Cold Temperature Tolerance of Trunk and Root Tissues in One- or Two-year-old Apple Rootstocks

Winter injury to the root systems of fruit trees can cause significant tree losses and yield reductions in the northern regions of the United States and Canada. To compare the root and trunk cold temperature tolerance, a series of experiments were conducted using ungrafted apple rootstocks. ‘Geneva® 11’ (G.11), ‘Geneva® 30’ (G.30), ‘Geneva® 41’ (G.41), ‘P.2’, and ‘Budagovsky 9’ (B.9) apple ( Malus × domestica Borkh.) rootstocks had root tissue hardiness similar to ‘M.26’, but ‘Geneva® 935’ (G.935) had greater cold-hardiness than M.26 when based on shoot regrowth in ungrafted trees. The LT 50 of M.26 and P.2 roots ranged from –12 to –14 °C. The LT 50 was –13 °C for B.9, –13.4 to –14.6 °C for G.30, and –12 °C for G.11. The LT 50 of G.41 was one of the highest in one experiment, –8 °C, and one of the lowest in another, colder than –15.0 °C. The LT 50 of G.935 roots was the lowest and ranged from –16 to –19 °C. Compared with M.26, trunk cold-hardiness in December was greater in B.9 and P.2 and was similar in G.30. Cold-hardiness of G.11 in December was mixed with less injury in the xylem but more injury in the phloem compared with M.26. In October, M.26 and G.935 trunks had little injury after exposure to –24 °C.