Improving Apple Color
Color management of apples is a topical issue for many growers at present. Foreground export color requirements on many cultivars have recently become stricter, and may continue to do so in the future. Furthermore, color banding has now been incorporated into the grading/packing standards of many of the widely grown cultivars, with an extra $3 to $4 per carton likely to be paid for fruit with superior red coloring. Therefore, producing apples with a high percentage of red color and high color intensity is vital if growers are to achieve the highest payments for their fruit.
Red color development in apples is due to the formation of anthocyanin pigments in the apple skin. Anthocyanin production, and therefore apple color is influenced by a range of environmental and management factors in the orchard. Temperature during the preharvest period influences red color development, with the critical coloring period being two to three weeks before harvest. Optimum temperatures for red color formation vary among cultivars, but are generally between 20 and 25 °C. Cool night temperatures (< 18 °C) from a few weeks before harvest enhance color development of the fruit, especially on poorly exposed areas, and are necessary to produce fruit with a high percentage of red color. However, if cool night temperatures are followed by excessive (> 30 °C) daytime temperatures, the positive effects on anthocyanin synthesis will be negated. Light levels are also an important factor in anthocyanin synthesis, but can be controlled to some extent through orchard management practices. This article will evaluate practices which influence red color development in apple and review steps which can be taken to increase color development in the orchard.
Cultivar and Site Selection
Cultivar selection is one of the first steps towards maximizing apple color on an orchard. Growers have a wide range of cultivars to choose from, including many red sports of commonly grown cultivars such as Royal Gala and Braeburn. Brookfieldä and Galaxyä are grown in many orchards as alternatives to Royal Gala, while Auroraä, Eveä, and Mahana Redä are receiving increased interest as highly colored alternatives to Braeburn.
If fruit coloration has been a consistent problem in particular cultivars, it may be worthwhile replacing them with high-color strains if they are available, or cease growing the variety altogether. This is a decision Fuji growers in the Central Valley, California have faced recently. Extreme daytime heat and high night-time temperatures have meant growers in this area are unable to produce the deep red colored Fuji that Asian markets demand. This is despite growers changing their pruning techniques and growing systems, and using reflective mulches. As a result, many growers are now growing Granny Smith or Gala instead of Fuji as these varieties are known to perform well under Central Valley conditions.
Because temperature has a substantial influence on color development, care should be taken to ensure cultivars are suited to the growing region. Although many apple varieties are grown throughout New Zealand, there is evidence to suggest certain cultivars may perform better in some areas than others. Growers have experienced coloring problems with Braeburn when grown in cooler areas such as Otago, while climate plays a big role in the color performances of Pink Ladyä in Australia. It has been suggested that Pink Ladyä requires both cold and warmth with high sunshine hours as fruit approaches harvest. Where fruit has been grown under these conditions in Australia, highly colored fruit has been produced with little effort put into other management factors. Therefore, when choosing cultivars to grow, it is important to consider the climate of the area and whether this matches the conditions required by the cultivar to produce high quality fruit.
Good canopy management is probably the most important factor in improving apple color. The correct style and severity of pruning is needed to open up the tree canopy to allow sufficient light through for fruit coloration. A compromise must be made so that enough wood is removed to achieve high light levels without reducing yield too much. Apples require a minimum of 50 percent full sunlight on fruits and leaves in order to achieve adequate red color, with superior coloring occurring at levels around 70 percent full sun. A textured canopy with filtered light is best for achieving good coloration, with growers running a risk of sunburnt apples if fruit are exposed to direct sunlight for long periods.
Good canopy management should be achieved through winter pruning alone, although summer pruning may be required on some cultivars and growing systems. High vigor varieties, such as Royal Gala may need summer pruning if winter pruning was not adequate, while trellised growing systems almost always require summer pruning. Timing is critical in achieving good results, with early shoot removal necessary to avoid sunburn and to allow time for fruit to color. Surplus water shoots and suckers should be torn out before they harden as unlike cutting, tearing removes the adventitious buds at the base, making regrowth less likely.
Crop loading is a significant factor for maximizing both fruit color and fruit size. Excessive crop loading (i.e., fruit clusters of 2 to 3 fruits) reduces fruit coloration through direct shading of neighboring fruit, or through competition for assimilates needed for coloration. A high fruit to leaf ratio may mean that the leaves cannot produce enough carbohydrates to color or size all of the fruit to their full potential. Fruit thinning is therefore needed to ensure the fruit to leaf ratio is within the desired range (i.e., at least 10 to 15 leaves per fruit) to reach an adequate size, and that apples are not in contact with each other so as to cause mutual shading. This may mean that fruit are reduced to ones or twos, but the overall result will be an increase in packout rates due to increased color.
Management of the soil nutrient status is important to ensure that excessive concentrations of nitrogen do not build up in the foliage and fruit.
High nitrogen can increase shoot production, leading to shading and therefore coloration problems. Studies have also shown nitrogen has a direct inhibitory effect on anthocyanin expression. Midsummer leaf nitrogen levels below 2.0 percent are usually best for full red color development, although such levels may reduce overall fruit size and adversely affect flowering and fruit set. Consideration should be given to postharvest fertilizer applications, as early summer nitrogen applications can be detrimental to color development.
Reflective mulches, placed on the ground between tree rows, have been used in some countries to increase red color in apples. The mulches, usually made from plastic or aluminum foil, increase both the percentage of red color and color intensity in apples without affecting other quality characteristics such as flesh firmness and soluble solids. In a trial carried out on Fuji apples in California, the percentage area of skin red color development was increased by 65 percent with an increase also seen in the color intensity using an aluminum foil under central leader trees. Reflective mulches increased the amount of the crop being packed in either Fancy or US#1 grade (75 percent) compared with the control (57 percent).
The mulches work by reflecting light back up into the tree canopy, thereby increasing the light available for anthocyanin synthesis. Experience in the United States has shown that mulches need to be put down only about one month before the predicted harvest date to achieve an improvement in color.
Using reflective mulches to increase color development shows good potential for use in New Zealand orchards, especially in seasons where color development is poor. Even though the cost of the material is relatively high (between $400 and $2000 per hectare, depending on whether plastic or aluminum is used), the increase in returns on blocks which exhibit poor color could justify its use.
Regulated Deficit Irrigation
Regulated deficit irrigation (RDI) is a technique that has been shown to increase the intensity of red color in apples. It involves supplying less water to the plant than it requires for maximum transpiration and growth, thereby inducing a low level of water stress. The vegetative growth of the tree is reduced, allowing more light to penetrate the canopy and color the fruit. RDI has been used primarily to manage vigor, but at Massey University when irrigation was restricted for the entire season, there was an increase in red color intensity of Braeburn apples. Severe water stress, however, may adversely effect color development.
A range of chemical products have been trialed for use as color enhancers of apples. Ethephon, an ethylene-releasing compound, has been used to advance ripening of early season cultivars such as Gravenstein. Ethephon works by stimulating the ripening process in the fruit, thereby advancing color development. This advancement in ripening makes it undesirable to use it on export fruit, but it may be useful in enhancing color on local market apples. However the eating quality of the fruit can be reduced due to the advancement of softening. Ethephon is generally used at rates between 100 and 600 ppm, anywhere between one and three weeks before harvest. By using low rates of ethephon (75 ppm) in combination with NAA 60 days before harvest, an increase in fruit coloration can be achieved without increasing fruit softening or drop.
Seniphos, a mineral mixture of phosphorous pentoxide, calcium oxide, and nitrogen, increases red color distribution and intensity in apples. Unlike ethephon, Seniphos does not stimulate other ripening characters, making it more desirable for use on fruit destined for long-term storage. Only one application is required two to three weeks before harvest, at a rate of one percent to achieve an improvement, although recommendations are for two to three applications before harvest. This makes it an ideal product to use late in the season if growers are experiencing low color development.
Another chemical that may improve fruit color is ReTain, a growth regulator which has been used on some apple orchards in the United States. It has been shown to delay harvest and in some varieties to result in fruit of superior color. While it has worked well in early-season varieties such as Royal Gala in the United States, its high cost has not been covered in low value cultivars like Red Delicious. ReTain may be suitable for use on selected cultivars, especially those of high value like Pacific Rose. Large-scale trials are currently being carried out to see the effect of ReTain has on apples grown in New Zealand climatic conditions.
In order to produce fruit that meets market demands, an integrated approach to color management is needed. Orchard management practices, such as cultivar choice, tree spacing, pruning and fruit thinning, need to be adapted to reflect the changing environmental fruit conditions and quality standards. Additional color improving techniques, such as RDI and the use of reflective mulches, need to be considered for use, especially in seasons unfavorable for apple color development. Furthermore, ongoing research must be supported to ensure improvements in apple color continue. By taking all of these factors into consideration and adopting an integrated approach to color management, growers should be able to improve the color of their fruit.
Sarah Gurnsey and Dr. G.S. Lawes
Institute of Natural Resources
Reprinted with permission from:
Tree Fruits Tasmania, No. 3, December 1999
Originally published in The Orchardist of NZ, March 1999