The incidence of bitter pit and blotch is associated with low fruit calcium status, whereas fruit with adequate calcium can be kept longer in storage after picking.

There are a number of factors that increase the risk of calcium related problems in apple fruit and it’s never just about one risk factor. Understanding calcium movement and distribution in the tree and what influences it, allows growers to better manage their trees so as to mitigate the risk of bitter pit and blotch.

Calcium availability and uptake

It is important to have sufficient amounts of calcium in the soil and a soil test can determine this. Calcium must occupy at least 65 per cent of the soil base exchange sites. Maintaining the pH between 6.0 and 6.5 will ensure the calcium in the soil is in a form that is available to plants.

Calcium movement into the tree is primarily via the transpiration stream. Calcium must be in the soil solution and be in contact with the roots. When the tree transpires, water flows into the root, transporting calcium with it into the tree. Good water management is therefore an important tool to achieve good calcium status in the tree.

Trees under water stress may suffer a shortfall in calcium as well as water. Conversely, if there is a sustained period of too much water, anaerobic soil conditions can develop which will severely reduce root function and therefore calcium uptake.

Ensuring there is a good balance of calcium with other competing cations (positively charged ion) such as potassium and magnesium in the soil is important. High amounts of these competing cations will reduce calcium uptake.  Calcium, being a cation, cannot exist alone in a soil solution without an anion (negatively charged ion) such as phosphate, nitrate and sulphate. These anions must also be balanced in the soil.

Calcium is primarily taken up by newly developed roots. Thus it is important to maintain a soil environment that sustains a healthy root system and promotes new root growth especially in the spring. Good drainage, nutrient and water management, along with organic matter, encourages a healthy and thriving soil ecosystem. Be careful with the overuse of chemicals.

Calcium movement and partitioning

Calcium moves around the plant in the transpiration stream via the xylem vessels. Calcium will therefore be deposited in plant organs that have the strongest demand for water; this is principally the leaves. New developing leaves are always going to be the strongest sink for calcium. The demand for calcium by the leaves is over 75 times more than the fruit.

Water flow and therefore calcium movement to the developing fruit is a little more complex.

Early fruit development (the first six weeks from fruit set) is the crucial window in which calcium moves into the fruitlets.  During early fruit development the surface to volume ratio of the fruit is high. This increases transpiration and helps water to move readily into the fruitlets taking calcium with it.  Also, the outermost layer of the young fruitlet’s skin has only a small amount of wax on it. This makes it more permeable to water movement.

Calcium absorption occurs for about six weeks after fruit set. Absorption then levels off and can slightly decline as fruit enlarges through to harvest. This is because there is a dilution effect as the total fruit calcium is now spread over a larger volume of fruit. Unlike other nutrients, like potassium and nitrogen, calcium does not move in the phloem and so will not re-distribute from the leaves to the fruit. Therefore, at this stage, the only way to get more calcium into the fruit is by regularly applying calcium sprays directly to the fruit.

Boron effect

Pollination is important in calcium uptake because apples with higher seed numbers accumulate more calcium.

Boron is required for pollen tube formation. If there are inadequate levels of boron, pollination is incomplete, which means less seed numbers and, in the worst cases, distorted fruit. Boron is best applied as a post-harvest and/or a pre-flower spray.

Seed numbers per fruit in relation to fruit diameter and calcium concentration

(Source: Hort Science 25: 351-353)

Risk factors

It’s usually never just one risk factor you need to manage.  The perfect storm for pit and blotch is the alignment of all risk factors. However, pit and blotch management can be complex whereby just one risk factor, if not mitigated enough, could be the one that will tip the fruit into pit or blotch disorders. Management is therefore about having a proactive plan to mitigate all risk factors.

Low crop loads lead to larger fruit and more tree vigour, with larger fruit resulting in a greater dilution of calcium. Crop variation within a block and within a row, as well as excessive vigour, block history, and hot/dry climates, are all key risks that need to be managed.

Tools such as pruning, crop load management, varying chemical thinning, the use of Regalis^®, water management (soil moisture monitoring), root pruning, nutrient management (soil and leaf test monitoring) and calcium sprays, allow a grower to proactively manage these risks. 

Bitter pit and blotch risk factors and possible management responses.

* Regalis® is a plant growth regulator. Its active ingredient is prohexadione-calcium.

Root pruning effect on vigour: The trees on the right were root pruned on one side, while the trees on the left were not root pruned. Notice the difference in the vigour.

Calcium sprays 

Once the fruit has entered the cell enlargement phase of growth (approximately six weeks after fruit set), the only means to increase fruit calcium status is to apply calcium sprays directly to the fruit.

Regular applications are important to top up the cells with calcium as they expand.  The key points for calcium spraying are:

• Aim for at least 25kg of calcium/ha for pit sensitive varieties.
• Split into 20 sprays over the season.
• Make sure each application uses the full recommended rates.
• Keep the program going regularly right up to just before harvest. Late calcium applications are just as important as early applications.
• Every calcium application is important as it just tops levels up a little more.
• Apply to get best absorption, higher water volume sprays are better.
• Use buffered formulations especially in hot climates.

Summary

Bitter pit and blotch incidence in the orchard is linked to low fruit calcium status. By understanding calcium availability, uptake and movement in the tree a grower can identify the key risk factors for pit and blotch incidence.

Nutrient imbalances and excessive vigour are important factors, however it is never just one risk factor that needs managing.  A coordinated and timely response is required to proactively manage and mitigate the incidence of these disorders.

Acknowledgements

APAL’s Future Orchards^® program is funded by Hort Innovation using the apple and pear industry levy funds from growers and funds from the Australian Government. AgFirst is a key Future Orchards partner.

About the author

Dean Rainham, Horticultural Consultant, AgFirst New Zealand, +64 3 528 0330, e-mail.

Source: APAL