Northern highbush blueberry (Vaccinium corymbosum L.) is sensitive to high temperatures, particularly during fruit ripening (Lobos and Hancock, 2015). After major heat events, reduction in fruit quality and physical disorders such as sunburn, fruit softening, and discoloration are commonly reported (Yang et al., 2019). To reduce the impact of high temperature on fruit, a number of blueberry growers in the northwestern United States either advance their harvest schedules to escape the heat or use overhead irrigation systems to cool the berries (Houston et al., 2018). Running overhead sprinklers or microsprinklers during heat events is an effective means of reducing temperature in blueberry (Yang et al., 2020a) and other fruit crops (Caravia et al., 2017; Greer and Weedon, 2014; Iglesias et al., 2002; Kliewer and Schultz, 1973; Parchomchuk and Meheriuk, 1996; Pelletier et al., 2016). As more growers begin to adopt these practices, some key questions are arising, such as when is the risk of heat damage critical economically in blueberries, and how can cooling practices be optimized to prevent the damage efficiently?
Previously, Yang et al. (2019) found that visual signs of heat damage occur in northern highbush blueberry when the surface temperature of berries reached 42 to 48 °C for 1.5 to 2 h in a sensitive cultivar and 3 to 3.5 h in a more tolerant cultivar. They also determined that the temperature of the sun-exposed berries was up to 7 to 11 °C warmer than the air temperature on hot, sunny days. Unfortunately, predictions of heat damage based simply on air-temperature measurements are not always accurate because there are other environmental factors, such as light intensity and wind, that affect the temperature of the plants and fruit (Cellier et al., 1993; Monteith and Unsworth, 2013; Saudreau et al., 2009). To estimate blueberry temperatures more effectively according to local environmental conditions, mathematical models based on energy balance are an option. In an energy balance model, the overall gain and loss of energy on an object is equal (Monteith and Unsworth, 2013). The energy flux of a fruit is estimated according to its geometry and surface characteristics, and is a function of the environmental conditions. Energy models have been widely used for predicting fruit temperature in a number of crops, including apple [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.], peach [Prunus persica (L.) Batsch], wine grape (Vitis vinifera L.), and fig (Ficus carica L.) (Cola et al., 2009; Evans, 2004; Li et al., 2014; Patiño et al., 1994; Pitacco et al., 2000; Saudreau et al., 2007; Smart and Sinclair, 1976), but they have not been applied previously to blueberry.
Energy balance models can also be useful for predicting the efficiency of cooling systems to reduce heat damage in fruit crops. For example, Evans (2004) provided equations for estimating skin and core temperatures in apple and was able to simulate changes in fruit temperature during cooling using an irrigation system with small spray nozzles. He calculated the amount of heat removed from the apples based on estimates of water interception, sprinkler spacing, and temperature differences between the fruit and irrigation water. A simple energy balance model was likewise used to predict potential water use during evaporative cooling in wine grape (Caravia et al., 2017).
In this study, we developed an energy balance model specifically for northern highbush blueberry. Our objectives included 1) developing a model for predicting blueberry fruit temperature based on the weather conditions, 2) evaluating the impact of different weather parameters on fruit temperature, and 3) predicting the efficacy of cooling on fruit temperature based on sprinkler specifications and cooling frequency. This information is needed to design effective management practices and strategies for preventing heat damage before harvest in blueberries.
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