Bitter Pit in Apples: What It Is, Why It Happens, and What You Can Do
- Phytech Team
- Jul 2
- 3 min read
Bitter pit is a frustrating problem where fruit looks great on the outside, then shows dark, sunken spots near the bottom of the apple just when you think it's ready for the market. Once the apple is cut open, there’s often corky, brown tissue under the skin. By that point, it’s too late. The damage is done.
The University of Wisconsin Extension suggests that weather and irrigation have a larger impact on bitter pit than all other factors, "followed by low crop load that will give you the highest risk of bitter pit." This means Phytech can truly help mitigate the impacts of bitter pit through irrigation and plant monitoring.
Read on to learn how.
What Causes Bitter Pit
Bitter pit is not caused by pests or pathogens, but rather by a lack of calcium in the fruit. For that reason, it’s often referred to as a disorder. The calcium deficiency causes the cell wall of the apple to break down, especially when the fruit grows too big too quickly. Rapid growth can dilute the calcium concentration in the fruit, increasing the likelihood of bitter pit.
Stress plays a role, too. Heat, drought, and irregular irrigation can all disrupt calcium uptake and movement in the tree. This is especially true for young trees or on vigorous rootstocks.
Some apple varieties are more sensitive to bitter pit, like Honeycrisp. The strong vegetative growth of the tree can pull calcium towards the leaves instead of the fruit.
How Growers Usually Treat Bitter Pit
There are a few well-known strategies for reducing bitter pit, most of which focus on improving calcium uptake and managing overall tree vigor. Calcium-related solutions like foliar sprays, dips, and balanced fertilization are important, but they won’t make much of a difference if the tree is too stressed to take up nutrients.
Since calcium moves with water through the xylem, poor irrigation practices can impair nutrient uptake. Even well-timed sprays can fall short if the tree isn't moving water effectively. That’s why managing stress through consistent irrigation is foundational.
One of the most effective ways to prevent bitter pit is by directly managing how much the fruit grows in the first place. When apples grow too quickly, especially at night, their cells expand rapidly. This can dilute calcium concentration in the fruit. Deficit irrigation is a proven method to slow fruit growth by creating mild, controlled water stress.
How Phytech Can Help
Controlling Apple Growth
One of the best ways to mitigate bitter pit is to regulate how much the fruit expands, particularly during the night, when most cell expansion occurs. This nightly growth depends on the water status of the tree at the end of the day. If the tree is fully hydrated at dusk, it has high turgor pressure, which promotes rapid cell enlargement. Larger cells result in lower calcium concentration per unit of tissue.
Phytech’s dendrometers give growers a window into this process by continuously measuring trunk diameter fluctuations, which correspond to daily cycles of water uptake and loss. The growth-shrink cycle reflects how much water the tree uses during the day and how much it replenishes overnight.
By interpreting these patterns, growers can determine whether trees are entering the night with excessive hydration, setting up conditions for overly rapid fruit expansion.
To counteract this, growers can use the data to apply moderate, daytime water deficits, enough to prevent excessive turgor pressure by evening without inducing chronic stress. Deficit irrigation limits overnight fruit cell expansion.
Monitoring Irrigation Carefully
Water stress isn’t inherently bad. Mild, controlled stress can be used as a tool. But prolonged or poorly timed stress can reduce calcium transport and compromise fruit quality.
By using the dendrometer signals to find the threshold point where growth is slightly limited but not halted, and aligning this with real-time soil moisture levels, growers can operate within an ideal physiological range. This enables enough stress to control fruit growth while still maintaining sufficient water flow to move nutrients like calcium.
Tracking Fruit Growth and Soil Moisture
Phytech’s fruit sensors provide another layer of precision by directly measuring the rate and timing of fruit expansion. Soil moisture sensors show the volumetric water content at six different depths in the root zone, helping growers ensure they are neither overwatering (which promotes excessive fruit growth) nor underwatering (which limits calcium movement).
This full picture allows for highly responsive irrigation scheduling that reflects real plant demand, not just assumed ET rates or calendar days.
For sensitive cultivars like Honeycrisp, this level of monitoring is especially valuable. With Phytech, growers can actively manage tree stress and fruit development, giving their crop the best chance of staying clean, crisp, and pit-free.
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