Cold Hardiness and Temperature Responses of Healthy and Mildew-Infected Terminal Buds of Apple During Dormancy

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Abstract
Cold hardiness of healthy and mildew[Podosphaera leucotricha]-infected terminal buds of apple trees (M. domestica ''Newtown'') was determined monthly during the dormant period Dec. 1, 1981 to March 1, 1982. The lowest survival temperature (LST) of each bud was evaluated by the combination of visual observation, forcing and grafting methods. The results obtained by using the 3 methods for the evaluation of LST agreed to within .+-.2.9.degree. C. The hardiness level of healthy buds was between -25.7.degree. and -26.7.degree. C during Dec.-Feb. and decreased to -18.9.degree. C in March. The hardiness level of mildew-infected buds fluctuated between -16.4.degree. and -21.6.degree. C during the entire dormant period. Incubation of terminal buds for 1 wk at -12.2.degree., -1.1.degree. +10.degree. C, and ambient (+3.3.degree. and +7.7.degree. C) temperature prior to freezing resulted in different survival patterns in Jan. and Feb. 1983.

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