Use of water mist to reduce the risk of frost damage in tree fruits

Climate variability and change have been major threats to global food security historically and will almost certainly continue to be threats in the future given the sensitivity of agricultural production systems to their surrounding environment. Recent changes in temperature and seasonality have significantly impacted commercial fruit production in the Great Lakes region. Michigan's sour cherry and apple production in 2012 was reduced by about 90% and 88%, respectively, compared to the previous year's production due to a series of spring freeze events (USDA, 2013). The timing of the seasonal warm up in the spring and resulting onset of phenological development is a key factor in determining potential cold damage risk for overwintering perennial tree fruit crops, as the vulnerability of vegetation to freeze injury increases rapidly with the stage of development. Application of water prior to the onset of growth has been used in the past to delay early vegetative development of temperate tree fruit crops. Evaporative cooling associated with this approach effectively reduces plant tissue temperature, slowing the rate of growth and leaving it less vulnerable to freezing temperatures. There are several potential drawbacks, however, including consumption of large quantities of water that could increase nutrient leaching along with elevated risks of plant disease risk. This study examined the potential effectiveness of water applied as a spray mist via a new plant management technology, the solid set canopy delivery system (SSCD), to suppress tree fruit bud temperatures and delay the phenological development of the buds. There were two major portions: 1) A detailed collection of field-based phenological and physiological observations associated with the operation of a prototype SSCD cooling system and: 2) Development of a deterministic model of tree fruit bud temperature that was used to examine the potential of water-based cooling of buds in Michigan. The observational study aimed to identify the timing and discharge rate of mist applications on cherry and apple trees was carried out in a growth chamber and at five Michigan orchards (apple at St. Joseph, Charlotte, and Hillsdale, sweet cherry at SWMREC, and sour cherry at Traverse City,) during the 2014, 2015, and 2016 growing seasons with automated instrumentation to monitor and control the water mist flow rate based on environmental conditions. Water mist was applied to apple and cherry buds via the SSCD system after the end of endo-dormancy until king bloom in the non-misted buds based on ambient air temperature and relative humidity. Overall, in three years of the field study misting delayed bloom by 4-9 days in apple and 7- 11 days in cherry, all using substantially less water than that reported in earlier studies; 8.4 to 26 cm/ha in apple and 5.5 to 10.8 cm/ ha in sweet cherry. The deterministic heat transfer model of a tree fruit bud was developed with observational data from growth chamber, potted plant and field-based studies. The model was calibrated using growth chamber data and validated using potted plant and field data. In a model validation study, model simulated one-minute bud temperatures were generally found to be in good agreement with observed bud temperatures, with overall mean average differences of -0.5±0.30C (lab observations) and -0.3±0.15 0C (field observations), mean absolute differences less than 10C and R-square values of 0.80 or greater. The model was then run with ten years of hourly climate data at three locations in major fruit-producing regions of Michigan (2006-2015). Overall, the model estimated a delay in bloom of misted buds by more than a week compared to non-misted buds, which translates into a potential reduction in the frequency of damaging freeze events of 50-75 %, and decrease in freeze injury severity by 10-60 % in misted apple buds and 45-100% in misted cherry buds. Collectively, the results suggest that the spray mist technique has promise as a straightforward and effective indirect frost control strategy with relatively few environmental impacts.