Mar 4, 2021

‘Calcium conundrum’ addressed as carrot growers tackle cavity spot

A quarter of a teaspoon of calcium per acre: 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 U.S. carrot growers, with problems compounded by unreliable results from the few fungicides approved for control.

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 defenses,” said David Marks, of Levity Crop Science, a British company specializing in bio-active crop enhancement.

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

“In a typical 400cwt/acre carrot crop that suffers complete loss from cavity spot, less than 900lbs of plant tissue would actually be calcium-deficient. The difference between healthy and affected tissue is tiny: four parts per million, or around one-eighth of an ounce of calcium per ton.

“Preventing susceptibility to cavity spot can hinge on around one-quarter of 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 behavior in the plant, Marks has not only explained the conundrum but developed a solution.

“Unlike most nutrients, calcium only moves upward through the plant,” Marks said. “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 characterizes locations such as fruits, roots and tubers.

“This explains why attempts to use calcium to correct physiological disorders is so ineffective,” Marks said. “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.”

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, 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 fertilizer incorporating LoCal technology,” Marks said. “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, Marks said 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 percent of routine fungicide applications had any effect.”

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

With a number of Levity’s products already available to U.S. growers, especially in the fruit and potato sectors, Marks hopes Albina will spark sufficient interest to support a U.S. registration.

Founded in 2011, Levity Crop Science is a British company that seeks to develop and commercialize sustainable agricultural products. Its definition of a sustainable product is one that improves crop production, reduces waste, increases yield and ultimately contributes to farmers’ profitability. Levity’s products are sold and used in countries around the world.