Sunburn

Sunburn in plants is not an infectious disease in the traditional biological sense, as it lacks a living pathogen. It is a strictly physiological disorder caused by excessive exposure to intense ultraviolet radiation and high temperatures.

Unlike fungal or bacterial infections, this process is purely abiotic. The plant loses its ability to perform photosynthesis in damaged areas due to the degradation of chlorophyll, which leads to the death of epidermal cells and underlying plant tissues.

Shade-loving plants or those suddenly moved from low-light conditions to direct sunlight are most susceptible. Fruits that become suddenly exposed after pruning or wind damage are also highly vulnerable to scorching.

There is a strong correlation between water availability and sunburn severity. Under water stress, plants cannot maintain effective transpiration, the process that normally cools the surface of leaves and fruits through evaporation.

Sunburn occurs instantly when radiation intensity exceeds the plant's antioxidant protection system, leading to rapid tissue collapse at critical thermal thresholds.

Woody plants, especially young saplings with thin bark, are highly sensitive to sunburn, as are leafy vegetables like lettuce or spinach. For fruit trees, damage typically affects apples, pears, and tomatoes on the sun-exposed side of the canopy.

In thin-barked trees such as cherries or young apple trees, sunburn can lead to cambium death, creating wounds that invite secondary pests and pathogenic fungi.

On tomatoes and peppers, sunburn appears as white, papery patches on the fruit skin, which may eventually rot as the compromised tissue becomes an entry point for opportunistic microbes.

The foliage of many ornamental plants can suffer significantly when transplanted without proper hardening, resulting in widespread chlorosis and necrosis of leaf blades.

On a larger scale, sunburn reduces the commercial value of crops, shortens the shelf life of fruits, and weakens the plant's overall immune system, making it more susceptible to other stressors.

The peak occurrence of plant sunburn is during the height of summer, typically from mid-June through early August. Sudden sunny days following prolonged wet periods are particularly dangerous as plants have not adjusted to high radiation levels.

In early spring, tree trunks often suffer from sunscald when bright sun reflects off snow, heating the bark during the day while freezing night temperatures cause rapid tissue cooling and cracking.

Summer heatwaves with low humidity accelerate plant dehydration, making it difficult for the plant to regulate its internal temperature, thereby increasing the risk of scorching even in resistant species.

The most critical time of day is between 11:00 AM and 4:00 PM, when UV radiation reaches its maximum intensity.

Plants under drought stress are at the highest risk because their natural cooling mechanism, transpiration, is impaired due to water shortages.

Early symptoms include light or yellowish spots on leaves, which gradually turn dry, brown, and brittle. The affected leaf tissue loses turgor and becomes thin and translucent.

On fruits, sunburn manifests as a light-colored patch with clear boundaries, eventually taking on a dry, parchment-like texture. These spots often become sites for secondary infections.

In trees, bark damage appears as vertical cracks, peeling, and discoloration of the bark to a lighter or reddish tone, indicating the death of the living cambium layer underneath.

• Loss of pigment (whitening) in affected areas.
• Desiccation and necrosis of plant tissues.
• Characteristic placement on the side exposed to direct sun.
• Splits and open wounds on tree trunks.

Severe cases can lead to premature leaf drop, which further exposes remaining fruits to sunlight, creating a cycle of increasing damage throughout the season.

The primary control method is hardening off transplants, which allows young plants to gradually adapt to natural UV levels before being moved into the open garden.

For vegetable and berry crops, using shade cloth is an effective way to diffuse sunlight and reduce surface temperatures of plants by several degrees.

Painting tree trunks with white lime wash or specialized white garden paint helps reflect sunlight, preventing the bark from overheating during hot days and fluctuating temperatures in early spring.

Consistent irrigation and maintaining optimal soil moisture levels are crucial for supporting plant transpiration, which provides the necessary cooling effect to prevent thermal damage.

Proper canopy management is essential; avoid excessive pruning that exposes large branches or delicate fruits to direct mid-day sun, ensuring that enough foliage remains to shade the plant's structure.