Rising Temperatures Cause Widespread Damage: Understanding Grape Sunburn

With the arrival of summer, grapevines in vineyards continue their growth cycle. Currently, vineyard operations are proceeding simultaneously with lateral shoot removal, leaf removal, and shoot tip cutting.

During this period, Germany experienced two days of extreme heat on June 26th and 30th in 2019, with maximum temperatures approaching or exceeding 40°C in some locations. For Germany, known for its cool climate, such high temperatures in late June are extremely unusual. Combined with the preceding period of minimal rainfall and extremely dry conditions, vineyards experienced widespread damage, particularly affecting young vines.

This damage is known as grape sunburn.

In reality, the grape berries have only just set and are not yet at a stage that could be called true fruit. However, this time, damage has occurred even to newly set berries that are still nearly in their floral stage.

This article provides a detailed explanation of grape sunburn, focusing on understanding this phenomenon and methods for prevention. In our current era of frequent climate change discussions and growing concerns about global warming, grape sunburn has become both a familiar and urgent problem requiring immediate attention.

Through this article, effective solutions to this problem should become clear.

Note: Damage occurring at this time of year can be difficult to distinguish between sunburn and drought stress, but under such extreme high-temperature conditions, both phenomena have essentially the same meaning, so this article treats them as "sunburn."

For an overview of grape sunburn, please also refer to our previous article "Grape Sunburn."

Is Grape Sunburn a Modern Problem?

When sunburn is identified as a problem, attention naturally turns to climate change and resulting global warming. This trend might lead some to perceive "sunburn" as a modern ailment that has emerged recently. However, reports of grape sunburn actually date back to the 1840s, making it far from a new type of damage.

On the other hand, it is true that reported cases of damage have increased in recent years. In Germany, 2016 saw extremely numerous and severe damage cases reported nationwide.

While not a modern disease, grape sunburn has increasingly become a focus of attention—this is the nature of the "grape sunburn" problem.

The Cause of Sunburn

As stated in our previous article "Grape Sunburn," the cause of grape sunburn is not "ultraviolet (UV) light" as commonly assumed, but rather "temperature."

More precisely, it is the combined effect of solar radiation and temperature.

The primary cause lies in temperature—that is, heat energy. The solar radiation factor works primarily by having the energy contained in absorbed sunlight convert to heat, thereby raising the surface temperature of grapes.

To provide a concrete example, on August 27, 2016, when the maximum air temperature at 2 meters above ground level recorded 34.8°C, the surface temperature of grape berries in German vineyards reached a maximum of 45°C.

As this demonstrates, or as anyone standing under direct sunlight can easily understand, the temperature an object experiences comes not only from the surrounding air temperature but also from heat energy directly absorbed at the surface.

Therefore, while the direct cause of sunburn is "temperature," "solar radiation" is identified as a factor that directly and indirectly influences that temperature.

The Mechanism of Sunburn

How exactly does this temperature change affect grapes, resulting in "sunburn"?

The locations affected by sunburn damage have been clearly identified. These are the "epidermis" in the grape skin and the "parenchyma cells" that exist beneath it.

When temperatures exceed a certain threshold, these tissues are affected and undergo structural changes. This results in the alteration of the waxy layer on the grape skin surface.

The waxy layer functions to prevent substances from penetrating from outside the skin into the fruit, while simultaneously preventing moisture and other substances contained in the fruit from evaporating to the outside.

In grapes affected by sunburn damage, the waxy layer in the skin undergoes structural changes that prevent it from performing these functions. In other words, it becomes unable to retain moisture within the fruit, allowing it to evaporate.

As a result, in clusters affected by sunburn damage, individual berries gradually lose moisture and become shriveled, ultimately taking on the desiccated appearance that is the typical symptom of sunburn damage.

Varieties and Timing Most Susceptible to Sunburn

Sunburn occurs in the epidermis and parenchyma present on the fruit skin surface. Therefore, varieties with larger berries are generally considered more susceptible to sunburn damage. This is thought to be because larger-berried varieties have a smaller proportion of skin relative to the volume of the grape berry, making them relatively more susceptible to heating.

There are also specific periods when sunburn damage is most likely to occur. The period just before set berries begin to soften is known as the most vulnerable time for damage. Additionally, the period when fruit ripening begins also tends to show increased susceptibility, and in these cases, sunburn can progress to what could be called "burns."

Conversely, the occurrence of clearly visible damage at the current stage—when berries have barely set—represents a quite exceptional situation.

Effective Countermeasures Against Sunburn

As already discussed, the cause of sunburn is excessively elevated grape surface temperatures. Therefore, quite simply, if we can lower the surface temperature of grape berries, this constitutes an effective countermeasure.

The first approach to lowering grape surface temperature is air circulation. According to research, directing air flow at grape rows at speeds ranging from 0.5 m/s to 4 m/s can reduce fruit temperature by up to 8°C.

Realistically, continuously directing air flow across entire vineyard areas with fans or similar equipment throughout hot daytime hours is difficult from both equipment and cost perspectives. Therefore, thinking more practically, the focus becomes how to effectively utilize natural wind. This makes vineyard site selection crucial.

Incidentally, irrigation for quick temperature reduction cannot be chosen as an option, as it risks promoting the spread of fungal diseases.

Varietal Solutions

The next consideration is selecting grape varieties that can cope with high temperatures.

At the time of writing this article, news broke regarding changes to permitted varieties in Bordeaux, France. While this development was presented as a strategy to address excessively high sugar levels and resulting alcohol increases due to warming rather than specifically for sunburn prevention, it has secondary connections to sunburn countermeasures.

Selecting grape varieties originally adapted to high-temperature environments is equivalent to planting varieties resistant to sunburn.

Viticultural Technical Countermeasures

As we have seen, vineyard site selection and choice of planted varieties hold significant importance for grape sunburn countermeasures. However, these measures are also difficult to apply to existing vineyards.

Therefore, let us examine effective countermeasures for vineyards where grapes are already planted. Effective measures for existing vineyards maintain the same basic approach—the key lies in preventing grape temperature increases.

The main methods currently proposed are the following two:

• Modifying trellis construction including training methods
• Adjusting leaf removal timing and extent

Let us examine each in detail.

Creating Trellises that Shade Grapes

This involves constructing trellises in an inverted triangular shape, using the grapevine itself as a canopy to prevent direct light from hitting the grape surface.

Additionally, what Germans call "Minimalschnitt" (minimal pruning)—a trellis system that fundamentally avoids pruning and training—is also considered effective for preventing sunburn.

Minimalschnitt, by its nature, constantly requires mechanized work, making it difficult for small-scale wineries to adopt due to equipment investment requirements. However, beyond sunburn prevention, it is attracting attention as a training method with distinctive characteristics for wine production costs.

Early Leaf Removal Timing as a Countermeasure

Since leaf removal itself increases opportunities for direct sunlight to hit grape surfaces, some argue that avoiding leaf removal altogether would be better for preventing sunburn.

This is actually a partially valid perspective—leaving leaves to create shade and prevent grape temperature increases is not impossible. However, in this case, poor air circulation can cause humidity to accumulate, potentially creating counterproductive effects. Additionally, moderate sunlight exposure increases aromatic compound content in fruit, and losing this benefit could result in risks greater than the sunburn risk itself.

Therefore, a more practical approach is advancing the timing of leaf removal.

Excluding exceptional cases like the heat wave that struck Germany and other parts of Europe, high-temperature periods typically occur from mid-July through early September.

By advancing leaf removal timing to around grape flowering time, we can secure a certain period between leaf removal and high-temperature onset.

During this period, grapevines continue growing, so by the time temperatures actually rise, some leaves will have emerged even in previously removed areas, changing the environment around grape berries. This is thought to enable obtaining positive effects from leaf removal while reducing negative impacts from temperature increases.

Countermeasures Using External Materials

Examples from Australia and other regions show that installing nets over grape fruit zones or applying powder-type covering materials that can reflect sunlight from grape surfaces are also effective countermeasures.

While both methods involve significant cost burdens and additional costs in terms of work efficiency and required labor time, necessitating cost-benefit calculations, they merit consideration as techniques that can be widely applied to existing vineyards.

Summary: Planning for Future Climate Change is Essential

Since grape sunburn results from grape surface temperature increases, the crucial point lies in preventing grape surface temperature elevation.

From this perspective, implementing site-based countermeasures becomes important for the future:

• Selecting sites with good wind flow when establishing vineyards
• Choosing north-facing slopes currently considered disadvantageous for vineyards

In addition to such measures, selecting and planting grape varieties with heat tolerance adapted to high-temperature environments also constitutes an effective approach. For planted varieties, a perspective of conducting replanting over several years will also be necessary.

However, if temperatures continue rising steadily in the future, it is clear that these countermeasures will eventually lose effectiveness at some point.

Therefore, rather than relying on any single method, combining multiple approaches and implementing countermeasures with as much margin as possible from now will be crucial for future sunburn prevention.

Appendix: Do Phenolic Compounds Provide Sunburn Protection?

Anthocyanins, the compounds responsible for color in red wines, are in a sense stress compounds produced when grape berries are exposed to sunlight. Therefore, actively conducting leaf removal to increase solar radiation reaching grapes is commonly practiced in vineyards when seeking to intensify grape color.

However, the quantity of phenolic compounds contained in grape berries does not influence the degree of sunburn.

Riesling, a white wine grape variety, is known to increase phenolic compound content in fruit as a self-defensive response when solar radiation increases.

Research focusing on this characteristic and sunburn severity measured phenolic compound content in juice pressed from Riesling grapes with varying degrees of sunburn damage. However, regardless of sunburn damage severity, no significant difference was found in total phenolic compound content among the respective juices.

Therefore, currently no evidence exists that phenolic compound content in grapes serves as a countermeasure against sunburn, and even red wine grape varieties containing abundant phenolic compounds can suffer sunburn damage if cultivation methods are inappropriate.

As described above, red wine grape cultivation tends to employ methods that actively expose grapes to sunlight primarily to increase anthocyanin content. However, given current conditions, countermeasures are undoubtedly necessary even for these grape varieties.