Influence of late autumn preconditioning temperature on frost hardiness of apple, blueberry and blackcurrant saplings

•Preconditioning is important for assessing FHmax of apple, blueberry and blackcurrant.•FHmax was reached at precondition temperatures between −3 °C and −7 °C.•Prolonged preconditioning increased FH in apple and blueberry but not in blackcurrant.•The low occurrence rate of LTE limits the use of DTA for blackcurrant. Maximum frost hardiness (FHmax) during dormancy is one key determinant for growth and productivity of woody horticultural plants in northern latitudes. Even though woody plants are known to be competent to reach FHmax when preconditioned at low temperatures in late autumn, it is not known the proper temperature to reach FHmax for different species and cultivars. That knowledge is needed for their use in different cultivation zones in Finland however. We examined the effect of different preconditioning temperatures (3, −3, −7 and −10 °C) and their durations (one or three weeks) on FH of two cultivars of apple (Malus domestica), three cultivars of blueberry (Vaccinium corymbosum) and three cultivars of blackcurrant (Ribes nigrum) to find out the condition for reaching FHmax. FH was assessed by controlled freezing tests at different temperatures. Frost damage was determined by electrical impedance (EIS), relative electrolyte leakage (REL) and differential thermal analysis (DTA) of stems, and by visual damage scoring (VD) of stems and buds. In apple, FH of stems was at the highest (-43 °C) at a preconditioning temperature of -7 °C and the lowest (−18 °C) at 3 °C as assessed by REL and EIS respectively. By DTA, low temperature exotherm (LTE) was observed at the lowest at around -40 °C in both apple cultivars after preconditioning at -7 °C. The highest level of FH in blueberry (from −39 °C to −45 °C) and in blackcurrant (from −39 °C to −50 °C) was found in all cultivars after preconditioning at −3 °C or −7 °C by EIS and REL. Some additional hardening especially in apple and blueberry was found with prolonged preconditioning for 3 weeks. FH estimates of stem by EIS and REL were quite similar, whereas VD typically under-estimated it as compared to EIS and REL. DTA results have minimal fluctuations compared to other methods. The use of DTA for blackcurrant is limited due to the low occurrence rate of LTE.