Graduate Thesis Or Dissertation
 

Predictions and Practices for Reducing Heat Damage in Northern Highbush Blueberry (Vaccinium corymbosum L.)

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  • Heat-related fruit damage is a common problem in the northern highbush blueberry (Vaccinium corymbosum L). This is particularly true in regions such as the northwestern United States, where summers are warm and dry, and daytime temperature regularly exceeds 32 oC. Millions of dollars of fruit damage are reported in blueberries grown in Oregon and Washington each year. To reduce heat damage, growers advance their picking schedules or use irrigation systems to cool their fields. While over-canopy sprinklers have been used traditionally to irrigate and cool blueberry during heat events, most new fields are irrigated by drip. Some growers are installing dual irrigation systems with micro-sprinklers for cooling. Currently, there is little information available on whether these systems can improve fruit quality in blueberries, and if so, how to operate cooling systems effectively. To address these problems, four studies were conducted from 2014 to 2016 in western Oregon. In the first study, berry temperature patterns and ultrastructure of the berry cuticle were examined at various canopy positions. A chamber-free convective heater was also used to determine the critical temperatures and heating times for fruit damage of two popular blueberry cultivars, ‘Aurora’ and ‘Elliott.’ Results showed that berries exposed to full sun were warmer and had thicker cuticle and wax layers than those in the shade. Mature fruit tolerated higher temperatures for a longer duration than immature fruit, and ‘Aurora’ was less heat-tolerant than ‘Elliott’. In the second study, cooling sprinklers and micro sprinklers were analyzed for their ability to reduce heat damage and to improve fruit quality. Effects of different cooling frequencies on reducing fruit temperature were also evaluated. Results showed that both sprinklers and micro-sprinklers were effective tools for reducing fruit temperature and improving fruit quality. Using micro-sprinklers with short cycles may be the best practice because these use significantly less water than sprinklers and keep fruit from getting too wet. In the third study, local weather data were collected to construct an energy balance model to predict blueberry fruit temperature. This model was later incorporated with different operational specifications to simulate fruit temperature changes under various cooling practices. The result successfully simulated diurnal fruit temperature patterns and identified the impact of major environmental factors on fruit temperature. Additionally, the model accurately simulated the outcome of different cooling practices. The last study investigated the relationship between blueberry fruit transpiration and Ca uptake in fruit. In this study, the density and functionality of fruit stomata were compared with fruit Ca uptake throughout developmental stages. Although this study did not directly relate to heat damage, cooling practices may potentially improve fruit transpiration during heat events. The data of this study indicated that blueberry fruit stomata density was low and mainly concentrated near the calyx. Fruit transpiration may mediate Ca uptake since the fruit stoma activity was concurrent with Ca accumulation patterns. Overall, the results of this work provide a better understanding of blueberry heat damage formation and lead to possible methods for problem resolution. By including more cultivar characteristics and farm specifications, this model can be applied as a forecasting tool to predict heat damage incidence and suggest feasible cooling practices. These outcomes may provide growers greater flexibility with adapting to changing climate conditions.
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