High tunnels are unheated, polyethylene film–covered greenhouse structures used around the world to reduce limitations imposed by harsh weather and temperature fluctuation (Lamont, 2009; Wells and Loy, 1993). High tunnels provide season extension for farmers who grow high-value crops, such as strawberry (Demchak, 2009). Strawberry plants are the most widely grown berry crop in high tunnel systems (Lamont, 2009). This system reduces leaf wetness from excess moisture, gray mold, and pest damage and provides environmental protection to improve percent marketability (Carey et al., 2009; Salame-Donoso et al., 2010).
A study comparing open-field vs. high tunnel systems for strawberry production in Wichita, KS, found that high tunnel production resulted in larger fruit, larger leaf area, greater leaves and shoot biomass, and fewer runners (Kadir et al., 2006). In Kansas, it was found that June-bearing plants in a high tunnel produced 5 weeks before open-field production with an average of 5 °C warmer crowns in comparison with open-field plants (Kadir et al., 2006). High tunnels can substantially improve marketable yields, shelf life, and extend the harvest season for strawberry fruit (Belasco et al., 2013; Kadir and Carey, 2004; Salame-Donoso et al., 2010). However, high tunnels require careful management to prevent excessive temperatures and humidity inside the structure during the warmest months (Jensen and Malter, 1995).
In open-field systems, there are several studies that highlight the importance of environment for growing strawberry successfully. Several studies have found that production temperatures, genotype, and irrigation are the most important parameters that regulate crown development and, consequently, fruit size (Connor et al., 2002; Hortynski et al., 1991; Wang and Camp, 2000). Temperature also affects the rate of nutrient uptake and metabolism, strawberry color development, and firmness. At higher temperatures, transpiration increases, which can increase nutrient uptake because of high light and optimal growing temperature (Kader, 2002; Paull, 1999). It has been reported that growing temperature is a major factor that can affect fruit size, which tends to decrease with temperatures above 29 °C (Kumakura and Shishido, 1994; Wang and Camp, 2000). Ideal crown development occurs at temperatures ≈13 to 29 °C, which encourages high yields with large fruit size. Temperatures above 29 °C encourage excessive crown development, which will potentially reduce subsequent fruit size, fruit weight, and overall plant growth (Hellman and Travis, 1988; Kumakura and Shishido, 1994; Masaru et al., 2016). June-bearing strawberry cultivars that were grown within a high tunnel had a greater fruit yield (weight, average size, and number) in comparison with the open-field production system (Kadir et al., 2006), and the authors suggested that this benefit is the result of a more conducive environment within the high tunnel in the central United States.
Kansas often receives daily temperatures above 29 °C from late June to early August (Kansas Mesonet, 2014). One solution to this challenge is using high tunnels with shadecloth and proper ventilation (Rowley et al., 2011). Another option is to use EC to reduce the internal plant temperature. In strawberry production, EC consists of an overhead sprinkler system installed between rows within the high tunnel. The plant cools as the water that was applied from the overhead sprinklers absorbs the heat energy and is converted into gas through evaporation (Thompson et al., 1993). The use of EC has been suggested as a potential method to decrease strawberry plant internal temperature and increase yields or postharvest quality of fruit (Koike et al., 2009; Lantz et al., 2010; Roos and Jones, 2016). Evaporative cooling or the technique of supplemental irrigation by microjet or sprinkler in addition to drip tape during plant propagation or at planting has shown to delay flowering and increase crown size, average fruit weight, and plant vigor of bare-root plants in Florida (Hochmuth et al., 2006a, 2006b).When this method was used by Dara et al. (2016) during plant establishment in commercial open-field production in California, it increased plant health by a reducing two-spotted spider mite (Tetranychus urticae) infestation and gray mold; however, no indication of yield or internal temperature changes was found. Evaporative cooling has also been used for orchards (Yang and Bryla, 2016) and commercial berry production systems (Evans, 2004; Parchomchuk and Meheriuk, 1996). In California, EC was effective in vineyards (Vitis vinifera) at increasing yield and delaying fruit maturity (Aljibury et al., 1975). It has also been used in apple (Malus domestica) production to enhance color and quality during storage (Evans, 2004; Parchomchuk and Meheriuk, 1996).
In the central United States, fall-planted June-bearing cultivars are typically used for open-field and high tunnel strawberry production because of their high-yielding harvest period that can last up to 6 weeks (Demchak et al., 2010; Kadir et al., 2006). However, many growers may wish to grow other crops in the fall and winter in high tunnels to maximize revenue. Furthermore, yields of day-neutral cultivars are typically higher than those of June-bearing cultivars because of the extended harvest season (Lantz et al., 2010; Rowley et al., 2011). In the high-elevation region of Utah with summer temperatures above 29 °C, it was found that peak high tunnel production of June-bearing cultivars starts 4 weeks before peak high tunnel day-neutral cultivar production (Rowley et al., 2011). In the high tunnel production system in Utah, day-neutral strawberry fruit are typically harvested from late May to mid-December, with total yields greater than June-bearing cultivars (Rowley et al., 2011).
Growing spring-planted day-neutral strawberry cultivars in a high tunnel system could be a new way for strawberry growers in the central United States to protect their crop while ensuring a longer harvest season than traditional open-field production. Furthermore, they could provide a viable rotation alternative for tomato (Solanum lycopersicum), which is the number one crop grown in high tunnels (Knewtson et al., 2010). Day-neutral fruit production begins in the late spring and lasts until midfall. Lantz et al. (2010) estimated the feasibility of high tunnel strawberry production in the northeastern U.S. market through an economic study, although the initial cost of the high tunnel was not included. They concluded that growing strawberry in the high tunnel was profitable during the 15- to 20-week harvest period if the day-neutral fruit yielded 0.75 to 1.25 lb/plant and sold for $2.00 to $4.00 per pound or yielded 0.60 lb/plant and sold for $3.00 to $4.00 per pound (Lantz et al., 2010). Premium prices for strawberry fruit occur in October and November, when production is low on a national scale (Belasco et al., 2013; Pollack and Perez, 2008) and may be driven further by consumer demand. Because day-neutral cultivars are planted in the spring, this system would allow growers to integrate fall and winter crops into the high tunnel system (Heidenreich et al., 2012; Rowley et al., 2011). By contrast, June-bearing cultivars have a higher opportunity cost when grown in high tunnels as they are typically planted in fall and eliminate valuable winter production space (Salame-Donoso et al., 2010).
Spring-planted day-neutral strawberry production in high tunnels in Kansas could be a profitable solution for specialty crop growers. However, extreme summer temperatures events in the central United States may negatively affect fruit yield and quality. Therefore, the objectives of this research were to 1) investigate the feasibility of spring-planted day-neutral cultivars in a high tunnel production system in the central United States as they relate to fruit yield and marketability as well as gray mold incidence, 2) identify day-neutral cultivars that are successful in this system, and 3) investigate the impact of EC on fruit yield and marketability.
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