Apple growers know that red coloration in many commercially important varieties is a requirement for sale. Lack of color development is often related to lack of maturity: the apples simply are not ready for harvest. But sometimes the apples are ripe and just haven’t developed the degree of red coloration required to sell to the commercial market.

Harvest is often delayed to allow red colors to further develop. Even direct-to-consumer farm stands hear complaints, and can lose sales, due to apples which don’t show the degree of color development customers expect.

No matter the situation, lack of apple color development can have frustrated growers seeing red.

“We all know the red color in apples is associated with sales. Grade is going to be higher if it is at least 50% to 60% of red skin color,” said Dr. Macarena Farcuh, University of Maryland Extension specialist, in a webinar hosted by the UMass Fruit Team.

Developing the desired red color without sacrificing apple quality can be a challenge. Farcuh’s research aims to help farmers see more red in their apple pigmentation – and eliminate the red they see in frustration when ripeness, harvest window and pigmentation development are out of sync.

Primary Colors

Holding ripe apples on trees to allow more red color to mature is not a solution, Farcuh said. Instead, those apples will have lower quality with a decrease in storage life; an increase in abscission from the tree pre-harvest will occur.

Pigment changes in apples are highly related to changes in sugar content. As the fruit ripens and sugar content increases, so too does red pigmentation, which is caused by anthocyanins. Chlorophyll and carotenoids cause the green and yellow/orange hues, and these background hues decrease as the fruit ripens.

Anthocyanin production is impacted not only by sugar content and maturation stage; it’s also affected by the nutritional state of the tree, its crop load, the use of plant growth regulators, daytime and nighttime temperatures and the intensity and quality of the sunlight available to the fruit. Development of red coloration in apples is complex.

The specific apple cultivar, as well as the strain, also determines the proclivity for red pigmentation to develop, with modern strains trending toward enhanced red coloration. Rootstocks too play an important role in the degree to which red coloration can develop.

Early season cultivars (such as Honeycrisp) are more susceptible to reduced red pigmentation development due to increasing temperatures than later season varieties. Anthocyanins develop (ideally) at daytime temperatures of 77º F with cool night temperatures of 59º pre-harvest. This allows a lot of daytime photosynthesis, yet the sugars aren’t totally broken down at night thanks to cool temperatures. This decreases respiration rates and promotes the retention of anthocyanins.

As nighttime temperatures rise above 68º, anthocyanin production slows down and the development of red pigmentation is minimized.

The biosynthesis of anthocyanins is dependent upon light-inducible enzymes. UV light, which has shorter wavelengths and higher energy levels than visible light, is essential for red color development.

Normally, any UV light that reaches the orchard floor is absorbed into the soil. Capturing that UV light and reflecting it back up into the tree canopy has previously been shown to improve red coloration in apples which are hanging in the lower one-third of the tree canopy.

Research Results

Farcuh’s research has shown that by placing reflective ground cover in the orchard rows three weeks pre-harvest, red coloration can be enhanced in the lower canopy. The result is an increase in the harvest of fruit with acceptable red coloration levels without any significant increase in fruit drop or impact on apple quality. The reflective ground covering also helps fruit to ripen uniformly from the top canopy to the fruits on the lowest limbs.

Reflective ground covers can be reusable metallic fabrics or single-use films. The researchers utilized films in Mid-Atlantic commercial orchard plots in Aspers, PA. Blocks of Honeycrisp grown on M9 rootstocks and Evercrisp® grown on B9 rootstocks were examined.

There were four replications of 20 tree blocks for each apple variety. Control blocks had no reflective ground coverings, while the treatment blocks had Oostende reflective film laid down between orchard rows for four weeks prior to the commercial harvest date.

Honeycrisps were harvested on three separate days in September, from only the lower one-third of the canopy – the area of impact for reflective fabrics. Measurements for fruit quality included hue angle levels using a color meter, red blush development as a visual measurement and chlorophyll content via a difference of absorbance (DA) meter.

Honeycrisp apples harvested from Oostende-treated blocks showed increased visible red pigmentation as well as a decrease in background hues of green and a decrease in chlorophyll skin content on the unblushed sides across all three harvest dates. Results were seen on both the blushed and unblushed sides of the apples. There was a significant increase in apples showing the needed 50% blush for commercial grading with the Oostende treatment.

Evercrisp apples showed hue angle decrease on the blushed sides, with darker red coloration, on Oostende plots, while unblushed sides did not differ from the control blocks. Visible skin pigmentation was greater than the 50% needed for top commercial grade on both the control and Oostende blocks, but Oostende blocks had greater red pigmentation overall.

Both UV light and photosynthetically active (PAR) light are necessary for red skin color development. The reflectivity of both from the ground back into the canopy were increased in all Extenday reflective groundcover blocks, with UV wavelengths showing a greater percentage increase in Honeycrisp from Extenday treatments. Evercrisp apples in Extenday treatment blocks showed an increased percentage of UV and PAR light reflectivity into the canopy as well.

Fruit drop percentages increased at later harvest dates across all treatments and cultivars. However, “no significant difference in fruit drop percentage throughout ripening, between Oostende and non-Oostende treatment, in any of the cultivars” was found, Farcuh said, indicating that Oostende does not increase fruit drop during ripening.

The researchers haven’t studied the effect in different orchard architectures or tree sizes. They did not look at different timeframes for applying the Oostende film prior to anticipated harvest date. They did not see any increased vole damage in blocks with Oostende.

While reflective films require some investment in money and labor, they are effective in enhancing the development of anthocyanins in apples – with no negative effects seen – and result in a higher percentage of apples which meet the coloration standards for commercial fresh sales at harvest.

“The use of Oostende does improve fruit skin red blush and canopy light distribution of commercially important apple cultivars,” Farcuh concluded.

You can view the webinar for more details at this link.