Reducing Bitter Pit in Honeycrisp Apples: A Focus on Calcium Management

A Focus on Calcium Management

Bitter pit is a major concern for apple growers, particularly for those cultivating the highly sought-after Honeycrisp variety. This calcium-related disorder not only detracts from the visual appeal of the fruit but also significantly affects its marketability, leading to potential financial losses.

What is Bitter Pit?

Bitter pit is a physiological disorder characterized by small, dark brown, sunken spots primarily found at the calyx end of apples. While bitter pit has a strong genetic basis—especially in Honeycrisp—calcium (Ca) remains essential for maximizing storage quality and minimizing fruit defects. The disorder is multi-faceted where the internal calcium content and balance, xylem functionality, genetics, crop load and abiotic stress can all influence bitter pit development. A lack of calcium moving through the xylem can lead to cell membrane leakage and cell death.

Key Symptoms:

• Dark brown, sunken spots on the fruit
• Internal cell breakdown due to insufficient calcium
• Symptoms may not be visually detectable until after 60 days of storage

The Economic Impact

The demand for Honeycrisp apples is high, making them profitable for growers. However, the appearance of bitter pit can render otherwise healthy-looking fruit unmarketable, leading to significant financial losses. Understanding and managing this disorder is crucial for maintaining profitability. There are many things about bitter pit that may be out of our control (genetics etc.), however, we often focus on fertility because we want to do what we can to reduce instances for our growers and calcium is a factor we can control.

Calcium Management: The Foundation of Prevention

Why Calcium Matters

Calcium is vital for maintaining cell stability and integrity. While there is no silver bullet for preventing bitter pit, there are several factors that growers can control:

• Crop Load: Manage the number of fruit per tree.
• Tree Vigor: Balance growth to avoid excessive vegetative development.
• Nutrient Ratios: Ensure a balanced nutrient profile to prevent deficiencies and imbalances.
• Soil Applied Calcium: Polysulphate®︎  as a natural mineral fertilizer that releases nutrients aligned with crop uptake.

To effectively combat bitter pit, establishing a strong calcium foundation is crucial. While foliar sprays can provide temporary relief, they are not a long-term solution because calcium needs to be available early, during cell division, without excessive amounts. Once cells begin to expand, calcium transport through the xylem becomes less efficient. Building a strong, soil-applied foundation is key so that we don’t over-rely on foliar. This is where Polysulphate comes into play. This multi-nutrient fertilizer not only provides calcium but also helps balance other essential nutrients in the soil. Here’s how it fits into your strategy:

• Application Timing: Apply Polysulphate early in the season to support root uptake during the critical cell division phase. Split applications at tight cluster and cell expansion (150 lbs./acre Polysulphate at each timing) reduced bitter pit. Use foliars to fine-tune Ca levels.
• Base Saturation: Ensuring proper base saturation ratios among all cations will enhance calcium availability and uptake.
• Foundation Over Foliar: Relying solely on foliar sprays can be a temporary fix; soil applications create a lasting foundation.

Additional Calcium Strategies:

• Balanced Fertility: Conduct soil tests to maintain pH between 6.5 and 7.0. Lime with high-calcium lime as needed.
• Soil Foundation: Use Polysulphate®︎ in the spring for additional calcium.
• Fertigation: Agrolution pHLow 15-6-27-3.3Ca, Agrolution 11-6-20-6Ca-1.5Mg, or Agrolution Zero P CalMag 15-0-15-5Ca-2Mg
• Foliar Adjustments: After establishing a strong soil foundation, fine-tune calcium levels with foliar sprays, starting one week after petal fall.

Managing Nutrient Interactions

To optimize calcium uptake:

• Monitor Boron and Zinc Levels: These nutrients are essential for calcium movement within the plant.
• Control Potassium and Magnesium: Excess levels of K or Mg can compete with calcium, further exacerbating the issue.
• Manage Nitrogen: Excessive nitrogen stimulates vegetative growth, increasing calcium demand in foliage and reducing availability for fruit.

The Bottom Line

Bitter pit presents a complex challenge for Honeycrisp apple growers, but effective calcium management can significantly reduce its incidence. By prioritizing soil and orchard health with products like Polysulphate®︎, along with balanced nutrition and careful management of crop load and tree vigor, growers can lay a strong foundation for calcium availability.

Sources:

University of Maryland Extension. (2023). Controlling bitter pit in apples: Best practices for growers (FS-2023-0701). https://extension.umd.edu/resource/controlling-bitter-pit-apples-best-practices-growers-fs-2023-0701/

Michigan State University. (n.d.). Bitter pit control in apples. https://www.canr.msu.edu/news/bitterpit_control_in_apples

Marini, R. (2009). Bitter pit management in Honeycrisp apples. Plant Nutrition, 32(5), 727-735. https://www.tandfonline.com/doi/abs/10.1081/PLN-120003934