Apple Red Skin Coloration: A Grower's Checklist

Color Importance

Red skin coloration is highly associated with sales, as vibrantly colored fruits are more appealing to the consumer's eye. In addition, darker fruit coloration is associated with increased nutritional value and antioxidant properties that have many health benefits, including potential cancer prevention. However, in the last few years, reaching the required 50-60% red skin coloration required by retailers, particularly in highly profitable early-season varieties such as Honeycrisp, has been a major challenge in the mid-Atlantic region.

Pigments Involved in Color Development

There are mainly three pigments concentrated in apple's skin that determine its coloration: anthocyanins, carotenoids, and chlorophylls. These pigments are synthesized through multi-step biochemical processes, or from a chain of chemical reactions. These processes are regulated, directly or indirectly, by factors such as genetic background, developmental stage of the fruit, nutritional status, canopy architecture, crop load, but also strongly affected by environmental factors such as temperature and light.

Anthocyanins

Anthocyanins are a set of phenolic compounds which are the main pigments in fruits, generating the characteristic blue, purple, or reddish hues. The content and composition of anthocyanins mainly determine the intensity and quality of red skin coloration in apples. Anthocyanins, as other phenolic compounds, are strongly associated with improved antioxidant activity in fruits and can also improve their nutritional value, hence increasing health benefits for consumers.

Anthocyanins biosynthesis in apple skin is developmentally regulated, taking place during the fruitlet stage as well as during fruit ripening, with the ripening stage being the most important. Furthermore, sugars are a key component of anthocyanins formation, making coloration strongly dependent on apple's sugar contents and composition.

Other Pigments

There are other pigments that contribute to the color of apples, such as carotenoids and chlorophylls. Carotenoids lend a yellow or orange color to fruit, with chlorophylls adding a green hue. Chlorophylls are concentrated in a greater proportion of unripe fruit and decrease as the fruit ripens.

More information on apple color from Penn State Extension: Fruit Color - Promoting Red Color Development in Apple

Environmental Factors and Apple Red Skin Coloration

The two main environmental factors that affect apple red skin coloration development are temperature and light.

Temperature

The ideal conditions for red color development in apples correspond to bright, clear days with temperatures of around 77ºF (25ºC) and cool nights (59ºF, 15ºC) throughout preharvest (3 weeks before harvest). When exposed to these conditions, trees are not stressed during the day, thus increasing their canopy photosynthesis and decreasing respiration rates at night. Increasing photosynthesis promotes the accumulation of sugars (carbohydrates) and decreasing respiration rates help to decrease the breakdown of sugars. Therefore, under the above temperature conditions, more carbohydrates are readily available. This is of crucial importance as sugars constitute the raw material for the production of anthocyanins.

Anthocyanin biosynthesis is suppressed at warmer temperatures (hot days (>90ºF; >32ºC) and warm nights (>68ºF, >20ºC)), thus making it difficult to produce apples with sufficient red skin coloration in hot seasons, especially early-harvest varieties such as Honeycrisp.

Light

Light intensity and quality (wavelength) are also crucial factors affecting apple red skin coloration during development and ripening. This is particularly important as all the chain of biochemical reactions involved in the synthesis of anthocyanins are induced by light. Skin color pigments are synthesized as a response to stressors such as sunlight, specifically in the ultraviolet (UV) radiation wavelengths. UV radiation wavelengths are shorter and higher energy than visible light that we see as colors (Figure 1) Therefore, the more that apples are exposed to the sun's rays—specifically UV wavelength—the more color that the fruits will develop on the skin. But this is only to a certain limit, as too much UV radiation can cause fruit sunburn, just like in human skin.

Figure 1. Radiant energy spectrum showing ultraviolet light, which promotes red coloration. Source: Lumenistics, 2012.

Sub-optimal temperature and light conditions present in the mid-Atlantic at the time that varieties such as Honeycrisp are harvested in recent years have resulted in a poor or marginal red skin coloration development in these varieties. As environmental conditions are difficult to control, the focus must be placed on managing the tree, its physiology, and uniform light distribution for improving coloration.

Site of Orchard Establishment and Apple Red Skin Coloration

Elevation

The altitude at which fruits are grown can also have an important effect on apple red skin coloration. To maximize fruit color, cooler and higher elevation areas are preferred, allowing for the necessary temperature differential between daytime and nighttime that will increase anthocyanin accumulation and thus the development of red skin coloration.

Aspect

Aspect, or the way that the sun hits the earth can also impact the development of red skin coloration. The sun's rays shine more directly on the southern than it does the northern side, therefore south-facing fruits receive much more UV radiation from the sun than a northern facing counterpart.

Soils

The inherent water- and mineral nutrient-holding capacity can affect red color directly, through nitrogen (N) availability (high N delays chlorophyll disappearance), and indirectly by promoting tree vigor and the potential for shading within the canopy.

Cultivars, Strains and Rootstocks and Apple Red Skin Coloration

The maturity date of a cultivar or strain determines the temperature range encountered at fruit maturity. Prolonged warm spells in August and September that have been experienced in recent years can result in the insufficient red color of early-ripening cultivars such as Gala and Honeycrisp. Warm harvest weather, combined with ever-stricter color grading standards may result in loss of value, particularly with older, less red strains of these cultivars. Thus, in recent years newer highly colored strains of Gala, such as Gale, Buckeye and Brookfield, have become predominant throughout the industry.

Until recently, the supply of Honeycrisp apples lagged behind the demand, which provided some insulation from the need to plant redder strains, but this phase of the Honeycrisp market has ended and strict red color standards are now being enforced. Red strains, such as Firestorm, Royal Red, and B42 Honeycrisp should be selected for replacing older selections that can no longer meet color standards (Figure 2).

Figure 2. Two red strains of Honeycrisp (Left and right) with the standard strain (center). Photo: J. Schupp, Penn State

Red strains of apples have received a bad rap owing to the mismanagement and poor eating quality of certain strains of Delicious. There is no evidence of a link between red color and poor taste of redder strains of other cultivars, including Gala and Honeycrisp. Sensory studies of B42 Honeycrisp and a standard strain conducted at Penn State showed that panelists could distinguish no difference between the strains in taste or texture and that they preferred the appearance of B42 over the standard strain.

Red strains may develop marketable red color before reaching the optimal fruit maturity to ensure good eating quality. Growers do need to make sure that the maturity of harvested fruit is appropriate for its intended use and storage period.

Size-controlling rootstocks reduce canopy volume and density, resulting in increased light distribution and improved red fruit color. Tree vigor must be strong enough to develop the full productive potential of the orchard, yet result in a calm tree at maturity. Growers should plant the most dwarfing rootstock that can be managed, given the inherent vigor of the soil and cultivar (Figure 3).

Figure 3. Even a dwarf apple tree can have poor fruit color in the lower canopy. Photo: Don Elfving, Washington State University

Mineral Nutrition and Apple Red Skin Coloration

Balanced mineral nutrition is critical to achieving high marketable yields of attractive red fruit. Although all mineral nutrients are needed to achieve balanced growth and fruit quality, two minerals stand out when focusing on red color—nitrogen and potassium.

Adequate leaf nitrogen is required for high productivity, but excess N is detrimental to red coloration. High N delays chlorophyll disappearance in the peel and results in a dull muddy color. Excess N also promotes vegetative growth which can result in shading and loss of red color. Cultivars differ in sensitivity to N. Gala color is less sensitive to N, and leaf N levels of 2.1- 2.3% will improve tree vigor and fruit size without being detrimental to color. Conversely, Honeycrisp, Jonagold, and Fuji are sensitive, and leaf N should be managed in the range of 1.8-2.0% to maintain good red color and prevent excessively large fruit.

Low potassium (K) results in poor fruit color. The optimal range for leaf K in apple is 1.5-2.5%, and it should be noted that apple fruit are high in K. This creates a need for active management of this mineral to replace that which is being removed by the crop. Correction of low K can produce a rapid and dramatic improvement in red fruit color, leading some to believe that "more K is better". There is no evidence that excess K can increase red color, but there is ample evidence to show that excess K can make bitter pit worse in Honeycrisp.

Crop Load Management and Apple Red Skin Coloration

An excessive crop causes trees to produce fruit with poor red skin coloration (Figure 4). The lack of red coloration is due to a shortage of resources, specifically sugars and nutrients reaching each fruit as a result of the increased competition. The lower sugar contents limit the fruit's ability to synthesize anthocyanins, thus preventing red skin coloration development. Excessive crop loads in varieties such as Honeycrisp are particularly harmful as the high depletion of sugars will affect fruit coloration development of the current year's production, and also the juvenile buds that will develop into next year's crop.

Figure 4. Effect of crop load density on Honeycrisp red coloration. Photo: J. Schupp, Penn State

Low crop loads, on the other hand, will promote tree vigor and vegetative growth, consequently affecting red skin color development as the few apples that are left on the tree will be shaded and thus will not receive the necessary sunlight required for anthocyanin biosynthesis. This will also limit total crop yield and profitability.

Therefore, crop load management must be targeted towards a balanced orchard. The optimal recommended crop load is around 7-8 fruits per cm² TCSA (Trunk Cross-Sectional Area) which will increase red skin coloration and maintain adequate fruit size while maintaining return bloom for the coming year.

Fruit Maturity Management and Apple Red Skin Coloration

Fruit maturity is also a critical factor affecting apple red skin coloration as anthocyanin synthesis is developmentally regulated, occurring mainly during fruit ripening. Therefore, controlling how and when apple fruit ripen will impact red coloration development. Ethylene, which is a gaseous plant hormone controlling ripening, can be regulated with the use of preharvest plant growth regulators, such as Ethephon, ReTain®, or Harvista®.

Ethephon (Ethrel®, Bayer Crop Science; MotivateTM, Fine Americas; Ethephon 2, Arysta LifeScience North America, LLC), which is an ethylene-releasing chemical, promotes fruit ripening and can help increase red coloration, but if temperatures are above optimal it will only advance maturity without increasing red coloration. Ethephon also accelerates fruit abscission and may negatively impact fruit storability.

ReTain® (active ingredient: Aminoethoxyvinylglycine (AVG), Valent USA) will inhibit ethylene production, delaying fruit ripening as well as color development.

Harvista® (active ingredient: 1-Methylcyclopropene (1-MCP), AgroFresh), will bind to ethylene receptors in the fruit, blocking its perception. This prevents the response to ethylene in the fruit, therefore, delaying ripening.

ReTain® and Harvista® will allow keeping fruit on the tree for a longer time, preventing fruit drop. If this delay shifts maturity into a cooler weather window, it may help fruit reach optimal coloration. Additionally, these chemistries increase maturity consistency throughout the orchard, which helps to manage fruit harvest windows.

Light Management and Apple Red Skin Coloration

Light is one of the critical factors affecting apple red skin coloration. However, undertake canopy light management with caution to avoid excessive exposure of fruit skin. Just like in human skin, the energy of sunlight can increase the potential for sunburn on the sun-exposed surface of apples. The heating effect of sunlight combined with the damaging effects of ultra-violet (UV) radiation are the main causes of sunburn in apples. Therefore, while it is beneficial for red skin coloration to expose the fruit to sunlight, it must be managed with limits.

There are several ways to manage sunlight in the canopy, with pruning and the use of reflective mulches being two of the most popular.

Pruning

Pruning is the long-established method of increasing light distribution in the canopy to promote red fruit color and other light-driven processes, such as flowering and fruit set. Conduct annual dormant pruning, making thinning cuts to remove entire limbs at their point of origin. This type of cut slows and limits the amount of regrowth, thus thinning out the branches and creating windows where sunlight can penetrate the canopy. Remove the largest limbs, especially in the top half of the tree to create a cylindrical or cone-shaped canopy. Then thin out additional limbs to space them out both radially and vertically. When tree vigor is strong, summer pruning to remove upright shoots and leaves can confer a temporary increase in sunlight. Summer pruning is typically undertaken about two weeks before the start of harvest to provide additional sunlight to promote red coloration.

Root pruning is another technique to reduce canopy vigor and improve red color. Root pruning is done with a tractor-mounted subsoiling blade or coulter wheel-mounted offset on a toolbar. Root pruned trees have shorter shoots and smaller leaves, contributing to increased light penetration. To be effective, root pruning is done early in the season just prior to or at the start of growth. Both sides of the row are cut at 16-18 inches from the trunk so as to remove enough roots to have an effect.

Plant Growth Regulators (PGRs)

PGRs containing the active ingredient prohexadione-calcium (PCa) (Kudos™,Fine Americas; Apogee™, BASF) can be used in combination with pruning to reduce vegetative growth and improve light distribution. PCa reduces shoot elongation, resulting in a more compact canopy with improved exposure to light.

Reflective Mulches

Groundcovers made with reflective material are a preharvest management tool that is used to improve the tree canopy light environment. Reflective groundcovers in apple orchards essentially bounce the light reaching the orchards floor back up into the canopy enhancing the capacity of apple trees to harness sunlight (Figure 5). This increases the amount of light reaching the apple fruit surface and exposes more apples to light within the orchard. This improved light environment is used by the tree to power its production of sugars, increasing fruit size, and red skin coloration. The use of reflective mulches can particularly increase light penetration in the inner and lower parts of the canopy, which are areas where generally less light is intercepted.

Figure 5: Deployment of Extenday reflective fabric to a block of Morning Mist ('Fugachee Fuji'). Photo: Thomas Kon, North Carolina State University

There are multiple types of reflective groundcovers in the market, such as white reflective fabrics (Extenday; ProLine), white film, or metallic films (Mylar). White reflective fabrics can last for up to seven years and can be rotated throughout the orchards; while white and metallic films have lower costs but usually are disposed of after one season. When purchasing, make sure that the chosen fabric reflects radiation in the UV range, as this is the wavelength that will promote red coloration. The establishment of these groundcovers is particularly crucial around 3 weeks before the anticipated harvest.

Summary

The importance of fruit color can't be underestimated, as it can increase sales at the marketplace and increase your profits. Not to be forgotten are the nutritional benefits from deeply colored skin, since a darker color indicates the presence of a high concentration of antioxidants. Replace older, less colored orchards with improved red strains when they become available. Since we cannot control temperature, it is important to control other factors of color development. Use leaf and soil analysis to maintain moderate tree vigor, with adequate levels of N and K. Use thinning cuts during dormant pruning to create a narrow canopy and reduce shading from excess branching. It is important to get a significant amount of UV radiation from sunlight in the last few weeks of ripening to get the best color. If standard methods are not working well, consider plant growth regulators to reduce vigor or alter the maturity window, or the installation of reflective mulches to maximize the UV radiation that hits the apples in order to capitalize on what the sun gives us.