Abstract
Abstract The cold resistances of 14 Malus clones were determined in early March annually from 1974 to 1978. Dormant twigs were cooled at −1.8°C/hr, samples removed at 5° steps from −15 to −45°, and thawed gradually. After thawing, impedance measurements were made and the twigs were held for 16 days under intermittent mist to determine recovery. Rankings of data on growth of twigs held under intermittent mist for 16 days after thawing were similar but not identical to direct impedance values obtained immediately after thawing. Tissue damage had no evident relation to shoot diameters. Surfically discernible root germs, which were present on Mailing 7 (M 7), M 26, M 27, Malling-Merton 106 (MM 106), and Cornell-Geneva 10 (CG.10), were killed at or above −15°. ‘Empire’ and Ottawa 4 were least sensitive and ‘Katherine’ and ‘Jonagold’ most sensitive to low temperatures. The responses of M 26 to low temperatures varied widely from year to year. We could discern no relationship between hardiness in late winter and timing of spring budbreak. The growth response method reported here appears to be closely related to whole plant response under field conditions.
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