Monitoring heatwave temperatures in vineyards: A preliminary study on simple method for tracking extreme heat and berry sunburn damage

Abstract

Climate change is intensifying heatwave events, causing severe damage to vineyards through berry sunburn browning and necrosis. While standard weather stations provide valuable climate data, they may not accurately capture the microclimate conditions at the surface of grape berries, particularly under direct solar radiation. On the other hand, instantaneous measurements can be made by thermal cameras and infrared thermometers but they require daily field visits, cannot capture temperature evolution throughout the day, and are unsuitable for monitoring multiple sites simultaneously. To overcome these constraints and characterize the complete thermal exposure profile during critical afternoon periods, this preliminary study evaluates simple temperature monitoring methods to track extreme heat conditions in vineyards. From June to September 2025, temperature sensors were deployed in four commercial Merlot vineyard plots and in an experimental plot (Vitadapt) in the Bordeaux wine region. Three sensor configurations were tested: one positioned on a grape cluster on the sun-exposed side of the canopy, one on a cluster on the shaded side of the canopy, and one in a shelter located within the vine canopy. Maximum temperature differences between full sun and sheltered sensors ranged from 4 to 10°C during peak heat events, which occurred between 15:00 and 18:00 with slight variations depending on row orientation, plot aspect, and slope. Shaded and sheltered sensors showed minimal differences except when morning solar radiation could reach the shaded sensor. The most relevant metric identified was the cumulative number of hours above critical thresholds (35°C, 40°C, 45°C, and 50°C). Regular monitoring of berry damage (intensity and frequency of sunburn, browning, and necrosis) was conducted throughout the season across the four commercial vineyards, with comprehensive assessments completed in late August for all plots. Damage affected 10 to 50% of bunches depending on the site, with damage intensity averaging 0.8 to 12.3% of berry surface per bunch. Results suggest that using full sun sensors or establishing relationships between shaded sensors and solar radiation, provides the most practical approach for tracking berry-damaging temperatures. This methodology could help viticulturists anticipate and respond to heat stress, though broader validation across additional sites, varieties and vintages is needed to refine temperature thresholds and improve advisory recommendations.

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Acknowledgments

We thank the participating viticulturists for providing access to their vineyards, Bordeaux Wine and Viticulture Experimental Facility. INRAE. https://doi.org/10.17180/75AW-3519 and the TEMPO Network tempo.pheno.fr for its support.