The Key to Cavity Spot Reduction – Understanding Calcium
Although the link between Cavity Spot infection and low calcium levels in carrots is well established, farmers often get little benefit from application of calcium fertilisers. In this article I will explore how carrots use calcium, why calcium fertilisation gives erratic results and ways to address this.
What is Cavity Spot?
Cavity spots are small lesions which can appear on anywhere on the carrot root. They are typically no larger than 13mm in diameter. Carrots with Cavity Spot are not saleable, and it can lead to reduced marketable yield, or in severe cases total crop loss.
What Causes Cavity Spot?
Cavity Spot is caused by localised infection by the pythium fungus which is induced by a deficiency of calcium. Symptoms become visible on carrots after 12 weeks when the carrots start to reach saleable size at maturity.
Most of the carrot is not affected – only small lesions. These lesions are typically in parts of the carrot that have a low level of calcium, and roots with high calcium status are far less likely to have pythium infection.
Why Does Calcium Deficiency Cause Cavity Spot?
Calcium’s main function within cell walls is to give cell wall rigidity & strength. The main symptom of calcium deficiency is the disintegration of cell walls and the collapse of affected tissues. It’s this tissue collapse that contributes to Cavity Spot, weak tissue that let’s pythium take hold where it otherwise would not be able to.
Can Fungicide be used for Cavity Spot?
Farmers can grow on the same field for many years, and only get cavity spot in some seasons despite the fungus being ever-present. Pythium is a weak pathogen that is not really capable of infecting healthy tissue, hence the strong correlation with low calcium status in infected lesions. A strategy of relying solely on fungicides to kill the pathogen then, is perhaps not the best way forwards. Getting calcium levels in the root clearly has a role to play in reducing infection rate.
Let’s quantify the problem.
Carrots don’t actually need very much calcium to avoid Cavity Spot, the quality problems associated with calcium result from tiny localised deficiencies. But these minor deficiencies (in terms of the portion of tissue affected) can make crops unsellable. In the case of Cavity Spot, the deficiencies are only in small lesions.
Whilst the carrot root may have small areas of calcium deficiency, the rest of the plant rarely has any calcium deficiency at all and is often precipitating out Ca from leaves as an excess.
Giving an example, if a 50MT/ha tonne crop of carrots had complete loss due to cavity spot, the actual quantity of Calcium deficient tissue (2% of each root is actually affected) is only 1000 Kg of carrot. The difference between the affected and healthy part of the carrot is typically only 4mg/kg (four parts per million).
Therefore, the amount of calcium required to prevent an entire 50 MT/ha crop of carrots from being susceptible to cavity spot is only 4g per/ha! This should raise a few questions for growers.
So, if carrots only need such tiny levels of calcium, why do calcium fertilisers not resolve Cavity Spot?
Why are small parts of the carrot deficient when the area right next to them isn’t? Why are these small areas of tissue deficient in Ca when there is no whole plant deficiency? and why doesn’t applying large amounts of calcium reverse the deficiency?
In order to answer these questions, it’s important to understand how calcium behaves in a plant. There are two factors to be considered – transport and absorption.
“Lack of phloem transport makes foliar calcium useless”
Unlike most other mineral nutrients, calcium is not phloem mobile and can only be transported through xylem. Calcium enters the plant with water and is transported upwards with transpiration, where it is either absorbed and stored, or is precipitated from the leaves as excess.
Calcium only moves upwards. This is why targeting and correct placement of calcium applications is so important. Calcium applied to leaves can’t correct problems in the roots. Therefore, foliar sprays of calcium fertilisers will never put calcium into carrots, it’s physiological impossible for the plant to move it down.
From a practical viewpoint this means that spraying carrots with foliar calcium feeds will never increase calcium in roots, no matter how good the product is on other crops. To reduce cavity spot, calcium needs to be applied to the roots.
But even when calcium is available in the soil we still get Cavity Spot, why?
Calcium is absorbed into cells using polar auxin transport, as auxin moves out of the cell, calcium enters. Parts of a plant that are low in auxin, can’t absorb calcium effectively, regardless of how much is available.
High auxin producing areas include new shoots, new flowers, and new leaves. Low auxin producing areas include fruits, roots and tubers.
This is why applying calcium to correct physiological disorders can be so ineffective. It doesn’t matter how much calcium is applied, parts of the plant with low auxin levels such as carrot roots can’t absorb it properly.
Auxins are used by plants to induce cell division in newly growing tissue. In carrots the root grows from the tip, so the new cells in the tip are high in auxin and pretty effective at absorbing calcium, as the cells age they are no longer dividing and auxin levels decrease so capacity to absorb calcium drops off. This explains why cavity gets worse farther from the root tip (the farther away from the root tip of a carrot the lower the auxin levels, and the lower the capacity to absorb calcium) and also why growing conditions effect cavity spot incidence so much (water status and temperature change transport, increasing likelihood of Calcium deficiency).
When plants are stressed calcium metabolism goes up, and carrot roots are not good at replacing it. This is why stressful growing conditions lead to higher Cavity Spot likelihood. Simply having calcium near the roots wont help, to get it into the carrot absorption must be improved.
How to Reduce Cavity Spot using Calcium.
Levity have developed chemistry that allows plants to absorb calcium in the absence of polar auxin transport. Our technology can help carrots absorb calcium when conventional sources would not be taken up, even in difficult growing conditions.
Albina is a granular calcium fertiliser that can be placed in the root zone and absorbed by roots. It contains Levity’s advanced LoCal chemistry that has been extremely effective on crops like tomato, strawberry and lettuce, but in a slow release prilled form that can applied at planting to root crops. It is the first product ever available that supplies slow release granular calcium with chemistry that allows active uptake by roots.
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Award winning scientist and experienced agronomist. With multiple patents, and proven track record of product development in biostimulants, pesticides and fertilisers.