Bitter pit is a physiological disorder that occurs on fruit, typically in the postharvest stage. The disease starts internally, showing nearly no external signs at harvest, progressing to sunken gray, brown or black spots on the calyx end after a few days to weeks in low temperature storage. The varieties that are most susceptible to bitter pit include: Honeycrisp, Delicious, Courtland, Fuji, Crispin, Empire, Idared, and Northern Spy.

Figure 1. Bitter pit symptoms on apples.

Causes

Bitter pit is predominantly caused by low calcium (Ca) levels in the fruit. Ca provides stability in the cell wall; when there is a deficiency in the fruit, physiological breakdown of cell walls can occur.

Bitter pit relates more to the distribution of Ca in the plant rather than the Ca level in the soil. Ca is absorbed by the roots and moves with water in the xylem through transpiration to the mature leaves. It is taken up into the fruit until mid-July; after which there is very little transport of Ca into the fruit. As the fruit grows, Ca is diluted with in the fruit.

Bitter pit is influenced by nutrient imbalances with Ca such as nitrogen (N), potassium (K), phosphorus (P), magnesium (Mg), and boron (B). The N:Ca ratio affects the occurrence of bitter pit more than the individual Ca level. High levels of Mg and K results in Ca deficiencies, therefore K and Mg foliar sprays can increase bitter pit occurrence. Typically the higher the nutrient ratio of N, K, Mg, P to Ca, the higher the susceptibility to bitter pit. From recent research from Cornell, Honeycrisp is more predisposed to bitter pit as it is found to have a higher leaf Ca level than Gala as well as lower Mg, P, and K levels in leaves than in Gala. The fruit has higher K:Ca, Mg:Ca, P:Ca, and (K+Mg):Ca ratios in Honeycrisp than Gala in both peel and flesh. Results concluded that Honeycrisp is more vulnerable to bitter pit due to more precise Ca partitioning.

Solutions for Bitter Pit

An excess or lack of soil moisture will affect the accumulation of Ca within plants. This water stress can be improved through irrigation and mulching for dry soils, and tilling for excessively wet soils. Adjusting the pH level (between 6 and 7) through liming or gypsum will help Ca uptake as the availability of minerals vary with the pH level; however, it will take a few years to make a difference. Some smaller actions can be taken such as controlling vigor and managing crop yields to help increase Ca uptake by fruit.

Foliar applications of Ca (calcium chloride or calcium nitrate) have been the most effective for increasing Ca in fruit than soil treatments because Ca is an immobile nutrient. It is recommended by Dr. Lailang Cheng, from Cornell that pre-harvest foliar sprays should begin shortly after petal fall at 1.8-2.4g/1000 litres (1.5-2lbs/100 gallons) of water for at least four applications and then changing the rate to 3.6-4.8kg/1000 litres (3-4lbs/100 gallons) of water for 2 mid-season sprays and continue until harvest. It is more important to make frequent sprays than applying sprays at a certain time during the season. Foliar application of sprays only benefit the fruit if the spray makes direct contact on the fruit, therefore good coverage is important.

Excess levels of N, P, Mg or K will increase Ca deficiencies because they compete with Ca therefore nutrients need to be kept at their optimum level. Ca imbalances with other nutrients predisposes cultivars such as Honeycrisp to Ca deficiencies.

There are a few different methods of detecting Ca deficiencies and bitter pit which include: nutritional status, maturity acceleration, veg growth/tissue analysis, Mg infiltration, and peel nutrient ratios. When testing Ca levels in vegetation, leaves need 1.3-2%. Mg (Mg chloride) infiltration will show evidence of Ca deficiencies 7-10 days after and is inversely related to Ca concentrations in fruit. Peel nutrient levels such as K:Ca, P:Ca, and K+Mg:Ca ratios may also help predict susceptibility to bitter pit and Ca deficiencies as they tend to be higher in some varieties, predisposing them to bitter pit.

References

  • Cheng, L. (2016). Getting the Right Nutrient Management. 2016 IFTA Annual Conference: Growing the Moneymakers: Fuji, Gala, Honeycrisp, pg 55-61.
  • Cheng, L., & Sazo, M. (2016). Bitter Pit of Honeycrisp: Physiological Causes and Mitigation Strategies [Abstract]. Cornell Fruit Field Day 2016: New York State Agricultural Experiment Station Geneva, pg 7-8.
  • OMAFRA (2009). Fact Sheet: Bitter Pit Control in Apples. Ministry of Agriculture, Food, and Rural Affairs. Retrieved from: http://www.omafra.gov.on.ca/english/crops/facts/00-009.htm
  • Retamales, J., & Valdes, C. (2000). Bitter Pit Prediction in Apples and the Commercial Use of Fruit Mg Infiltration. Washington State University Tree Fruit Research and Extension Centre. Retrieved from: http://postharvest.tfrec.wsu.edu/pages/EMK2000B
  • Schwallier, P., & Irish-Brown, A. (2011). Bitter Pit Control in Apples. Michigan State University Extension. Retrieved from: http://msue.anr.msu.edu/news/bitterpit_control_in_apples
  • Washington State University (2005). Market Diseases of Apples, Pears and Quinces: Bitter Pit. Washington State University Tree Fruit Research and Extension Centre: Postharvest Information Network. Retrieved from: http://postharvest.tfrec.wsu.edu/marketdiseases/bitterpit.html
  • Witney, G. (1997). Fruit Mineral Uptake and Augmentation in the Orchard: Postharvest Quality Implications. Washington State University Tree Fruit Research and Extension Centre. Retrieved from: http://postharvest.tfrec.wsu.edu/pages/PC97B