2019 ASHS Annual Conference

Temperate fruit trees such as apple, pear, cherry, and peach require dormant season cool temperatures to complete endodormancy so that flowers fertilize and leaves emerge normally in the spring. Recent and continuous global rises in mean annual temperatures are increasing the probability of marginal winter cold in traditional fruit production areas where many fruit tree cultivars risk poor flower set, irregular vegetative budbreak and/or early phenology (bloom). An example where the global increase in mean annual temperature has affected winter chilling is in the southeastern U.S., significantly impacting peach fruit set and cropping in 3 of the last 5 years. Continued temperature increases are predicted through the 21^st Century. Fruit growers must 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 predetermine if chilling might be insufficient. Several compounds and fertilizers currently used with limited success are dormancy breaking agents, which include hydrogen cyanimide, Erger®, thidiazuron, potassium nitrate and calcium ammonium nitrate, but labelling remains a roadblock. Timing and rates are also critical when using these compounds, thus temperature models need refinement. Other management tools become necessary if dormancy issues persist into spring. For stone fruits, these include delayed pruning, cooling by whitewashing or water, retaining short shoots, and delaying thinning. The current trend of record global temperatures suggest that new dormancy breaking technologies and replacement of cultivars with new ones requiring much less chilling will be necessary to grow deciduous fruit crops in an increasing warmer environment. Fortunately, annotation of the Prunus genome is discovering genes and gene complexes (QTLs) that regulate dormancy, which breeders hope to incorporate into new cultivars via Marker Assisted Breeding that will lead to climate adapted fruit trees in the future.