Growers are being encouraged to consider applying calcium to their apples ahead of harvest, to help tackle the negative effects that recent spells of heat and drought stress may have on their trees, in order to minimise the chances of bitter pit while in storage.

According to Scott Garnett, ICL senior speciality fertilizer agronomist, the lack of calcium in the fruit itself, rather than the tree as a whole, leads to the development of black and brown lesions indicative of bitter pit, when in storage.

AHDB fertiliser guidelines state that the risk of bitter pit depends on the potassium to calcium ratio. If it is higher than 30:1 in air stored Cox or controlled atmosphere stored Bramley, or 25:1 in air stored Bramley, then commercially significant losses and bitter pit are likely.

However, there are several control measures, including calcium application, that will help to mitigate this risk. 

“Thinning of the fruit will help to restore the vigour of the plant and the apples produced will be higher quality. And as calcium favours residing in the leaves, rather than the fruit, it’s important to prune excess growth. This ensures that the apples receive as much calcium as possible, and the majority of the tree’s energy goes into producing fruit,” says Scott.

Calcium can be applied throughout the growing season, but the late summer or early autumn is the most important period to help maintain fruit firmness and storage quality ahead of harvest.

Scott mentions that one of the biggest problems facing the top fruit industry, is the fact that it is extremely difficult to encourage foliar applications of calcium to penetrate the apple’s skin, and this is where bitter pit stems from.

“But trials have shown that when using Agroleaf Liquid Calcium+, a combination of amino acids, plant sugars and a penetrating surfactant, the calcium is moved past the epidermis level of the skin, and bitter pit is significantly reduced.”

As calcium application is a preventative treatment for bitter pit, it’s essential that growers make their application before it’s too late.

“The deficiency isn’t visible while the fruit is on the tree, and there’s nothing growers can do to restore their health once the symptoms show in storage. The apples will be worth nothing to the grower,” adds Scott.