2019 ASHS Annual Conference

Loss of cold hardiness occurs during unseasonably warm periods during winter and can lead to xylem and cambial tissue damage in trunks and limbs of fruit trees. Apple rootstocks can vary in their ability to maintain cold hardiness during mid-winter exposure to warm temperatures. In the third year of a multi-year study, we measured hardiness in 3 Geneva cultivars and 1 advanced selection from the Vineland series using 2-year-old shoots that were cut from trees. Shoot were placed in buckets of water and exposed to 4, 10 or 21 °C for 2-days. Two-year-old shoot segments were then cut into 6-cm pieces and exposed under controlled conditions to temperatures ranging from -20 to -38 °C. Some segments were left unfrozen to serve as a control. Hardiness was measured as oxidative browning.
M.9, G.41 and V.6 shoots not exposed to warm temperatures were hardy to -38°C, and G.935 showed partial xylem and cambial injury at -30°C and colder. Prior exposure to 21°C caused loss of xylem hardiness at temperatures of -30°C and colder in M.9, at -35°C and colder in V.6, but no change in G.41 and G.935. Exposure to 21°C caused loss of cambial tissue hardiness at -40°C in G.41 which had mild injury and in V.6 which had severe injury. Cambial hardiness in M.9 and G.935 were unchanged compared to the control. Exposure to 10°C did not cause a measurable loss in hardiness in M.9, G.41 or G.935, but caused an increase in cambial injury in V.6 at -38° C .