How to Reduce Bitter Pit in Apples

Whilst bitter pit is linked with localised low levels of calcium it is characterised as a physiological disorder, and not a calcium deficiency. There is a reason for this distinction and it should inform best agronomic practice on how to reduce bitter pit in apples.

What is Bitter Pit?

Bitter Pit is a physiological disorder of apples which results in dark spots on the skin. The spots are strongly associated with low calcium levels which lead to collapsed tissue in the lesions where it occurs. Bitter pit symptoms are most severe when crops suffer from stressful growing conditions.

What causes Bitter Pit in apples?

Bitter Pit is caused by poor cell division in developing fruit. When cell division is weak calcium cannot be properly absorbed leading to local calcium deficiency in the characteristic spots. Although symptoms appear when the fruit starts to reach full size, they are caused during the early stage of development becoming visible later.

Although Bitter Pit is more prevalent in parts of fruit with lower levels of calcium, however it is rarely the case that the crop as a whole is calcium deficient even on crops with high levels of bitter pit. There are frequently no signs of calcium deficiency in leaf analysis, or even in most parts of the fruit with bitter pit lesions.

Indeed when we consider the amount of fruit calcium needed to prevent bitter pit we can see that far too much calcium is used for ‘prevention’ of what is in terms of calcium required an extremely small problem.

So how much Ca is needed to prevent the disorder? Depending on variety and location fruit calcium levels need to be above 5 – 5.4 mg Ca/100g fresh weight (Terblanche et al, 1980). Most apples are close to this without any correction, with the difference between calcium level in fruit with and without bitter pit being typically between 2 and 4 ppm.

If we look at a crop with 100% bitter pit and do some simple calculations we can see just how little calcium is needed to make the difference. If the crop is 30MT per Ha, and all the fruit has bitter pit lesions only part of that fruit has bitter pit lesions (2%), making 600 Kg of deficient tissue. If the difference in calcium content between the bitter pit affected tissue and the rest of the fruit is 4ppm then this makes the amount of calcium implicated in the problem 2.4g.

This means a 2.4g calcium deficiency created 30MT of crop loss. When we see how small the level of extra calcium needed to prevent bitter pit risk, it raises questions about why we apply so much calcium to prevent it.

To understand why we get these small areas of calcium deficiency, we need to understand more about how calcium is picked up, moved around and absorbed by plants.

Unlike most other nutrients Calcium is not phloem mobile, this means plants move it in xylem with water with calcium moving through the plant is in the direction of water flow, moving through the plant from roots to leaves via transpiration stream.

This makes calcium movement susceptible to water availability, and weather conditions hence bitter pit increases when water flow is restricted by growing conditions. It also makes getting applied calcium to fruit tricky, as fruit is low in transpiration so receives little throughput of calcium when compared to other parts of the plant like leaves.

Absorption of calcium is linked to presence of the auxin hormone. Tissues high in auxin absorb calcium easily, but tissues low in calcium have difficulty absorbing calcium no matter how much is available. This makes absorption of calcium rather than availability of calcium the main driver in bitter pit susceptibility in apple crops.

When we look at the physiology of the fruit, we can learn a little about best timings for reducing bitter pit incidence. Auxin levels are highest in fruits between flowering and when the fruit is 5mm in size, this is the main period of cell division (which is powered by auxins).

As the fruit gets bigger it is focused on increasing the size of cells rather than creating cells, and auxin levels drop making it progressively harder to get calcium into fruit as it increases in size.

Given the physiology of apples the best way to reduce bitter pit is to focus on timing, placement and absorption rather than quantity and frequency of application.