Evaporative Cooling with Sprinklers to Reduce Heat-related Fruit Damage in Northern Highbush Blueberry

Hot and sunny weather can cause a considerable amount of fruit damage in blueberries and results in millions of dollars of crop loss each year. The objective of this study was to evaluate the efficacy of using sprinklers to reduce the damage. The study was conducted for 2 years in a mature planting of northern highbush blueberry (Vaccinium corymbosum L. ‘Elliott’) located in western Oregon. Treatments included sprinkler irrigation (at night) and cooling when daytime air temperature reached set points of either 90 or 95 °F, sprinkler irrigation (at night) with no cooling, and drip irrigation with no cooling. For cooling, sprinklers were turned “on” for 15 min every hour until ambient air temperature dropped below the set point. Heat damage occurred primarily on sun-exposed berries and happened at both green and blue stages of fruit development. Early damage resulted in poor fruit color at harvest and produced large crevices on the berry surface. Berry temperatures were usually higher than the air temperature between 1000 and 1800 HR and reached up to 117 °F in sun-exposed clusters and 100 °F in shaded clusters. Berry temperatures dropped to < 90 °F within the first 15 min of cooling and never exceeded ambient air temperature during the cooling cycle. While the percentage of fruit with heat damage was low even without cooling (< 2%), cooling reduced the damage to nearly 0% each year, and increased the size of the berries. However, cooling had no effect on yield, fruit firmness, soluble solids concentration, titratable acidity, thickness of the waxy bloom on the fruit, or concentration of phenolic compounds in the fruit. Based on these results, cooling should begin at the “late green fruit” stage at critical temperatures ≥ 95 °F.