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Moving Beyond Calcium Sprays

A summary of a presentation by Lee Kalcsits, WSU Horticulture. Summary by Tianna DuPont, WSU Extension. April 18, 2018.

Bitter pit is a major challenge for Honey Crisp producers world-wide. Dr. Lee Kalcsits, WSU Assistant Professor of Horticulture spoke at the 2018 International Tree Fruit Association Conference in New Zealand sharing the latest research on how to manage this challenging disorder.

Why is calcium important?

We all know that calcium is a key player in the saga of bitter pit. But why is it so important? Calcium is important for cell structure, Kalcsits explained, it binds with pectins in the area between cells called the middle lamella. Calcium pectate glues the cells together. As fruit ripens and ethylene is produced, calcium pectate breaks down. How quickly it breaks down depends on the amount of calcium present. Cells with high calcium resist breakdown maintaining healthy cells for longer.

Why does fruit often have low calcium?

Calcium from the soil travels through the plant in the xylem following the transpiration stream. As transpiration pulls water through the plant, calcium from soil water moves with it, ending up in either the leaves or the fruit. Because leaves transpire more water than fruit, proportionally more calcium ends up in leaves and less in fruit. Calcium is relatively non-mobile and so once it is in a leaf it cannot move into the fruit. On the other hand, potassium is mobile and the plant can re-mobilize potassium from the leaves to fruit creating an imbalance of K:Ca in the fruit.

How is fruit with bitter pit different from a healthy fruit?

Fruit Structure. Kalcsits is using a number of sophisticated research tools to learn about the structure of bitter pit affected fruit and how they differ from healthy fruit. One tool is Advanced Light Source synchrotron at the Lawrence Berkeley National Lab in Berkeley, California. The machine is “a bit like a high-powered CT machine,” Kalcsits explained. By scanning the fruit, he can create images down to the cellular level to understand differences in fruit structure that are related to bitter pit. He is finding differences in structure between healthy and bitter pit fruit.

Nutrient Distribution. Not only the structure but also the nutrient distribution is different in healthy and bitter pit fruit. Kalcsits used a device to scan fruit and map the K:Ca levels in fruit tissue. Healthy fruit had a lower K:Ca ratio than bitter pit fruit. But significant within fruit variation can lead to variable results depending on sampling site for elemental analysis.

How can we manage bitter pit?

Soil: Fill the tank. Generally, young Washington soils have sufficient calcium but it is important to test and make sure there is sufficient soil calcium and the calcium is in good balance with other nutrients in the soil. Calcium sprays are designed to supplement calcium taken up from the soil.

Soil: Good conditions for uptake.  If roots are not healthy due to disease or inundated soils calcium available in soils will not be absorbed into the plants.

Calcium sprays. Calcium sprays are an important input. See WSU Crop Protection Guide for recommendations. When is the best time to spray calcium? Early in the season the fruit has a fuzzy texture with less wax and so more of the calcium spray sticks per area. As the season goes on more total calcium sticks because there is more surface area. What is the efficiency of calcium absorption from sprays? Dr. Kalcsits recently did an experiment using calcium isotopes to measure the absorption of calcium into the fruit. He found that at all five time points they measured in the season they were getting some calcium absorption into the fruit. About 25% of the calcium was getting into the peel and cortex and about 10% was getting into the cortex.

Crop load management. Dr. Kalcsits recently collaborated with Dr. Stefano Musacchi and Dr. Sara Serra where they did an experiment looking at crop load levels in Honeycrisp apples. In agreement with other studies, they found that Honeycrisp has a particularly narrow target cropload of 5 to 7 fruit per cm2 tree cross sectional area (TCSA).  Below 4 (large fruit) had a higher incidence of bitter pit. Above 7 there was poor color development and poor return bloom. Fruit from higher cropload trees which had less bitter pit had a lower K:Ca ratio in the fruit but this needs to be balanced against poor fruit quality and return bloom if trees are cropped too heavily.

Water.  Even with appropriate calcium sprays and crop load Dr. Kalcsits found that some orchard blocks still had high levels of bitter pit, as high as 38% in one block studied. He found that when calcium sprays and cropload were controlled for, fruit weight was important. Plant water status contributed to final fruit weight when all other factors are controlled. While too little water stresses the tree and does not allow it to move water with its load of calcium into the tree’s leaves and fruit, luxurious water consumption resulted in large fruit with higher bitter pit. “We need to manage water to an optimal value,” Dr. Kalcsits explained. His future research is pursuing what the optimal tree water potential and irrigation timings may be.

Additional Resources

Calcium Absorption during Fruit Development in ‘Honeycrisp’ Apple Measured Using 44Ca as a Stable Isotope Tracer. L Kalcsits, G van der Heijden, M Reid, K Mullin – HortScience, 2017


Lee Kalcsits

Assistant Professor, Tree Fruit Physiology

Tree Fruit Research & Extension Center, Wenatchee, WA

509.663.8181 ext 229


Tianna DuPont, WSU Tree Fruit Extension Specialist

Tree Fruit Research & Extension Center, Wenatchee, WA, (509) 663-8181

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