with calcium, limiting calcium uptake by the plant.
• Excessive nitrogen, tree vigour, pruning, as well as low fruit loads will increase bitterpit incidence.
• Try to achieve uniform moderate tree vigour with annual moderate crop loads.
• Maintain irrigation to reduce drought conditions as well as calcium stress in the tree as calcium is translocated throughout the plant in the xylem not the phloem.

It is worth considering a Gypsum application in the early spring to provide luxury levels of calcium alongside your base fertiliser requirements. Then apply calcium foliar sprays to trees regularly, starting a week after petal fall.

For further information and advice, talk to your local fertiliser advisor.

Gypsum is BioGro organic certified. 

Gypsum - it all starts with healthy soil.

References

1. Michigan State University, by Philip Schwallier, Amy Irish- Brown,2001, Bitterpit control in apples - MSU Extension
2. Michigan State University, by Eric Hanson, 2012 Using gypsum in fruit production - MSU Extension
3. University of California, by Janet Caprill, Maximizing Calcium Uptake In Apples - UC ANR Small Farms Network