Strawberry is an important source of early-season income on farms across New England (Bornt et al., 1998; Grubinger, 2012), where it is estimated there are more than 1000 acres in production on 884 farms (U.S. Department of Agriculture National Agricultural Statistics Service, 2018) and locally produced strawberry fruit are highly regarded by consumers. Short-day cultivars (June-bearers) have long been popular in the region for their winterhardiness and low establishment costs (Black et al., 2002; Pritts and Handley, 1998), but regional dependence on these cultivars has prevented growers from participating in the commercial strawberry market outside their brief 4- to 6-week fruiting period each year (Pritts and Handley, 1998).
Day-neutral (DN) strawberry cultivars are less affected by daylength and typically continue to flower as long as temperatures remain between 40 and 85 °F, resulting in a substantially longer fruiting period compared with short-day plants (Pritts and Handley, 1998; Rowley et al., 2010). Furthermore, DN plants produce ripe fruit ≈10 weeks after planting, not the following year as with short-day plants (Pritts and Handley, 1998), reducing the period of crop management before financial return (Bornt et al., 1998).
DN cultivars have largely been developed for regions of the United States with substantial acreage in strawberry production, namely California and Florida (Lawrence et al., 1990). However, following the adoption of the plasticulture production system (Poling, 1993), early field trials in North Carolina showed that cultivars could produce as much as 1.02 kg/plant annually on the east coast of the United States (Ballington et al., 2008), suggesting great potential for DN production across the country. Recently, cultivar evaluations conducted from the mid-Atlantic region of the United States to as far north as Quebec, Canada, indicate that DN productivity is influenced by cultivar, site, and growing season, and annual marketable yields have ranged from between 189 and 950 g/plant (Condori et al., 2017; Lewers et al., 2017; Petran et al., 2016; Pritts, 2017a; Pritts and McDermott, 2017; Van Sterthem et al., 2017; Weber et al., 2018). Using a standard plasticulture plant spacing of 17,424 plants/acre (Lantz et al., 2010) such yields equate to between 8137 and 40,902 kg·ha−1, well exceeding the 5900 lb/acre (6613 kg·ha−1) harvested by New England growers (U.S. Department of Agriculture National Agricultural Statistics Service, 2018). This strongly suggests that DN strawberry cultivars may not only extend the fruiting period, but also offer high annual yields compared with short-day plants.
There are only a limited number of commercial farmers growing DN cultivars in New England. Anecdotally, the cultivars Albion, San Andreas, and Seascape are the most commonly grown, with select farmers reporting yields exceeding 25,000 lb/acre. While there have been exploratory trials at universities in the region (Handley, 2008; A. Radin, personal communication), to our knowledge, no replicated studies have been published and yield data are not available. Studies outside New England show tremendous variation in annual yield among sites, highlighting the need for region-specific research. For example, ‘Seascape’ was among the lowest yielding cultivars in Minnesota, but among the highest yielding in Maryland and New York (Lewers et al., 2017; Petran et al., 2016; Pritts, 2017a).
Several studies have found that low tunnel protective structures increase the marketable yield (grams per plant) and/or the percent marketable yield, especially late in the season and following weather events (Lewers et al., 2017; Petran et al., 2016; Pritts, 2017a; Resende et al., 2010; Van Sterthem et al., 2017). This suggests that low tunnels may be a useful tool for protecting fruit marketability in regions with frequent precipitation events, such as throughout the northeastern United States, but little is known about low tunnel effects on plant growth and development.
Of particular interest is runner initiation, which has been cited as a barrier for the adoption of the plasticulture cultivation system (Handley et al., 2009). While runner removal is standard practice in plasticulture production (Voth and Bringhurst, 1990), many growers converting from the perennial matted-row cultivation system [where runners are not removed (Poling and Durner, 1986)], find the task costly and time consuming because substantial labor is required to remove runners by hand (Handley et al., 2009). The gibberellic acid synthesis inhibitor prohexadione-calcium (commonly known as Apogee; BASF Corp., Research Triangle Park, NC) has been approved in Canada to assist with runner management (BASF Canada, Mississauga, ON, Canada). However, despite trials in Maryland, Massachusetts, and Maine showing that applications can significantly reduce runner initiation and even improve fruit yields in strawberry (Black, 2004; Green and Schloemann, 2010; Handley et al., 2009), the product has not been approved in the United States. Thus, other tools are needed.
Lewers (2013) has reported observing a reduction in runner emergence under low tunnels in field trials in Maryland, but to our knowledge, these effects have not been documented in any published studies. However, they are in line with other research showing the cultivars Chandler and Sweet Charlie produced fewer runners under high tunnel protected culture (Kadir et al., 2006). Total leaf area, shoot biomass (grams), leaf number, and the number of branch crowns have also been affected by high tunnels (Kadir et al., 2006), suggesting that protected culture may also influence plant size and even planting density.
Furthermore, recent experiments comparing the microclimate under low tunnels with the traditional open bed environment have reported that season-long air and soil temperatures are greater under low tunnels (Condori et al., 2017; Van Sterthem et al., 2017). Warmer temperatures have been associated with greater yields and an extended fruiting season, especially in the mid-Atlantic region where temperatures are milder than the northeastern United States (Condori et al., 2017; Lewers et al., 2017; Van Sterthem et al., 2017). Given these findings, it is important to understand the effect of low tunnels on microclimate and season duration in the northeastern United States, where summers are hot but nighttime temperatures can drop precipitously in the fall. Of particular interest is whether low tunnels increase temperatures during the summer months even when low tunnels are vented, and whether they provide any thermal insulation at night during the fall months, when they may be used to assist in season-extension. Experiments conducted in warmer regions suggest low tunnels do not provide a substantial buffer in nighttime temperatures (Condori et al., 2017; Van Sterthem et al., 2017), but since most studies present season-long averages, not daily fluctuations in temperature, this remains unclear.
The objectives of this study were to quantify the effects of low tunnel structures and DN cultivar on yield, fruit marketability, and fruit and plant characteristics. We were also interested in the impact of low tunnels on air and soil temperatures. Toward this end, we evaluated five relatively well-known DN cultivars: Albion, Monterey, Portola, San Andreas, and Seascape, as well as three additional cultivars that may be candidates for our region: Aromas, Cabrillo, and Sweet Ann. Plants were grown on two production systems (called cover treatments): open beds and low tunnels. Data were collected on yield, fruit weight, the fruiting pattern of cultivars throughout the season (called fruiting pattern), fruit SSC, runner emergence, plant height, plant diameter, and average, maximum, and minimum air and soil temperatures.
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