COLD-HARDINESS OF DORMANT BUDS OF GRAPE CULTIVARS - COMPARISON OF THERMAL-ANALYSIS AND FIELD SURVIVAL

Thermal analysis (TA) was used to evaluate dormant bud cold hardiness of nine Vitis cultivars weekly during the 1993-94 dormant period. TA h ardiness estimates were expressed as either mean low-temperature exoth erm 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 ran ged from a mean of 15% with 'Concord' to 100% with 'Viognier'. With th e exception of 'Viognier' and one of two 'Cabernet Sauvignon' clones, field mortality levels were accurately bracketed by TA estimates of LT 10, MLTE, and LT90 values, which had been obtained in the week precedi ng the freeze. 'Viognier' bud hardiness was overestimated by almost-eq ual-to 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 m isinterpretation of secondary bud exotherms as due to primary buds.