Effects of High Temperature Disorders on Horticultural Produce

Fruits and vegetables face many threats during their growth and after harvest, but one of the most underestimated is high temperature stress. When horticultural produce is exposed to excessive heat or intense sunlight – whether still on the plant or during postharvest handling – a range of physiological disorders can develop. These include bleaching, scalding, sunburn, uneven ripening, and desiccation. The result is degraded quality, reduced shelf life, and significant economic losses for growers and suppliers alike.

What are high temperature disorders? 🔗

High temperature disorders in horticultural produce are physiological breakdowns – not caused by pathogens or mechanical damage, but by the adverse effects of excessive heat and solar radiation on plant and fruit tissue. When temperatures climb beyond a crop’s tolerance threshold, cells can be injured or killed, pigment formation is disrupted, and moisture is lost rapidly. These disorders are particularly common in tropical, arid, and semi-arid growing regions, but they increasingly affect temperate areas during summer heatwaves as well.

The key factor is that fruit surface temperatures can be 10-18°C higher than the surrounding air temperature when exposed to direct sunlight. This means that even when air temperatures are moderate, the fruit surface can reach damaging levels if solar radiation is intense enough.

Sunburn: the most common high temperature disorder 🔗

Sunburn is the most widely recognised high temperature disorder affecting horticultural produce. It develops when fruit skin or tissue is exposed to a combination of high solar radiation and elevated temperatures for a prolonged period. The damage is irreversible once it occurs, affecting both the appearance and internal quality of the fruit.

Three types of sunburn 🔗

Research has identified three distinct categories of sunburn, each with different causes and symptoms:

Sunburn necrosis is the most severe form. It occurs when fruit surface temperatures reach approximately 52°C (125°F) for at least 10 minutes. At these extreme temperatures, cell membranes lose integrity and the tissue dies. The affected area appears as dark brown or black necrotic spots, typically visible within one to four days of heat exposure.

Sunburn browning is the most common type. It occurs at slightly lower fruit surface temperatures – generally above 46°C (115°F) depending on the cultivar – combined with high UV radiation exposure lasting at least 60 minutes. The affected skin develops yellow, brown, bronze, or dark tan patches on the sun-exposed side of the fruit. These spots may not appear until a few days after the damage, and they can worsen during cold storage, turning a deeper brown over time.

Photooxidative sunburn is different from the other two because it occurs at much lower temperatures. It happens when previously shaded fruit is suddenly exposed to full sunlight – for example, after pruning, after a storm strips away leaf cover, or when heavy fruit loads shift branches. The fruit tissue, unaccustomed to intense light, becomes bleached and turns completely white. This type of damage can occur at temperatures as low as 31°C (88°F).

Sunburn scald in apples: a closer look 🔗

Apples are among the most susceptible fruits to sunburn injury, making them a textbook example of high temperature disorders. Sunburn scald in apples is characterised by brown to black patches on the sun-exposed side of the fruit, typically the southwest-facing surface that receives the most afternoon heat and light.

Light-skinned cultivars such as Granny Smith are more vulnerable, as are cultivars like Honeycrisp. According to research from Washington State University, the threshold temperature for sunburn browning in apples ranges from 46°C to 49°C (113°F to 120°F) at the fruit surface. Because developing fruit cannot dissipate excess radiation as effectively as leaves, their surface temperatures can climb 10-15°C above the ambient air temperature.

Water stress further increases susceptibility – the transpiration of moisture from the fruit surface normally provides some cooling effect, and when water availability is limited, this natural defence is reduced. Heavy crop loads can also contribute, as overloaded branches may bend and suddenly expose previously shaded fruit to direct sunlight.

The economic impact is substantial. Apple-growing regions in Washington State alone report annual losses of up to US$100 million due to sunburn. Globally, losses ranging from 10% to 50% of the harvest have been documented in major apple-growing countries including Australia, Chile, Spain, Turkey, and South Africa.

Scalding and bleaching in vegetables 🔗

High temperature disorders are not limited to tree fruits. Vegetables such as tomatoes, peppers, cucumbers, and squash are frequently affected by sunscald – the vegetable equivalent of sunburn.

In peppers, for instance, fruit exposed to direct sun during hot weather can become extremely hot compared to fruit protected by a dense canopy. The combination of high air temperatures and rapid fruit expansion after rainfall makes peppers especially vulnerable. The damaged area typically appears as a sunken, bleached or tan lesion that can become a point of entry for secondary pathogens, compounding the losses.

Cucumbers are susceptible to sunburn when skin surface temperatures exceed approximately 38°C (100°F), while peppers begin showing damage above approximately 41°C (105°F). The fruit surface on the sun-exposed side heats up well beyond the surrounding air temperature, and the resulting tissue damage ranges from light discoloration to complete bleaching and necrosis.

In leaf scald, another related disorder, hot and windy conditions cause water loss from leaf surfaces faster than the roots can replace it. Cells along the leaf margins are damaged first, producing a characteristic scorched appearance. This is particularly common when air temperatures exceed 35°C (95°F) and humidity is low.

Uneven ripening: a hidden consequence of heat 🔗

While sunburn is visually obvious, uneven ripening is a subtler but equally costly consequence of high temperature stress, particularly in tomatoes.

When tomatoes are exposed to sustained temperatures above 32°C (90°F), the production of lycopene – the red pigment – is inhibited. Carotene, the yellow-orange pigment, is somewhat more heat-tolerant. This is why heat-stressed tomatoes often develop yellow or orange patches that fail to ripen to a uniform red, a condition known as blotchy ripening or yellow shoulder disorder.

Research published in Plant Physiology has shown that high temperatures around 38°C suppress the expression of ripening-related genes, including those responsible for ethylene production, lycopene synthesis, and cell wall softening. This means the fruit’s entire ripening programme is disrupted at a molecular level – not just the colour, but also the texture and flavour development.

Related disorders include gray wall (where outer fruit walls turn brown and collapse), internal whitening (white corky tissue inside the fruit walls), and white tissue disorder caused by high bed temperatures under plastic mulch. These conditions are worsened by potassium deficiency, as potassium plays a critical role in pigment formation and overall fruit quality under heat stress.

How high temperatures affect ripening biology 🔗

At the cellular level, elevated storage or field temperatures accelerate respiration and ethylene production, leading to faster but uneven ripening. A study on post-harvest tomato behaviour found that increasing the storage temperature from 18°C to 26°C reduced average shelf life by about four days and increased fungal susceptibility by approximately 11% across 41 genotypes tested. The combination of accelerated metabolic activity and uneven pigment development makes heat-stressed tomatoes difficult to market.

Desiccation and moisture loss 🔗

High temperatures greatly accelerate desiccation – the loss of moisture from fruits and vegetables. When produce is exposed to heat after harvest, the rate of transpiration and evaporation increases sharply. This leads to shrivelling, wilting, weight loss, and a decline in visual appeal and texture.

Desiccation is particularly damaging for leafy vegetables, berries, and thin-skinned fruits that have high surface-area-to-volume ratios. But even thick-skinned fruits like citrus can suffer. In citrus, sunburn disrupts oil glands in the peel, leading to accelerated water loss and reduced fruit growth.

The problem is compounded in regions without adequate cold chain infrastructure. In many developing countries, produce moves from the field to the market under ambient temperatures that may exceed 40°C, with no cooling or shade. Under these conditions, the combined effects of sunburn, desiccation, and accelerated senescence can result in postharvest losses exceeding 35% in field conditions.

Economic impact and quality degradation 🔗

The economic consequences of high temperature disorders are far-reaching. Sunburned and scalded produce is often downgraded or discarded entirely during sorting and packing. Even mild sunburn browning – while not always immediately visible at harvest – can progress during cold storage, leading to further losses.

In apples, sunburn-related losses can range from 10% to as high as 50% of the harvest depending on the cultivar, growing conditions, and year. Associated disorders like lenticel marking, cracking, russeting, and bitter pit are often worsened by prior sunburn damage. Internal quality parameters such as firmness, soluble solids, and titratable acidity are also altered in sunburned fruit, affecting both fresh market value and processing suitability.

For vegetables, the losses from sunscald are compounded by secondary infections. Damaged tissue serves as an entry point for bacteria and fungi, accelerating decay. Field losses from sunscald and associated rot can exceed 35%, while protected cultivation under screen houses can reduce these to below 5%.

Prevention and management strategies 🔗

Managing high temperature disorders requires a combination of cultural, physical, and chemical approaches. No single method is foolproof, but integrating several strategies can significantly reduce damage.

Canopy management and shading 🔗

Maintaining a healthy, dense leaf canopy is the first line of defence. In fruit trees, careful pruning ensures that fruit remains shaded without reducing overall light for photosynthesis. For vegetables, cultivar selection matters – varieties that produce thick canopy cover naturally show lower sunscald incidence. In commercial settings, shade nets have been shown to have a protective effect against sunburn compared to unshaded controls, and can reduce sunscald losses from over 35% to under 5%.

Evaporative cooling 🔗

Overhead sprinkler systems that wet the fruit surface in cycles – allowing it to dry and cool through evaporation – can effectively reduce fruit surface temperatures. This method is widely used in apple orchards and works well for temperature-dependent sunburn. However, it does not protect against UV radiation, and water quality must be monitored to avoid mineral deposits on fruit surfaces.

Reflective particle films 🔗

Application of white particulate coatings such as kaolin clay, calcium carbonate, or talc directly on fruit surfaces helps reflect, scatter, and block solar radiation. Kaolin-based particle films reduce fruit surface temperature and improve light distribution within the canopy. Foliar sprays of kaolin (4%) or glycine betaine (50 mM) have been shown to decrease sunburn incidence in citrus while also improving fruit quality.

Proper irrigation and mulching 🔗

Avoiding water stress is critical because well-hydrated fruit has a greater capacity for transpirational cooling. Mulching reduces soil temperature, conserves moisture, and reflects radiant heat when reflective materials are used. Under black plastic mulch, surface temperatures can exceed 65°C (150°F), radiating damaging heat onto nearby plants – so mulch type selection matters greatly.

Harvest timing and postharvest handling 🔗

Harvesting during cooler parts of the day and moving produce into shade or cold storage quickly can prevent additional heat damage. For tomatoes affected by uneven ripening, harvesting early and ripening indoors at controlled temperatures is an effective workaround. Leaving harvested bins of fruit in direct sunlight should always be avoided, as fruit temperatures can rise rapidly after being detached from the tree.

The growing relevance of heat disorders under climate change 🔗

As global temperatures rise and extreme heat events become more frequent, high temperature disorders are no longer a regional concern limited to arid climates. In 2021, apple production areas across the Northern Hemisphere were hit by severe heatwaves, causing widespread sunburn damage that had measurable impacts on fruit quality and storage performance. Growers in traditionally temperate regions are now encountering sunburn problems that were once associated only with warmer climates.

This shift makes understanding and managing high temperature disorders more important than ever – for students of horticulture, for commercial growers, and for anyone involved in the postharvest supply chain.

What do you think? With climate change intensifying summer heat events worldwide, which mitigation strategy – shade netting, reflective coatings, or evaporative cooling – do you think holds the most practical promise for small-scale growers? And could breeding heat-tolerant cultivars eventually reduce our reliance on these physical interventions?

References

1. https://postharvest.ucdavis.edu/disorders/apple-sunburn
2. https://treefruit.wsu.edu/sunburn-in-apple-and-strategies-to-mitigate-it/
3. https://treefruit.wsu.edu/orchard-management/environmental-stress-management/
4. https://www.sciencedirect.com/science/article/abs/pii/S0925521422003210
5. https://www.canr.msu.edu/news/hot_and_sunny_days_promote_sunscald_in_peppers_and_other_vegetables
6. https://ag.umass.edu/vegetable/fact-sheets/tomato-physiological-ripening-disorders
7. https://pubmed.ncbi.nlm.nih.gov/12226253/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC8623658/
9. https://www.tandfonline.com/doi/full/10.1080/15538362.2019.1605558
10. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sun-scald
11. https://attra.ncat.org/true-love-tomatoes-and-trouble-managing-heat-stress-in-tomato-production/