Growers of Honeycrisp are familiar with the fight to prevent bitter pit development. Although Honeycrisp is arguably the most well-known variety with bitter pit issues grown today, “it also happens in many other varieties,” Bernardita Sallato, Washington State University, said during her presentation “Root Health, Fertility, Water and Crop load Effects on Bitter Pit” during the recent WSU 2022 North Central Washington Tree Fruit Days.

Sallato is a tree fruit Extension specialist and has been researching bitter pit for 18 years. Green spot in WA38 apple cultivar is probably due to the same issues as bitter pit, as is cork spot in pears, Sallato said. And no matter what you call it, different cultivars are more susceptible to developing the same deficiency of calcium. Bitter pit “is a cellular deficiency,” she said, and not due to a lack of calcium in and of itself.

The symptoms of bitter pit are due to the oxidation and necrosis of plant tissue caused by deficiencies in calcium on the cellular level. Cell leakiness, as a result of not enough calcium being available, causes structural failure which leads to the oxidation. This then causes black spots to form on the apples’ skin and in the flesh. This dry, dark tissue has a bitter taste.

Roles of Calcium in Bitter Pit

Calcium works to bind the middle lamella, a structural element. It is also found around the membrane of the cell walls. These are two different types of calcium and are not interchangeable. A deficit of structural calcium can fail in susceptible varieties and cause bitter pit.

Calcium which is found around the cell membrane is available to be released into the cell in reaction to environmental stressors, particularly later in the growing season, Sallato explained. Calcium from the membranes responds to “any environmental impact into the fruit,” such as too much shade, heat stress or insect damage.

In this second role, the calcium in the membranes serves as a “signaling element” and moves from the cell membranes and into the cell when certain conditions are met. There, it’s involved in a reaction, such as protein development. Once it is no longer needed, and the reaction is completed, this calcium goes into the cell vacuoles, where it becomes non-mobile and inactive.

If activation of the calcium available around the membranes happens later in the growing season, a calcium deficiency can occur and cause structural issues which lead to leakiness and bitter pit development.

When the total calcium in the fruit is measured, both forms of calcium are measured. This is “not a really good indicator” of bitter pit potential, she said. Although the two forms of calcium are related to bitter pit development, they are not available at the same time, and the inactivated calcium stored in the vacuole cannot be used to prevent bitter pit development.

Calcium Transport & Availability

Calcium is taken up by root tips and travels via the xylem. Calcium itself is not very mobile in the plant. The plant’s capacity for taking in calcium is similar to the ion exchange capacity seen in soils.

Cell division happens in the early season. This is when calcium availability in the soil can be targeted. Later in the season, when fruit matures, cells grow in size, but no more cells are created.

After about 40 days, the xylem’s functionality decreases in some apple cultivars. Longevity of the xylem tissue is related to transpiration and movement of calcium into the fruit. There is also a reduced capacity of the cell membrane to retain calcium as the season progresses.

Other nutrients, such as nitrogen, magnesium and potassium, travel into the plant and fruit via the phloem, which remains active and can move elements both in and out of the plant.

How to Influence Calcium

The soil calcium level, the root growth, the ability to transport calcium into the fruit and the influencing of cellular calcium can provide growers means of regulating available calcium. In varieties more susceptible to bitter pit, management of these still may not be enough to prevent all bitter pit.

In the soil, a cation balance between potassium and calcium is important. If both potassium and calcium are low, they can be supplied early in the growing season. Too much potassium will compete with the uptake of calcium, and very high potassium levels will increase soil pH. This pH increase will cause drainage and soil moisture issues and will impact calcium uptake even further by affecting root growth.

Root tips are the key factor in allowing calcium uptake into the plant. Early in the season, adequate root growth and healthy roots, along with appropriate moisture levels, are needed for calcium uptake. Roots cannot grow in saturated soils, and if there are not root tips, then calcium – which is passively taken into the plant via the tips – will be prevented from entering the xylem.

Older roots have a layer which calcium cannot penetrate. If calcium uptake is prevented, other nutrients which travel via the phloem will still be taken into the plant and an imbalance of nutrients will occur.

Early root growth will promote calcium uptake. Calcium is needed during cell division, which only occurs early in the growing stages. Root growth occurs just after shoot growth. Shoot growth is believed to utilize stored calcium reserves, so new calcium uptake is needed for cell division. Accelerating root growth using plastic mulch may offer growers a means of increasing calcium uptake.

Calcium uptake via the xylem is driven by transpiration. When fruit is developing during cell division, there is transpiration from the fruit and calcium enters the fruit. The fruit does not transpire once it has developed cell walls and is maturing. “We have a window of opportunity to get calcium into the fruit,” Sallato said.

Seeds are thought to be a driving force which gets calcium into the fruit, moving calcium from high concentration in the seeds and into the fruit itself. Thinning trees too soon may prevent uptake of calcium and cause bitter pit concerns. Thinning decreases transpiration, so calcium uptake is inhibited.

“We have to manage crop load. Vigor and crop load go together,” she said. “This is the way I feel we need to manage bitter pit in our orchards.”

Excessive vigor means that there are reduced numbers of seeds early in the season, preventing calcium uptake. Gibberellic acid (GA), released during shoot growth, also inhibits the uptake of calcium into the fruit. Plant growth regulators can be utilized to inhibit GA.

Too much vigor also can cause excessive uptake of potassium and nitrogen, larger fruits and can create shade conditions. These factors will decrease calcium availability to fruit and increase bitter pit development. Rootstock selection is one way to manage vigor, with B9 being a lower vigor rootstock shown to have lower levels of bitter pit. In general, dwarf rootstocks show less bitter pit, she said.

Once cell division has ceased and calcium can no longer be taken into the fruit, growers can focus on preventing oversized fruit, slowing the uptake of nitrogen, magnesium and potassium and reducing stress. Late season applications of PGRs may be able to prevent stress and reduce bitter pit, and research trials are underway at WSU to examine this possibility.

“Do everything that you can to prevent stress conditions,” Sallato said.

Bitter pit is influenced by more than two dozen factors and is complicated. It is genetic, although cultivar susceptibility can change due to growing environment. This means there are measures growers can take to reduce the prevalence of bitter pit: maximizing calcium uptake during cell division and minimizing calcium loss due to excessive vigor or crop load.