By Amy Grundling
Sunburn is a physiological disorder that occurs on fruit species, such as apples, due to excess exposure of sunlight . This disorder causes large crop losses of up to 50% of the total crop yield in the South African apple industry . Depending on the type of cultivar, the symptoms of sunburn can vary from dark brown spots to white patch discoloration. The affected areas also create an easy entrance point for pathogens, causing the internal quality of the fruit to decrease. This combination results in decreased quality making the fruit unmarketable. Therefore, it is necessary to understand the causes of sunburn and how its effect on apples can be mitigated.
Types of Sunburn
1. Sunburn Necrosis: creates a brown or black necrotic spot on the fruit surface. This type of damage occurs when the fruit surface temperature reaches 52 ± 1◦C for ten minutes and longer . The high temperature of the fruit surface causes the denature of proteins, which decreases the integrity of the membrane. Damage can occur between a few millimeters to numerous centimeters deep in the sub-epidermal tissue. These symptoms will be visible after one to four days after exposure.
2. Sunburn Browning: is the most common type of sunburn that affects attached apples that are exposed to the sun. This type of sunburn causes brown, yellow and bronze spots on the exposed surface of the apple. The discoloration is due to the decrease levels of chlorophylls and anthocyanins and increased levels of carotenoids and quercetin glycoside in the surface . Sunburn Browning is a result of high solar radiation that increases the fruit surface temperature to a specific minimum temperature. Unlike Sunburn Necrosis, Sunburn Browning do not cause damage in the sub-epidermal tissue.
3. Photo-oxidative Sunburn: occurs on apples that have grown under the shade and then suddenly becomes exposed to solar radiation . This type of sunburn creates white patches on the surface of the fruit. These symptoms occur due to the fruit that are not acclimated to high solar radiation. The sudden exposure to sunlight can take place after thinning and pruning. The induction factors of Photo-oxidative Sunburn are the production of reactive oxygen species (ROS) and solar radiation in viable range of wavelengths; contrasting to Sunburn Necrosis and Sunburn Browning which is influenced by the maximum fruit surface temperature.
Indirect Factors that Influence the Severity of Sunburn
There are also many indirect factors that influence the severity of sunburn on apples. These factors include climate, geographic locations, previous exposure, soil, cultivar and crop cover. Low relative humidity with high temperature increases the atmospheric water demand. This causes higher levels of water stress, which increases the sensitivity of apples for stress-induced disorders. Wind velocity have an impact on the temperature of the air surrounding the fruit. Increased air movement reduces the heat by convection cooling. The susceptibility of the cultivars to sunburn also play a great role. Different cultivars have different susceptibility to sunburn due to the following factors: solar absorptivity, interception of solar energy, temperature tolerance, photostability, tolerance to UV radiation and the ability to acclimate. Cultivar such as Granny Smith and Jonagold are highly susceptible. Moderate susceptible cultivars include Fuji, Golden Delicious and Braeburn. The least susceptible cultivars are Pink Lady, Idad and Topaz.
The geographic location influences the probability of sunburn due to the elevation and latitude that impacts the climate of a specific region, as well as factors that influence the intensity of solar radiation such as aspect. South Africa have a combination of subtropical, arid and mediterranean regions. These regions experience clear summer skies, high temperature and high evaporative demand. This causes high levels of sunburn in the apple industry. Orchard management also play an important role in sunburn. Sunburn is more likely to occur in high-density orchard, for example. The fruit will be more exposed to solar radiation due to less canopy cover. Training system with open pruning also increases sunlight interception.
How to Prevent Sunburn
There are three main techniques, namely climate-ameliorating, suppressants and chemical, that can be used to decrease sunburn damage. Climate-ameliorating entails technological techniques that changes the micro-climate surrounding the fruit . Evaporative cooling is used to mitigate heat stress, through overhead sprayers. Heat is reduced through the evaporation, which decreases the fruit surface temperature. However, evaporative cooling does not reduce the damage done by UV radiation . Therefore, Sunburn Browning damage does not decrease under this system. The calcium and magnesium carbonates that are deposited on the surface of the fruit by water can also have a negative impact on the fruit appearance. Another downside of the evaporative cooling system is the high cost of installations, intensive management and high-water requirement. Due to these factors the South African apple industry do not use this system.
In South Africa shade nets are more frequently used. High-density polyethylene (HDPE) nets are placed over the tree canopy. The shade net reduces solar radiation interception experienced by the apples which in turn decreases fruit surface temperatures. The colour and density percentage of the material determines the effectiveness of the shade net. The downside of shade nets is the reduction of colour development in the red cultivars. However, the protection shade nets provide against excessive solar radiation and hail outweighs the negative effects.
Suppressants, such as particle films and sunscreen, are materials that are sprayed on the fruit. Particle films are composed out of kaolin clay, hydrated magnesium silicate or calcium carbonate . These white inorganic products reflect solar radiation from the surface of the fruit by increasing its albedo. The particle films wash of easily during the rain. Therefore, regular reapplication is necessary. Another drawback is the difficult removal of white residues from the calyx areas. This causes the fruit to decrease in market value due to health concerns. The sunscreen that is used on apple consist out of organic -chemical and physical inorganic particles . The sunscreen absorbs high-intensity UV wavelengths and then dissipated trough the emission of long wavelengths. Sunscreen needs to be reapplied periodically due to the rapid surface growth of fruit.
It is clearly seen that sunburn is a great risk for the farmers in the apple industry. It prevents farmers to export their products as international markets highly values visually and aesthetically appealing products. It is therefore important for South African apple producers to satisfy the consumers demand through by preventing sunburn on apples. South Africa have different climatic conditions, geographic properties and limited resources than other apple producing regions. It is therefore necessary for the South African apple producers to develop systems and use products that are specific for their needs. Through reducing sunburn, the 50% of yield loss can be prevented. This will not only decrease food waist, but also increase the farmers income through exports.