Two selections and two cultivars of red raspberry (Rubus idaeus L.) we
re evaluated for cold hardiness in vitro. Tissue-cultured shoots were
exposed to temperatures from 0 to -18C and samples were removed at 2C
intervals. Injury was assessed by a visual rating of tissue browning a
fter freezing. Only shoots subjected to step-wise acclimation at low t
emperatures before freezing revealed significant differences among the
four types in the lowest shoot survival temperature. Acclimation trea
tments increased the lowest survival temperatures of in vitro shoots b
y a mean of 3.1C. The hardiness obtained from this screening method ag
reed with that of winter survival in the field. Ranking, from the most
to least cold hardy, was 'Boyne', Gu 72, Gu 63, and 'Comox'.