Critical temperatures and heating times for fruit damage in northern highbush blueberry

Authors: YANG, FAN-HSUAN - Oregon State University; Bryla, David; STRIK, BERNADINE - Oregon State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2019
Publication Date: 12/12/2019
Citation: Yang, F., Bryla, D.R., Strik, B.C. 2019. Critical temperatures and heating times for fruit damage in northern highbush blueberry. HortScience. 54(12):2213-2239. https://doi.org/10.21273/HORTSCI14427-19.
DOI: https://doi.org/10.21273/HORTSCI14427-19

Interpretive Summary: Heat damage is becoming a prevalent problem for many blueberry growers in the northwestern United States. The region, which includes Oregon and Washington, is the leading producer of blueberries in the country. In 2015, these two states produced a combined total of 91,104 t of blueberries (36% of the total U.S. production) . However, the Washington blueberry industry lost an estimated $10 million of fruit that year due to heat and inadequate water for cooling and irrigation. Similar losses were reported in Oregon. Extreme weather events such as this have become more common in the region over the last two decades. Options for reducing heat damage with drip are currently limited. To contend with this problem, some blueberry growers are installing dual irrigation systems and use micro-sprinklers to cool the berries and drip tubing to irrigate the plants. Currently, there are many questions regarding the use of micro-sprinklers to reduce heat damage, including the temperature at which cooling is needed. Therefore, the objective of the present study was to characterize and determine the critical temperatures and heating times for fruit damage. The work revealed that berries fully exposed to the sun were much warmer than those in the shade, which explains why heat damage usually occurs on the upper part of the canopy. It also showed that heat damage occurred on both green and blue-colored berries, with necrosis and softening being the two most prevalent types of heat damage. Results indicated that mature fruit have higher critical temperatures and tolerate longer periods of heat exposure than immature fruit. This is possibly due to thicker skin and more wax on the mature berries. Heat tolerance differed between two cultivars tested. ‘Aurora’ was more heat sensitive and had a lower critical temperature than ‘Elliott’. These results provide information for developing cultural practices to reduce blueberry heat damage.

Technical Abstract: Over-canopy sprinkler systems are often used to cool blueberry fields in the Pacific Northwest, 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 in northern highbush blueberry. An initial study conducted in western Oregon in a mature planting of late-season ‘Elliott’ blueberry revealed that heat damage was typically observed within 1-3 d after an extreme heat event. Damage occurred primarily on sun-exposed berries and happened at both green and blue stages of fruit development. A subsequent study was conducted to determine whether the critical temperature for heat damage differs between green and blue fruit in 'Aurora' and 'Elliott' blueberry. Berries were heated using a chamber-free convective unit and were exposed for up to 4 h to berry temperatures of 42, 44, 46, and 48 C. ‘Aurora’ was more susceptible to heat damage than ‘Elliott’, particularly when the berries were green. Green fruit of ‘Aurora’ were damaged within 2-2.5 h at 42-46 C and 2 h at 48 C. After 4 h, ‘Aurora’ had damage to 17% of the green berries on the cluster at 42 C and as much as 59% damage at 48 C. Green fruit of ‘Elliott’ had no damage at 42 or 44 C and 14% and 24% damage after 4 h at 46 and 48 C, respectively. Neither cultivar had any damage on blue fruit at 42 C, and both had < 4% damage after 4 h at 44 C. ‘Elliott’ also sustained minimal damage to blue fruit at 46 C but had nearly 20% damage after 4 h at 48 C. In ‘Aurora’, damage occurred to blue fruit within 2 h at 46 and 48 C, which increased to 19% and 30% after 4 h at these respective temperatures. 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 > 32 C during the green stages of fruit development and > 35 C during ripening.