Improving Colour Formation in Apples

As an apple crop gets closer to harvest it can be a frustrating time for farmers watching crops hit the right size but not form enough colour and sugars to be picked. Often early harvest is where a grower can get the best price in the market, and of course the longer the time to get the whole crop off the orchard the higher the labour costs involved.

Here we look at the role of molybdenum in fruit maturation, and showcase how Sulis a product developed by Levity CropScience has been helping growers in the UK and US improve colour in apples.

Ethylene does not directly produce colour in apples

Ethylene is the hormone most associated with fruit maturity in apples, and many of the methods and products used to encourage maturity focus on either applying ethylene or stressing the trees to encourage its production.

However ethylene is not the only ripening hormone, and its effects on colour and brix levels are not direct. Ethylene’s role in the ripening of apples is primarily that of fruit softening.

Ethylene stimulates cell wall hydrolase genes to produce polygalacturonase (Tacken et al, 2010). This is why use of ethylene promoters to stimulate colour and brix are associated with loss of fruit firmness. Ethylene also stimulates removal of calcium from cell walls, which is why apples have higher incidence of bitter pit both pre and post harvest when there are stressful growing conditions.

ABA is the primary hormone involved in colour formation in apples

It is not however ethylene that is responsible for triggering colour and brix formation in apples, that process is stimulated by a different hormone ABA (Sata et al 2018).

When ethylene is produced or applied to a crop the trees also start to produce ABA, which then stimulates the enzyme activity that produces maturation.

What is the link between molybdenum and maturation of apples?

Mo deficiencies are not only transient and short-lived, but virtually impossible to detect: quite different from copper or sulfur deficiencies, for example, where the leaves tell the whole story.

Molybdenum is a limiting factor in synthesis of ABA, to make it plants must have adequate levels. However simply upping levels of moly in the crop is not enough to ensure the plant can make enough ABA to improve fruit maturation.

Sulis can help improve colour in apple crops

Levity have developed Sulis, a product that gives the plant the molybdenum it needs to power ABA production, alongside ‘Blush’ a biostimulant that ensures rapid uptake and utilisation by the crop and switches on the enzyme systems that power maturation.

Importantly Sulis achieves this without also triggering a reduction in fruit firmness as it contains cell wall stabilisers that counter the effects of ethylene and prevent loss of calcium from fruit.

Sulis is proven to improve colour, brix and earliness in multiple UK and US field trials.

This 2014 trial on Gala in Kent the main apple growing region of England used 2 2L/Ha applications of Sulis and compared fruit colour. Apples from the Sulis treated blocks (left) were significantly more red than apples from the control blocks.

This graph taken from the same study on Gala shows proportion of fruit colouration split between green, yellow and red and showed a significant shift towards red and yellow and a large reduction in green.

As well as producing better colour, the brix levels were also significantly higher. Additionally in blind taste tests the Sulis treated apples were consistently selected as having better flavour.

Good results were also obtained on a number of other varieties including Zari, Braeburn and Tentation with all varieties showing significant improvements in both colour and brix levels.

In the Zari trial the control plots required four picks, whereas all fruit was harvested in two picks in the Sulis treated plot. This not only meant higher priced produce for the farmer but also gave significant labour savings.

US trials (carried out in partnership with Omex US) compared Sulis to commonly used treatments for improving colour (Potassium based) and for bitter pit reduction (calcium based treatments). The trials measured colour and brix but also measured other parameters to study the effect on quality by observing changes in quality problems associated with low calcium and decreased cell strength.

In Honeycrisp (a variety prone to bitter pit sulis reduced pre-harvest bitter pit to zero and post-harvest bitter pit to just 2% (compared to 30% for other treatments).

US trials also showed significant improvement in earliness, colour and brix on a number of varieties including Honeycrisp, Envy, Cosmic Crisp, and Gala.


To stimulate colour and brix ahead of harvest, apply Sulis at 2L/Ha as soon as fruit starts maturation, repeating the application at 7-10 day intervals.

For more information on Levity’s products, research and trials on apples check out our apple guide

Sulis Fertiliser Bottle

Please contact us for more information about purchasing Sulis.

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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.

Author: David Marks

Managing Director, Linkedin Profile