Sunburn

Sunburn in plants is a non-infectious physiological disorder resulting from excessive exposure to solar radiation and high temperatures. Unlike pathogenic diseases caused by fungi or bacteria, sunburn is a manifestation of cellular stress and thermal damage. It occurs when the energy absorbed by plant tissues exceeds their capacity to dissipate it, leading to the destruction of pigments and cell structures.

This condition affects a wide range of horticultural and agricultural crops, including fruit trees like apples, as well as vegetables like peppers and tomatoes. Sunburn typically appears on the exposed sides of fruits, leaves, or young trunks, manifesting as bleached, scorched, or necrotic patches. These lesions significantly reduce the marketability and yield of the produce.

The biology of this phenomenon is closely linked to the plant's inability to maintain a stable internal temperature under extreme light intensity. When the heat load surpasses the cooling effect of transpiration, the cell membranes are damaged, and oxidative stress leads to programmed cell death in the affected area. Over time, these weakened tissues become entry points for secondary pathogens.

Development and spread of sunburn are intensified by specific environmental conditions: sudden exposure of shaded tissues to direct sunlight, high air temperatures, low humidity, and calm winds which limit convectional cooling. Cultural practices that expose sensitive parts of the plant, such as drastic pruning, can also trigger widespread injury during peak summer months.

Effective management and control strategies focus on reducing solar load and enhancing the plant's resilience. Key measures include:

• Installation of shade cloths to reduce direct light intensity during the hottest hours.
• Proper irrigation management to ensure adequate moisture for transpiration-based cooling.
• Application of kaolin-based reflective sprays or white latex paint on trunks to reflect solar radiation.
• Strategic pruning techniques that avoid exposing sensitive fruits or inner branches to direct sun.