Extreme Heat on Grapevines: Physiology, Impacts and Management

Hot and dry weather events have become increasingly common across Australian wine regions, prompting questions from grapegrowers about the impacts on vine performance and fruit quality and what management strategies might help mitigate negative effects.

Sunburn and Berry Damage

All grapevine varieties are susceptible to sunburn, though the degree of susceptibility varies with variety and canopy condition.

• Exposure to high temperatures post-veraison is particularly damaging because berry skin pigments are developing; however, pre-veraison heat can desiccate berries into hard “pebbles”, especially those on the outer edge of a bunch.

• Sunburn can occur rapidly — berry surface temperatures of 40–43 °C for as little as five minutes can cause visible damage, and maximal injury may accumulate over several days of exposure.

Consequences of sunburn include:

• Brown lesions on berry skins

• Shrivel and drying of whole bunches

• Increased susceptibility to fungal bunch rots due to compromised skin integrity

Preventative strategies: Maintaining an adequate leaf layer around bunches through appropriate canopy management, nutrition and irrigation is the most effective means of avoiding sunburn. If a canopy is already compromised, early season applications of vineyard sunscreen products (e.g., particle-film technologies such as kaolin clay) have shown promise in trials in regions such as the Hunter Valley and Riverland. Consultation with your winery or fruit purchaser is essential before using such products.

Shrivel and Heat Physiology in Shiraz

Varieties like Shiraz, which are known to be sensitive to berry shrivel, are particularly vulnerable under elevated temperatures late in the season.

Mechanisms and impacts include:

• Increased shrivel: Warmer conditions during late ripening accelerate the onset of cell death in the berry mesocarp and increase shrivel rate.

• Stomatal responses: Shiraz grapevines may develop larger stomata under heat stress, a physiological adaptation that supports evaporative cooling when water is available. However, this mechanism breaks down under water deficit, removing a key means by which vines protect tissues from heat stress.

Heat Effects on Fruit Composition: Anthocyanins and Sugar Balance

High temperatures during ripening can delay the onset of anthocyanin accumulation in red cultivars such as Shiraz and Cabernet Franc, with consequences for colour development relative to sugar accumulation (i.e., colour-alcohol balance).

This disruption of anthocyanin-to-sugar balance can result in wines with higher alcohol but lower colour intensity — an important quality consideration for red wine styles.

Management strategy:

• Regulated deficit irrigation established shortly before veraison can promote earlier or faster anthocyanin accumulation and help partially restore colour–sugar balance in heat-affected vineyards.

Drivers of Earlier Ripening: Climate vs Vineyard Management

Growers frequently observe earlier ripening under hot and dry conditions. The AWRI explains that this pattern can be driven by both climatic factors and soil water status.

Key mechanisms include:

1. Increased temperatures — Elevated average temperatures accelerate phenological development, advancing budburst and flowering, while the relative duration of flowering to veraison and veraison to harvest remains largely unchanged.

2. Reduced soil moisture — Dry soils contribute to earlier ripening via:

   • Increased root production of the hormoneabscisic acid (ABA), which signals shoot growth cessation and ripening onset.

   • Rapid soil warming, which advances phenological stages such as budburst in spring.

Practical Takeaways for Vineyard Management

Soil moisture — Adequate water supply before and during heat events helps preserve canopy function (cooling by transpiration) and protects bunches from direct solar exposure.

Canopy — Avoid excessive leaf removal during hot spells; use judicious canopy manipulation to balance protection from sunburn with airflow to mitigate disease risk.

Irrigation — Apply water ahead of heat events and, where possible, during cooler parts of the day to maximise rootzone moisture without compounding evaporative loss.

Consider products carefully — clay based technologies can reduce radiative heating but must be discussed with your grape purchaser regarding residues and winery requirements.

Further Reading and References

This article synthesises findings from the AWRI and linked research including:

• Bonada, M., Sadras, V.O., & Fuentes, S. (2013) on heat effects and berry cell death.

• Sadras, V. & Moran, M. on stomatal responses and anthocyanin–sugar balance disruption.

• Crothers & Downey on sun exposure and wine quality.

• Greer & La Borde on sunburn effects.

• Additional vineyard heatwave management fact sheets from Wine Australia and AWRI.