Calcium deficiencies contribute to certain fruit disorders in apple, including bitter pit. Bitter pit is a physiological disorder of apple fruit that has caused serious losses in certain apple varieties for many years. The visible signs may be only slight indentations in the skin with no change in color. The skin over these depressions usually takes on a deeper green color than the surrounding skin, and finally, the disorder appears as small, brown, desiccated pits. The pits may be few in number to numerous, and although they may extend over much of the fruit surface, they are more prevalent on the calyx end of the fruit. A transverse section through the spot will reveal brown, dry, spongy tissue just beneath the skin. The disorganized tissue is much softer than the unaffected fruit tissues. This disorder may not be evident at harvest but can develop in storage, resulting in extensive fruit loss. Bitter pit is more likely in some cultivars, such as Honeycrisp, Cortland, Northern Spy, and Granny Smith.
Figure 1: Honeycrisp apple with small pits at the calyx end of the fruit due to calcium deficiency during apple fruit development
When the trees are young, fewer and larger fruit are produced which are more prone to bitter pit. Additionally, excessive nitrogen, potassium or fluctuating soil moisture can increase the likelihood of bitter pit developing.
If you plant cultivars that are susceptible, thinking through control options before you plant is important. In commercial orchards, foliar application of calcium products such as calcium nitrate, have been shown to be effective in reducing the incidence of bitter pit. Avoiding biennial bearing through crop load management is important to produce an even crop every year. This can help reduce fruit size and thus reduce bitter pit.
Further reading:
Professor Emily Hoover, University of Minnesota
Dr. Adriana Telias