Thermal analysis (TA) was used to evaluate dormant bud cold hardiness of nine Vitis cultivars weekly during the 1993–94 dormant period. TA hardiness estimates were expressed as either mean low-temperature exotherm temperature (MLTE) or temperatures lethal to 10% (LT10), 50% (LT50), or 90% (LT90) of dormant bud sample. A destructive freeze on 19 Jan. 1994 presented an opportunity to compare dormant bud field survival with laboratory estimates of bud hardiness that had been derived from TA. Vineyard air temperatures of –24C caused primary bud kill that ranged from a mean of 15% with `Concord' to 100% with `Viognier'. With the exception of `Viognier' and one of two `Cabernet Sauvignon' clones, field mortality levels were accurately bracketed by TA estimates of LT10, MLTE, and LT90 values, which had been obtained in the week preceding the freeze. `Viognier' bud hardiness was overestimated by ≈1.5C, and the hardiness of `Cabernet Sauvignon clone UCD#6' was underestimated by <1C. The discrepancy with `Viognier' may have been related to prior destruction of primary buds by bud necrosis and the misinterpretation of secondary bud exotherms as due to primary buds.
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