Strategies for producing temperate tree fruit under increasing winter temperatures

Temperate deciduous fruit tree species such as apple, pear, cherry, and peach require dormant season cool temperatures to complete endodormancy so that flowers set and leaves emerge normally in the spring. Recent global rises in mean annual temperatures have increasingly resulted in marginal winter cold in traditional fruit production areas where many fruit tree cultivars risk poor flower set and irregular vegetative budbreak. Fruit growers are trying to adapt to the increased frequency of unusually warm winters using both proven and experimental cultural practices to both induce and break dormancy in deciduous trees using water, nutrients, hormones, chemicals, cultivar selection, and pruning methods. Current tools include on-site, real-time weather data to calculate chill hours, units or portions, so that winter chilling can be monitored and prediction models developed to assist in timing dormancy breaking horticultural practices. Historical and current data can also be combined and analyzed for El Nino or La Nina weather patterns to determine if chilling might be insufficient. Several compounds and fertilizers are currently used as dormancy breaking agents such as hydrogen cyanimide, Erger^®, thidiazuron, potassium nitrate and calcium ammonium nitrate but consistent effectiveness remains elusive. Timing and rates are critical when using these compounds, thus temperature models and adjuvants will need to be improved. Other management tools become necessary if dormancy issues persist into spring. For stone fruits, these include delaying pruning as late as possible to delay cambial activity in inadequately chilled buds, cooling by whitewashing or water to reduce tree (bud) temperatures during a protracted bloom to aid fertilization, retain more short shoots since those buds have a lower chilling requirement, and delay thinning of marginally chilled trees until fertilized fruit can be differentiated from non-pollinated ones. The recent record global temperatures suggest that new dormancy breaking technologies and replacement of cultivars with those requiring much less chilling will be necessary to grow deciduous fruit crops such as peach in an increasingly warmer 21^st century.