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Over-canopy sprinkler systems are used to cool northern highbush blueberry ( Vaccinium corymbosum  L.) fields and maintain fruit quality in the northwestern United States, but more information is needed to determine exactly when cooling is needed. The objective of this study was to identify the critical temperatures for heat damage to berries and for effective evaporative cooling. An initial study conducted in western Oregon in a mature planting of late-season ‘Elliott’ blueberry revealed that heat damage was typically observed within 1 to 3 days after an extreme heat event. Fruit damage, including softening, shriveling, and necrosis, occurred during both green and blue stages of development and was found primarily on sun-exposed berries, which on hot, sunny days (&gt;35 °C) were 7 to 11 °C warmer than the ambient air temperature. A subsequent study was conducted to determine whether the critical temperature for heat damage differed between the green and blue fruit stages. In this case, ‘Aurora’ was compared with ‘Elliott’ blueberry. Berries were heated using a chamber-free convective unit and were exposed for up to 4 hours to berry temperatures of 42, 44, 46, and 48 °C. When the berries were green, significant damage was visible at each temperature within 1.5 to 2 hours in ‘Aurora’ and 3 to 3.5 hours in ‘Elliott’. Damage of green berries increased with time and temperature, and after 4 hours, ranged from 17% to 59% of the total berry number in the cluster in ‘Aurora’ and 10% to 24% in ‘Elliott’. Fruit damage at the blue stage was less than at the green stage and was only significant at 46 and 48 °C (within 3.5 to 2 hours, respectively) in ‘Aurora’ and at 48 °C (within 2 hours) in ‘Elliott’. Wax and cutin layers thickened on the berries as they progressed from green to blue, which perhaps increased their tolerance to heat at later stages of development. Based on these results, northern highbush blueberry fields should be cooled at air temperatures &gt;32 °C during the green stages of fruit development and &gt;35 °C during ripening.

Critical Temperatures and Heating Times for Fruit Damage in Northern Highbush Blueberry