Pre- and Post-Harvest Management of Bitter Pit on Apples

Introduction

There is an ongoing need to use management practices that create a balance in orchards, minimizing the effects of the many interacting internal and external factors that contribute to bitter pit (Wooldridge, 2001). This article summarizes the main pre- and post-harvest management factors that can be manipulated to reduce the risk of bitter pit, and suggests further actions. Additional information can be found in the references.

Pre-Harvest Management

General Principles

• Avoidance: The fundamental principle of bitter pit control is avoidance. Consult records to establish past trends in cultivar and orchard susceptibility.
• Correlations: Bitter pit is positively correlated with fruit size, tree vigour, fruit nitrogen (N), potassium (K), and magnesium (Mg).
• Calcium Focus: Bitter pit develops where fruit tissues contain inadequate calcium. Management focuses on providing adequate calcium and avoiding factors (like irregular irrigation or excessive thinning) that lead to large fruit, which can dilute or withdraw calcium.

Orchard Nutrition

• Soil Management: Optimize soil conditions for calcium uptake before planting and maintain them throughout the orchard’s life. This does not replace the need for calcium sprays.
• Young Trees: Apply smaller, more frequent applications, with relatively higher nitrogen to promote growth, while maintaining nutrient balance.
• Bearing Trees: Key activities include:
  • Nitrogen and potassium in spring
  • Nitrogen in autumn
  • Calcium sprays during fruit expansion
  • Correction of pH and soil phosphorus in autumn
• Acidification: Long-term fertilization, irrigation, and herbicide use can acidify soil. Liming neutralizes acidity and balances base cations (calcium, magnesium, potassium).

Soil and Orchard Parameters

• Guidelines: Established for bitter pit control (Terblanche et al., 1980). The aim is to produce fruit free of bitter pit at harvest and able to survive storage and transport.
• Analysis: Both soil and leaf analysis are essential, with refined guidelines (Kotze, 1996).
• Nitrogen Management: Shoots compete with fruit for calcium. Excess spring nitrogen increases vegetative growth. Balance nitrogen between spring and autumn.
  • Calculation:
    Kg N / ha = (1.5 x yield) + 5
    (Yield in metric tons/ha; max 80 kg/ha, min 35 kg/ha)
  • Adjustment: Apply based on tree vigour:
    • Vigorous: Full bloom and after harvest
    • Normal: At flowering, after harvest, plus half-rate six weeks after flowering
    • Weak: Inspect roots/irrigation, correct issues, then apply at flowering, six weeks after, and after harvest

Plant Stress

• Reduce Stress: Maintain constant, adequate water, good soil aeration, moderate temperatures, and low wind. Overhead cooling’s effect is inconclusive, but frequent canopy wetting may reduce transpirational water loss and localized calcium depletion.

Tree Vigour

• Balance: Excessive vigour and low crop loads increase risk. Maintain long-term balance between yield, fruit size, and vigour. Thin early and avoid severe winter pruning. Select rootstocks suited to cultivar, soil, and management style.

Crop Load & Fruit Size

• High Crop Load: Producing moderate-sized fruit reduces risk compared to small crop loads and large fruit.
• Fruit Size: Larger fruit are more susceptible, likely due to calcium dilution. Low crop load and alternate bearing increase risk.

Foliar Calcium Application

• Sprays: Pre-harvest calcium sprays reduce bitter pit and improve resistance to rotting. At least six sprays are required; up to 12 is common. More sprays generally increase fruit calcium and reduce bitter pit.
• Application: Maximize uptake by wetting the entire fruit surface, using effective adjuvants. Calcium nitrate is effective, but alternatives exist. Sprays are important when cool, wet conditions limit calcium uptake.
• Disadvantage: Calcium sprays can cause blemishing (“calcium spot”), often due to water impurities. Some cultivars are more susceptible.

Timing of Calcium Sprays

• Uptake Peaks: Research (Lotze & Theron, 2005) shows calcium uptake peaks at ~40 days after full bloom (end of cell division) and again at 90 days (end of shoot growth). The latter is most significant for bitter pit control.
• Best Practice: Early, frequent sprays with correct concentration and adjuvant are needed in high-risk situations and sensitive cultivars.

Post-Harvest Management

Fruit Harvest Maturity

• Timing: Fruit picked before optimum maturity are more prone to bitter pit. Lowest levels are in fruit picked after the climacteric. Optimum maturity is based on changes in specific indices (van der Merwe, 1996).

Post-Harvest Dipping

• Calcium Dips: Dipping in calcium chloride or nitrate increases near-surface calcium but can cause blemishing and lenticel damage. Dipping is seldom needed if orchard factors and calcium sprays were optimal.

Storage

• Senescence Delay: Low-temperature and low-oxygen storage slow biological reactions and respiration, delaying senescence. Fruit quality is determined in the orchard. Keep high-risk fruit separate and market early.

Cold Chain

• Essential: Cooling must occur as soon as possible after picking, and the cold chain must be maintained during all handling, storage, and transport.

References / Further Reading

1. Kotze, W.A.G., 1996. Soil and nutritional factors affecting apple fruit quality. In J.C. Combrink (ed.), Integrated management of post-harvest quality. ARC Infruitec-Nietvoorbij, Stellenbosch, South Africa.
2. Lotze, E. & Theron, K.I., 2005. Acta Hort. (in press) Dynamics of calcium uptake with pre-harvest sprays to reduce bitter pit in ‘Golden Delicious’.
3. Terblanche, J.H., Gurgen, K.H. & Hesebeck, I., 1980. An integrated approach to orchard nutrition and bitter pit control. In D. Atkinson et al. (eds.), Mineral nutrition of fruit trees. Butterworths, London, pp. 71-82.
4. van der Merwe, J.A., 1996. Controlled and modified atmosphere storage. In J.C. Combrink (ed.), Integrated management of post-harvest quality. ARC Infruitec-Nietvoorbij, Stellenbosch, South Africa.
5. Wooldridge, J. 2001. Bitter pit and its control: a South African perspective. Unpublished Handbook, ARC Infruitec-Nietvoorbij.