Colored sweet bell peppers (Capsicum annuum) are a category of large, blocky peppers that are horticulturally mature when green, but continue to ripen to physiological maturity in colors including red, yellow, orange, purple, white, brown, or black (Simonne et al., 1997). Fresh bell peppers are an important source of ascorbic acid and provitamin A, with green, red, and orange peppers having the highest concentrations of these antioxidants (Simonne et al., 1997). While green bell peppers still dominate consumer preferences, markets exist for colored bell peppers, particularly orange, red, and yellow (Frank et al., 2001), and fresh market demand for colored peppers has been increasing (Jovicich et al., 2005).
Colored bell peppers are usually priced two to three times higher than their green counterparts (Jovicich et al., 2005), which compensates for the increased time needed for peppers to go through the period of coloring that lengthens crop exposure to adverse environmental conditions that can damage the fruit and lead to reduced yield and quality (Day, 2010). Jovicich et al. (2005) found greenhouse-grown colored bell peppers were worth up to five times more than those produced in open field conditions, which may explain why colored bell pepper production takes place extensively under protection (Lopez-Marin et al., 2012). Peppers are among the top five most common crops grown in Midwest high tunnels (Knewtson et al., 2010a); and though more recent data are needed on production trends, the underlying assumption is that high tunnel production of colored bell peppers in the Midwest is increasing.
A high tunnel is a solar-heated, passively ventilated, plastic-covered structure that is used to produce high-value specialty crops (Jett, 2017; Lamont, 2009), and small-scale growers serving local markets are the primary users of high tunnels (Carey et al., 2009; Zheng et al., 2019). In the Midwest, high tunnels are an especially important tool for growing solanaceous crops (Carey et al., 2009; Lamont, 2009) because they extend the growing season substantially (Lamont, 2009; Reeve and Drost, 2012) while increasing yield (Waterer, 2003) and improving fruit quality (O’Connell et al., 2012). These improvements in crop production are achieved, in large part, because of the protection afforded by the high tunnel from adverse weather including rain, wind, and hail (Lamont, 2009).
High tunnels maintain a higher soil temperature throughout the duration of the growing season, which improves crop growth (Gent, 1992; Knewtson et al., 2010b), and air temperatures within high tunnels increase the total number of growing degree days (GDD) and the rate at which GDD accumulate (Both et al., 2007; Waterer and Bantle, 2000). Increased GDD accumulation accelerates solanaceous crop growth, development, and ripening (Both et al., 2007; O’Connell et al., 2012; Waterer and Bantle, 2000), and harvest from high tunnel crops can occur 2 to 5 weeks ahead of crops grown in open-field conditions (Kaiser and Ernst, 2012). This acceleration of growth becomes especially important for colored bell pepper production because the development of mature fruit color can take an additional 20 to 30 d after the fruit has reached the mature green stage (Vidigal et al., 2011).
While high tunnels are an important production tool for solanaceous crops, farmers continue to struggle with management challenges within high tunnels, as recently reported by Bruce et al. (2019). Challenges within high tunnel production systems are wide-ranging (Bruce et al., 2019; Zheng et al., 2019), but they include management of heat-related stress (Díaz-Pérez and Smith, 2017). Excess heat can increase bell pepper flower and fruit abortion (Bosland and Votava, 2000; Deli and Tiessen, 1969) as well as increase the incidence of physiological disorders including sunscald (Barber and Sharpe, 1971) and blossom-end rot (BER) (Olle and Bender, 2009).
Bell peppers are adapted to average growing temperatures between 18 and 29 °C (Swiader and Ware, 2002). Night and daytime temperatures above 24 and 32 °C, respectively, lead to flower abortion and stalled fruit set (Bosland and Votava, 2000; Swiader and Ware, 2002). Unfortunately, flower abortion and fruit loss of bell peppers because of high temperatures are common problems in the United States (Bosland and Votava, 2000). The term “sunscald” has been used to define a general category of fruit tissue injury that results from direct exposure to solar radiation, and the physiological disorder can cause economically important losses in bell pepper production (Barber and Sharpe, 1971). BER is a symptom of a localized calcium deficiency, commonly seen in fruit of tomatoes and peppers (Taylor and Locascio, 2004); and several factors, including genetics, growth rate, irrigation regime, and relative humidity, have been shown to have an effect on BER incidence on fruit (Coolong et al., 2019; Taylor and Locascio, 2004). The presence of BER makes fruit unmarketable, and losses because of BER have been reported as high as 35% in the southeastern United States (Coolong et al., 2019). While a large-scale field producer may be able to absorb a larger percentage loss without economic impact, small-scale growers who have the added expense of the high tunnel must work to optimize production and avoid losses related to excess heat.
In temperate climates, producers experience heat-related crop challenges within high tunnels in the summer production months (Díaz-Pérez and Smith, 2017). Techniques to manage excess heat within high tunnels include the use of either forced or natural ventilation (Zheng et al., 2019), whitewashing of the high tunnel (Díaz-Pérez and Smith, 2017), and shadecloth (shade netting) (Díaz-Pérez and Smith, 2017; Drost and Maughan, 2018).
While ventilation including gables, fans, and roof vents have been identified as important tools for heat management in high tunnels (Zheng et al., 2019), these may be cost-prohibitive for many small-scale growers. The use of shadecloth may be the most economically feasible option on many farms (Díaz-Pérez, 2014; Drost and Maughan, 2018).
Shadecloth (or shade netting) is typically made of woven or knitted plastic materials such as high-density polyethylene or polypropylene (Castellano et al., 2008) and is commonly black (Stamps, 2009). Shadecloth has been shown to improve yield (Ambrózy et al., 2016; Elad et al., 2007; Selahle et al., 2015) and postharvest quality of sweet bell peppers (Ambrózy et al., 2016; Kong et al., 2013; Mashabela et al., 2015; Selahle et al., 2015).
Shadecloth is cited as a tool to manage excess heat and solar radiation (Stamps, 2009), but the recommendations for level of light reduction and color of shadecloth vary. Many recent studies of the effects of shading on colored bell pepper production have been conducted outside of the United States (Ambrózy et al., 2016; Díaz-Pérez and Smith, 2017; Elad et al., 2007; Kong et al., 2013; Mashabela et al., 2015; Selahle et al., 2015) or in regions excluding the Midwest (Day, 2010; Díaz-Pérez 2013, 2014). Furthermore, many studies are conducted in open-field conditions (Ambrózy et al., 2016; Day, 2010; Díaz-Pérez, 2013, 2014; Kong et al., 2013), which makes it more difficult to predict plant performance under shade within high tunnel production systems. Research from the Southeast region using shade structures in open-field conditions suggests the optimum shade level to improve bell pepper plant health and yield is between 30% and 47% (Díaz-Pérez, 2013, 2014); however, shade recommendations for Midwest high tunnel pepper production are unknown.
The purpose of the present research was to define responses of colored pepper production to high tunnel shadecloth use in the Midwest. Our objectives were to 1) test black shadecloth with light-reduction levels that were within the ideal (30%) as well as the upper limit (50%) of the current recommendations, and 2) evaluate the performance of seven commercially available bell pepper cultivars with four different colors at maturity—orange, purple, red, and yellow—while assessing their response to the shade treatments.
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