Ben-Min Chang,* Brad Estergaard, and Steve Marsh
*Agriculture & Agri-Food Canada, 4200 Highway 97 S, Summerland, BC V0H 1Z0, Canada (ben-min.chang@agr.gc.ca)

Extreme cold events occurred in the Okanagan Valley, British Columbia, in December 2022 and January 2024. At Summerland Research and Development Centre, the lowest temperature reached -22.7°C in 2022 and -25.2°C in 2024. In 2022, grapevines were exposed to air temperature below -18°C for 42 hrs. In this area, the lethal temperature of buds was monitored regularly by differential thermal analysis. Before the 2022 cold snap, the lethal temperature to kill half of Merlot buds was -22.6°C in a commercial vineyard. The lowest temperature recorded was -21.5°C in the same area. With the known lethal and lowest temperatures recorded, the estimated bud damage rate was <50% at this site. However, the observed damage rate was 77%. This suggested that exposure time might be critical. The procedure of differential thermal analysis was modified to simulate 24 hrs exposure at -14, -16, -18, and -20°C. The canes were sampled from a Merlot vineyard in Summerland, BC in 2024 before the cold event and stored at 1 ± 3°C in a walk-in cooler until use. Forty buds were tested in each treatment. When the low temperature exotherm was detected, the timestamp was recorded simultaneously. After the 24 hrs treatment at constant temperature, the freezer temperature was ramped down to -33°C at a rate of 4°C/hr to kill all surviving buds. The cold hardiness of an additional 54 buds was also evaluated by the standard method. The results showed the lethal temperature was at -22.4°C measured by the standard method. However, six hours of exposure at -18°C was sufficient to damage 50% of the buds. The preliminary data suggests shorter exposure time is required at lower temperature to inflict bud damage. The exposure time should be considered when estimating bud damage rate.

Funding Support: Agriculture & Agri-Food Canada