British scientists crack ‘calcium conundrum’ for carrot cavity spot

Less than a teaspoon of calcium per hectare: all that’s needed to reduce susceptibility to cavity spot in carrots. Yet calcium applications of several times that rate still fail to control the disease.

Now a team of British scientists claims to have solved this ‘calcium conundrum’, offering growers the ability to tackle cavity spot without recourse to fungicides, liming or cumbersome crop rotations.

A severe disease that can render the entire crop unsaleable, cavity spot remains one of the most damaging diseases for carrot growers around the world. Calcium has often been implicated in the disease, but growers have struggled to get consistent results from either soil or foliar applications of the macronutrient.

“Because the pathogen responsible for cavity spot, Pythium, is relatively weak, it can only attack cells with imperfect defences,” explains Dr David Marks, of Levity Crop Science, a British company specialising in bio-active crop enhancement.

“Calcium gives the cell wall rigidity and strength. When levels fall, the fungus takes advantage.

“In a typical 50t/ha carrot crop that suffers complete loss from cavity spot, just 1 tonne of plant tissue would actually be calcium-deficient. The difference between healthy and affected tissue is tiny: four parts per million, or 4mg/kg.

“Preventing susceptibility to cavity spot can hinge on around 4g/ha – that’s less than a teaspoon of calcium per acre.”

Understanding why these tiny amounts of calcium are not delivered by in-field calcium applications is the ‘calcium conundrum’. But after studying calcium’s behaviour in the plant, Dr Marks has not only explained the conundrum but developed a solution.

“Unlike most nutrients, calcium only moves upward through the plant,” he says. “When absorbed through the roots, it goes to the leaves where it’s either stored or lost from the plant as excess.

“Meanwhile foliar calcium applications follow the ‘Las Vegas’ maxim: what happens to the leaves, stays in the leaves. Foliar calcium doesn’t correct root deficiencies.”

Yet soil applications are also fraught with difficulty. To absorb calcium, plant cells require high levels of auxin, a plant hormone. Auxin levels vary: areas of high production include new shoots, flowers and leaves, while low auxin synthesis characterises locations such as fruits, roots and tubers.

“This explains why attempts to use calcium to correct physiological disorders is so ineffective,” explains Dr Marks. “The role of auxin is to induce cell division in fresh tissue. In carrots, where the root grows from the tip, those new tip cells have lots of auxin: calcium absorption is good. Older cells further from the tip have less auxin, so absorb less calcium.”

Dr Marks’ explanation is borne out in practice. Severity of cavity spot gets worse with distance from the root tip (less auxin in the cells), changes in growing conditions (water status and temperature affect calcium absorption) and later harvesting (over-mature carrots have less auxin).

To resolve the problem, Dr Marks and his team turned to a group of naturally occurring compounds that stimulate calcium uptake in the absence of auxin. These ‘calcium transport stimulants’ had been successfully used to control a calcium-related plant disease, bitter pit, in apples – a chemistry technology that Levity termed LoCal.

“In this instance, we created a granular calcium fertiliser incorporating LoCal technology,” says Dr Marks. “Albina is a slow-release prill that can be applied at planting to root crops. It is the first product to supply slow-release granular calcium, via chemistry that allows active uptake by roots.

“Resolving the calcium conundrum, it releases calcium slowly over the growing season. A carrot crop gets calcium in the right place, at the right time, in the right form.

“Albina provides an opportunity to reduce the reliance on fungicides, creating a healthier carrot crop that denies Pythium the opportunity to take a hold.”

In the United Kingdom, Europe’s second-biggest carrot producer, Dr Marks says growers are rightly concerned that only one fungicide, metalaxyl, is approved for cavity spot control. “There are anxieties about best practice in product stewardship, particularly as anecdotal evidence suggests only 50 per cent of routine fungicide applications had any effect.”

UK carrot producers who have used Albina have reported strong results, Dr Marks reports, with commercial trials backing up the findings.

Dr Marks hopes these results will spark interest from growers in other major markets to support further registrations of Albina beyond the UK.