Sunburn in apples is a significant issue for apple growers, leading to considerable economic losses and affecting fruit quality. This condition results from excessive exposure to sunlight, causing damage to the apple’s skin and underlying tissues. Understanding the causes, symptoms, and management strategies for sunburn in apples is crucial for mitigating its impact and ensuring high-quality fruit production.

Causes of Sunburn in Apples

Sunburn in apples occurs when the fruit is exposed to intense sunlight, particularly during periods of high temperatures and low humidity. Several factors contribute to the risk of sunburn:

1. High Temperature: Prolonged exposure to temperatures above 30°C (86°F) can cause thermal injury to the fruit’s skin.
2. Intense Solar Radiation: Direct exposure to sunlight, especially during peak hours (10 AM to 4 PM), increases the risk of sunburn.
3. Low Humidity: Dry air conditions exacerbate the effects of high temperatures and intense sunlight.
4. Fruit Positioning: Apples located on the outer parts of the tree canopy are more susceptible to sunburn due to direct sunlight exposure.

Symptoms of Sunburn in Apples

Sunburn manifests in apples in various forms, depending on the severity of the exposure. The primary types of sunburn include:

1. Sunburn Necrosis: This is the most severe form, where the affected area turns brown or black, leading to cell death and tissue collapse.
2. Sunburn Browning: A less severe form, where the skin develops a yellowish or bronze discoloration without significant tissue collapse.
3. Photooxidative Sunburn: This form occurs when apples are suddenly exposed to sunlight after being shaded. It leads to bleaching of the skin, turning it white or silvery.

Impact on Fruit Quality

Sunburn significantly affects the aesthetic and commercial value of apples. Damaged fruit is often downgraded in quality, resulting in reduced marketability and economic losses for growers. Additionally, sunburn can predispose apples to secondary infections by fungi and bacteria, further compromising fruit quality.

Management Strategies

Effective management of sunburn in apples involves a combination of cultural practices, orchard design, and protective measures:

1. Tree Training and Pruning: Properly training and pruning trees to create a balanced canopy can reduce the exposure of fruit to direct sunlight. This practice ensures that apples are shaded during peak sunlight hours while still receiving adequate light for photosynthesis.
2. Use of Protective Netting: Installing shade nets over apple orchards can significantly reduce the risk of sunburn by filtering sunlight and lowering ambient temperatures.
3. Sunburn Protectants: Applying sunburn protectants, such as kaolin clay or specialized sunblock formulations, can create a protective barrier on the fruit’s surface, reflecting harmful UV rays and reducing heat absorption.

Several commercial products are available to help reduce sunburn in apples. Here are some options:

• Raynox® Sunburn Protectant: This product helps prevent sunburn browning on apples without leaving a visible residue. It’s effective under extreme sunburn conditions and doesn’t negatively impact beneficial insects​.
• Surround® WP: Made from kaolin clay, this particle film helps reflect sunlight and reduce the fruit surface temperature. It is widely used in apple orchards to mitigate sunburn and can also help with pest control​.
• Purshade®: This product contains calcium carbonate, which forms a reflective barrier on the fruit surface. It helps in reducing heat stress and sunburn and can be used throughout the growing season​.
• Screen® Duo: This sprayable product provides a protective layer against UV radiation and excessive heat, reducing the incidence of sunburn. It’s designed to be easy to apply and rainfast​.
• Parka®: It supplements the fruit’s natural cuticle, reducing the risk of sunburn by reflecting harmful UV radiation and preventing heat stress damage. It is made from food-grade ingredients, making it safe for workers and leaving no residue on the fruit.

4. Irrigation Management: Adequate irrigation, especially during hot and dry periods, helps maintain tree health and reduces the risk of sunburn. Soil moisture should be monitored regularly to ensure consistent water supply.
5. Foliar Sprays: Certain foliar sprays containing calcium and antioxidants can enhance the apple’s resilience to sunburn by strengthening cell walls and reducing oxidative stress.

Several commercial products are available to help reduce sunburn in apples. Here are some options:

Calcium-Based Foliar Sprays

• Stopit® (Yara North America, Inc.): Stopit is a foliar spray that supplies calcium directly to the fruit, helping to improve cell wall structure and reduce the risk of sunburn. Regular applications are recommended during the growing season to maintain adequate calcium levels in the fruit.
• Foli-Cal®: is a calcium-based foliar spray that enhances fruit firmness and reduces physiological disorders. It can be applied throughout the growing season and is particularly effective when used as part of an integrated fruit management program. Strengthens cell walls, improves fruit quality, and reduces sunburn and post-harvest issues​.

Antioxidant-Based Foliar Sprays

• Sunshield™: Sunshield is a foliar spray containing antioxidants that protect fruit from oxidative stress caused by sun exposure. It is applied to the fruit surface to create a protective barrier against UV radiation and high temperatures. Reduces sunburn damage, improves fruit color, and extends shelf life by mitigating oxidative stress​.
• KDL® (Potassium Dextro-Lac): is a foliar nutrient spray that includes antioxidants and potassium, enhancing the fruit’s natural defenses against environmental stress. Regular applications during the growing season help maintain fruit health and resilience. Protects against sunburn, improves fruit firmness, and supports overall tree health​.

Combination Products

• Nutri-Cal®: is a foliar spray that combines calcium with other essential nutrients and antioxidants. Used as part of a regular foliar feeding program to enhance fruit quality and reduce sunburn risk.Strengthens cell walls, reduces oxidative damage, and improves overall fruit health and marketability​.

Sunburn in apples is a complex issue influenced by environmental factors and orchard management practices. By understanding the causes and symptoms of sunburn, apple growers can implement effective strategies to mitigate its impact. Employing a combination of cultural practices, protective measures, and advanced technologies can help ensure high-quality apple production and reduce economic losses associated with sunburn.

This article was posted in Apples and tagged Amaya Atucha, Apples, Photooxidative Sunburn, Sunburn, Sunburn Borwning, Sunburn Necrosis.