Superficial Scald in Apples: Strategies and Solutions
What is superficial scald in apple fruits and what causes it?
Physiological disorders correspond to abnormalities that can occur in any of the apple tissues, and can result in loss of quality, marketability, and profitability, leading to increased loss and wastage of apples. These disorders are caused by abiotic factors such as genotype/genetic background, maturity at harvest, orchard/preharvest factors, seasonal variations, and postharvest storage conditions. It is important to mention that pathogens or mechanical damage do not lead to physiological disorders.
In particular, the physiological disorder of superficial scald in apples is the result of a chilling injury. Chilling injuries occur during cold storage at temperatures below the optimum range. During apple cold storage fruits can accumulate a-farnesene, a volatile compound present in the fruits’ wax layer. This compound can cause superficial scald when it oxidizes with atmospheric oxygen. Therefore, superficial scald generally develops during cold storage (more than 3 months in storage) but it is increased 3-7 days after taking the fruit out of cold storage.
Superficial scald is only restricted to the skin of the apples and usually on the shaded side. The symptoms appear as brown patches on the skin of the apple, which are diffuse (no defined edges between affected and unaffected skin) irregular, and light brown to dark brown to black in color (Fig 1). Superficial scald can also be accompanied by the development of a rough texture of the fruit.
What factors cause or contribute to apple superficial scald development?
There are many factors that can cause or contribute to the development of superficial scald of apples. These factors include fruit maturity, the genetic background of apples, preharvest and environmental factors, as well as postharvest factors.
Regarding fruit maturity, it has been reported that fruits that are harvested too early, i.e., when they are not yet mature, are more susceptible to developing superficial scald. This can be related to a higher oxidation of alpha-farnesene.
The genetic background of the apple is also a key factor in terms of the susceptibility to the disorder, as development of superficial scald varies depending on the type of cultivar. It is now known that there are some apple cultivars that are very resistant to superficial scald such as Golden Delicious and Gala, while Granny Smith is highly prone to its development. Differences in the wax coating of the apple influence gas exchange and the dissipation of toxic compounds, explaining these differences.
With respect to preharvest factors, it has been reported that apples that are grown with high potassium, high nitrogen and low calcium levels have a higher risk of developing superficial scald. The size of the apple also plays a role, as bigger apples have more susceptibility to get scald. Additionally, superficial scald is more likely to develop in the shaded area of the fruits or in fruits that lack light exposure.
Additionally, environmental factors, such as orchard temperatures are crucial as fruits are highly susceptible to developing superficial scald when these have been exposed to temperatures >77°F during the month before harvest. Furthermore, apples that have received less than 150 cumulative hours below 50°F are also prone to developing the disorder. It is also known that orchards with low moisture content or drier climates have a higher risk of developing superficial scald.
Postharvest factors such as mechanical damage and ethylene exposure can also affect the susceptibility to superficial scald. Ethylene is a natural plant hormone that accelerates fruit ripening and production of a-farnesene. So, if the fruit is exposed to ethylene, it is more susceptible to superficial scald. On the other hand, mechanical damage like bruising or other physical damage to apples can increase their susceptibility to superficial scald because it disrupts the natural protective barriers of the fruit, making them more vulnerable to scald development.
What are the practices that can be implemented to reduce apple superficial scald development?
The most widely used method to reduce superficial scald in apples is the application of antioxidants within the first week of harvest to prevent the accumulation of a-farnesene oxidation products. Particularly, one of the most widely used is diphenylamine (DPA), which is a synthetic antioxidant insoluble in water.
Ethylene control can also be used to reduce superficial scald. Apples should be stored in a low-ethylene environment because this environment inhibits the production of scald-causing compounds like alpha-farnesene. To achieve this, the use of 1-methylcyclopropene (1-MCP, SmartFreshTM, AgroFresh Inc) is recommended. 1-MCP blocks ethylene perception, thus can delay fruit ripening, reduce spoilage and prevent the development of certain disorders, including superficial scald of apples.
Ultra-low oxygen controlled atmosphere (ULO-CA) storage is another method used to reduce the risk of superficial scald. This technology involves reducing oxygen levels to 0.5% or less, while maintaining elevated carbon dioxide levels, which slow ethylene production, inhibit the synthesis of alpha-farnesene and other volatile compounds that cause superficial scald, and additionally inhibit microbial growth.
Ventilation is one of the most common practices used to reduce superficial scald in apples. Maintaining adequate ventilation in apple storage facilities helps regulate temperature and humidity levels, thereby reducing the risk of superficial scald. This is achieved by maintaining air circulation in the storage area, which dissipates accumulated heat and moisture. In addition, proper ventilation prevents the accumulation of carbon dioxide and ethylene gasses, which are also known to contribute to the development of superficial scald.
The use of calcium sprays can be used pre harvest to increase the calcium content in the fruit, which strengthen cell walls and reduce the risk of superficial scald development.
Lastly, cultural practices, such as proper pruning and thinning of the apple trees, can improve the microclimate around the fruit and reduce the risk of scald development. These practices can improve air circulation around the fruit, avoiding shaded areas in the canopy that can promote superficial scald development later. Thinning also helps to reduce competition among the fruit for resources and promote uniform ripening.
This article appears in September 2023, Volume 14, Issue 7 of the Vegetable and Fruit News.