Bitter Pit in Granny Smith Apples: Calcium Deficiency Solutions

A 12-year-old Granny Smith apple tree has developed black spots on approximately 20% of its fruit over the past two seasons. Standard fungicide treatments with sulfur and captan have failed to improve the condition. A commercial apple grower suggested calcium deficiency as the cause.

Identifying the Problem: Bitter Pit vs. Fungal Disease

The symptoms described—dark spots on fruit that don't respond to fungicides—strongly suggest bitter pit (also called blossom end rot in some contexts), a physiological disorder caused by calcium deficiency rather than a fungal pathogen. This explains why sulfur and captan applications were ineffective.

Why Calcium Deficiency Occurs

Calcium moves through the xylem with water transpiration. Factors that disrupt this flow include:

• Irregular watering (drought stress followed by heavy irrigation)
• Excessive nitrogen fertilization promoting vigorous vegetative growth
• High potassium or magnesium levels competing with calcium uptake
• Root damage or poor root development
• Rapid fruit enlargement outpacing calcium supply

Effective Treatment: Foliar Calcium Sprays

The most direct correction method is applying calcium directly to developing fruit and foliage:

1. Product selection: Look for calcium chloride or calcium nitrate formulations at garden centers. Products marketed for "blossom end rot" on tomatoes work equally well on apples.
2. Timing: Begin applications when fruit are young (shortly after petal fall) and continue at 10-14 day intervals through the growing season.
3. Application: Follow label rates carefully—excess calcium chloride can cause leaf burn. Apply in early morning or evening to maximize absorption.
4. Coverage: Thoroughly wet fruit surfaces and foliage.

Cultural Practices to Support Calcium Uptake

• Maintain consistent soil moisture with mulch and regular irrigation
• Avoid excessive nitrogen fertilization, especially in early summer
• Conduct a soil test to check pH and nutrient balance
• Prune to improve light penetration and air circulation

With consistent foliar calcium applications and improved cultural practices, next season's crop should show significant reduction in bitter pit incidence.