Planting and high-tunnel management.

Experiments were conducted at two sites: University Farm of North Carolina Agricultural & Technical State University, Greensboro, NC (USDA Hardiness Zone 7b) and Small Farm Unit of the Center for Environmental Farming Systems, Goldsboro, NC (USDA Hardiness Zone 8a). Soil types were Enon fine sandy loam and Wickham loamy sand at Greensboro and Goldsboro sites, respectively. High tunnels [Greensboro (MCS Tuff high tunnel; Morgan County Seeds, Barnett, MO), Goldsboro (30 Budget Plus; Atlas Greenhouse Systems, Alapaha, GA)] at both sites were constructed in Aug. 2014 with north–south orientations. High tunnels were 30 ft wide and 96 ft long, with 8- and 6-ft side walls at Greensboro and Goldsboro sites, respectively. Both structures were covered with a single layer of 6-mil greenhouse-grade polyethylene film (FarmTek, Dyersville, IA). At Greensboro, the land was in the second year of organic transition, the previous crops were organically managed strawberries (up to June 2014, then no crops were planted before construction). In Goldsboro, the land had been managed organically for more than 10 years; the previous crops were organically managed sweet corn (Zea mays) before construction.

Ten strawberry cultivars were evaluated in the study: San Andreas and Albion are day-neutral cultivars; Florida Radiance, Benicia, Camarosa, Camino Real, Chandler, Strawberry Festival, Sweet Charlie, and Winterstar are June-bearing cultivars. Plugs of ‘San Andreas’, ‘Florida Radiance’, ‘Benicia’, ‘Camino Real’, and ‘Winterstar’ were conventionally raised (Norton Creek Farms, Cashiers, NC), while plugs of the other cultivars were organically raised (Cottle Strawberry Nursery, Faison, NC).

Strawberry plugs were transplanted on 30 Sept. 2014 on raised beds. Six beds, measured at 30 inches wide and 4 inches high with 4-ft center-to-center bed spacing, were made inside each high tunnel. The beds were covered with 48-inch, 1.25-mil embossed black plastic mulch (Berry Hill Irrigation, Buffalo Junction, VA). One 8-mil drip tape with 8-inch emitter spacing was placed in the middle of each bed (T-Tape Systems; Rivulis Irrigation, San Diego, CA). A completely randomized experimental design with three replications of each cultivar was used at both sites. Each experimental plot contained 18 strawberry plants that were planted in two rows and staggered 14 inches apart.

High tunnels were closed when night temperatures were below 55 °F and opened during the day when temperatures reached 70–75 °F. Double layers of 1.5 oz/yard² rowcover (Gro-Guard; Atmore Industries, Atmore, AL) were laid directly on the plants at night for frost protection when outside temperatures were predicted to drop below 32 °F.

Fertility and pest management.

Both high tunnels were fertilized preplant with feathermeal (12N–0.5P–0K; Nutrimax, Greensboro, NC) at 60 lb/acre nitrogen (N), sulfate of potash (0N–0P–42K; Compass Minerals, Overland Park, KS) at 83 lb/acre potassium (K), soluble borate micronutrient (Beau-Ron^®D; Drexel Chemical Co., Memphis, TN) at 0.98 lb/acre boron (B), and 10,000 lb/acre organic compost (Brooks Contractor, Goldston, NC). The organic compost had average properties of 6.9 pH, 2.91 mS·cm⁻¹ electric conductivity, and a carbon to nitrogen ratio of 10.9:1. Estimated available nutrients for the crop are 0.6% N, 0.03% phosphorus (P), 0.2% K, 10.3% calcium (Ca), 0.1% magnesium (Mg), and 0.1% sulfur (S). Plants were fertigated from March to May with fish emulsion (2N–0.9P–0K; SF Organics, Thomson, IL) at a weekly rate of 0.4 lb/acre N.

Organic Materials Review Institute listed pesticides potassium salts of fatty acids (M-Pede; Dow AgroSciences, Indianapolis, IN), Isaria fumosorosa Apopka Strain 97 (PFR-97; Certis US, Columbia, MD), and Bacillus thuringiensis sp. kurstaki (DiPel; Valent BioSciences Corp., Libertyville, IL) were applied in a rotational scheme at rates based on product labels to control two-spotted spider mites (Tetranychus urticae) and aphids [green peach aphid (Myzus persicae), melon aphid (Aphids gossypii), and strawberry aphid (Chaetosiphon fragaefolii)] at both sites. In addition, predatory mites [Phytoseiulus persimilis (Spidex; Koppert Biological Systems, Howell, MI)] at a rate of 1.4 adults/ft² were introduced on 23 Jan. and 30 Jan. 2015 at Greensboro to control two-spotted spider mites. Landscape fabric was laid in row middles to control weeds at Greensboro, whereas hand weeding was conducted at Goldsboro.

Data collection.

Ripe strawberry fruit were harvested from 6 Nov. 2014 to 28 May 2015 and from 7 Nov. 2014 to 4 May 2015 at Greensboro and Goldsboro, respectively. Harvests were conducted once per week from November to February and twice per week in March, April, and May. Marketable and cull fruit (misshaped, damaged by diseases or insects, cold injured, or sunburned) were counted and weighed separately at each harvest. Following frost events from 18 to 20 Feb., percentages of damaged open blossoms were recorded on six randomly selected plants in each plot on 23 Feb. 2015. Plant canopy diameters were measured in early December, February, March, April, and May on six randomly selected plants in each plot. Border plants were avoided. Throughout the production season, runners were counted and removed on a weekly basis. At Greensboro, TSS contents of three typical marketable strawberry fruit per cultivar per replication were measured at six harvests (late February, early and late March and April, and early May) as described previously (Salamé-Donoso et al., 2010). A handheld refractometer (Spectrum Technologies, Aurora, IL) was used for the measurements. Fruit TSS was measured once at the Goldsboro site during peak harvest in the middle of April. About 18 of the youngest fully expanded strawberry leaves and petioles were sampled from each experiment plot on 30 Apr. 2015 at the Greensboro site. Plant samples were sent to the North Carolina Department of Agriculture & Consumer Services (Raleigh) for nutrient analysis. Data loggers (WatchDog 1650; Spectrum Technologies) were set at the height of plant canopies inside and outside of high tunnels to automatically record air temperatures at a 30-min interval throughout the season.

Statistical analyses.

Analysis of variance was performed using the Proc Mixed procedure of SAS (version 9.2C for Windows; SAS Institute, Cary, NC). Tukey’s honest significant difference test (α = 0.05) was conducted for multiple comparisons of different measurements among strawberry cultivars.

Yield.

Cultivar effects on whole-season strawberry yields differed between the two locations. Significant differences among cultivars were observed at Greensboro, but not Goldsboro (Table 1). ‘Florida Radiance’ had the highest total yield (657 g/plant), followed by ‘Winterstar’ (total yield 585 g/plant) and ‘Chandler’ (total yield 569 g/plant) at Greensboro. ‘Winterstar’ and ‘Chandler’ also ranked second and third among the evaluated strawberry cultivars at Goldsboro in terms of the total yields, whereas Benicia was the top producer at this site with a total yield of 800 g/plant. Day-neutral cultivar Albion and June-bearing cultivar Sweet Charlie had the lowest yields at both locations; their total yields were 342 and 396 g/plant at Greensboro, and 440 and 332 g/plant at Goldsboro, respectively (Table 1). Comparing yield performances between the two day-neutral cultivars, Albion had lower total and marketable yields than San Andreas, although the difference was not significant. These results are similar to those reported by Ruan et al. (2011); however, higher yields of ‘Albion’ in contrast to ‘San Andreas’ were noticed in Washington State (Hoashi-Erhardt et al., 2012).

Table 1.Monthly marketable and total yields (g/plant) of 10 organically managed strawberry cultivars grown under high tunnels in the 2014–15 season at Greensboro and Goldsboro, NC.

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Although ‘Winterstar’ exhibited the second highest total yield, its marketable yield ranked fourth, after ‘Benicia’, ‘Chandler’, and ‘Camino Real’ at both locations. When averaging among cultivars, marketable yield accounted for 75% and 66% of total yields at Greensboro and Goldsboro sites, respectively (Table 1).

Harvest of day-neutral cultivars started earlier than that of June-bearing cultivars at both locations. The first harvest was from ‘Albion’ on 6 Nov. (Greensboro) and 7 Nov. (Goldsboro), and 14–17 d later for ‘San Andreas’. Among the June-bearing cultivars, Florida Radiance matured the earliest, with the first harvest occurring 87 and 79 d after transplanting at Greensboro and Goldsboro, respectively. This was about 2 weeks earlier than ‘Winterstar’ and a month earlier than other June-bearing cultivars (Table 2).

Table 2.First harvest dates and time after transplanting of 10 organically managed strawberry cultivars grown under high tunnels in the 2014–15 season at Greensboro and Goldsboro, NC.

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Peak harvest occurred in April for all strawberry cultivars evaluated in the study (Table 1). At Goldsboro, marketable yield in April accounted for more than 70% of entire season’s harvest for June-bearing cultivars and 53% and 63% for day-neutral cultivars San Andreas and Albion, respectively. At Greensboro, the number ranged from 38% for ‘Strawberry Festival’ to 71% for ‘Benicia’ (Table 1). Results of the study confirmed previous observations that high tunnels advanced strawberry harvests by 4 to 5 weeks (Kadir et al., 2006; Rowley et al., 2010b) considering peak harvests normally occur in May for open-field strawberries in North Carolina (Poling et al., 2005). In March, ‘Winterstar’ had 73 and 65 g/plant total yield at Greensboro and Goldsboro, respectively, the highest among other cultivars in this month. However, marketable yields were only 34 and 36 g/plant at Greensboro and Goldsboro, respectively. Deformed fruit developed from frost damaged flowers was the primary reason for low market yield from Winterstar, as well as other cultivars in March.

Released in 2009 by the Institute of Food and Agricultural Sciences, University of Florida, ‘Florida Radiance’ has become one of the major strawberry cultivars in Florida and some other early spring production regions worldwide (Chandler et al., 2009; Ohlemeier, 2011; Whitaker et al., 2012). Although ‘Florida Radiance’ was categorized as a short-day cultivar, it develops more flowers under long-day conditions and requires less chilling for flower initiation than typical short-day cultivars (Whitaker et al., 2013). Because of this unique characteristic, ‘Florida Radiance’ is recommended for protected culture systems (Whitaker et al., 2013). Using ‘Florida Radiance’ as one parent, ‘Winterstar’ was also developed by University of Florida, and released in 2011 (Whitaker et al., 2012). ‘Winterstar’ shares many desirable characteristics with ‘Florida Radiance’. In the present study, ‘Florida Radiance’ was superior in early yield compared with ‘Winterstar’; however, ‘Winterstar’ had more consistent yield than ‘Florida Radiance’. ‘Benicia’ is a newly released cultivar by University of California, which is marketed for its high yield and large fruit (Gasic and Preece, 2014). Planting area of the cultivar has increased from 62 to 1165 acres within 2 years in the Oxnard district of California (Burfield, 2013). Currently, there is very little information regarding growing ‘Benicia’ under other production systems.

Vegetative growth.

Canopy diameters of strawberry plants slightly declined from December to February and increased from March to May at Greensboro (Table 3). ‘Albion’ had the smallest canopy size in most months except in December at Greensboro. Throughout the season, ‘Albion’ and ‘San Andreas’ increased canopy sizes by 4.8 and 8 cm, respectively, while more than 10 cm growth was observed on most June-bearing cultivars. ‘Chandler’, ‘Florida Radiance’, and ‘Winterstar’ had canopy diameters of over 36 cm in May. They also developed the most branch crowns and had the highest values on fresh weights of leaves and stems at the end of the season (Table 4). Similar trends of the growth of plant canopies were observed at Goldsboro (data not shown), but ‘Camarosa’, ‘Benicia’, ‘Camino Real’, and ‘Chandler’ exhibited higher values on fresh weights of leaves and crowns at the end of the season (Table 4).

Table 3.Canopy diameters of 10 organically managed strawberry cultivars grown under high tunnels in the 2014–15 season at Greensboro NC.

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Table 4.Number of branch crowns and fresh weights of leaves and stems of 10 organically managed strawberry cultivars grown under high tunnels in 2014–15 season at Greensboro and Goldsboro, NC.

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Small canopy sizes of day-neutral cultivars suggest plant density can be increased to maximize yield in limited spaces under high tunnels. By comparing yields of ‘Albion’ grown at four plant spacings, Portz et al. (2011) noticed that there was no significant difference in strawberry yields between 10- and 12-inch spacing (spacing was measured within rows, between plants, and between rows). With canopy diameter of 24.1 cm at Greensboro, plant spacing of ‘Albion’ could be reduced from the present 14-inch to 10- or 12-inch spacing without sacrificing yield.

Developing three to four branch crowns is important to achieving adequate yield in the annual hill plasticulture system (Poling et al., 2005). A range of 2.2 to 5.3 branch crowns were formed at the end of the season in the present study. ‘Albion’ and ‘Sweet Charlie’ developed fewer than three branch crowns, and ‘Chandler’ and ‘Winterstar’ had more than four branch crowns at both locations (Table 4). Few runners were developed before April. This was particularly true for strawberries at Greensboro. At Goldsboro, ‘Camino Real’ had relatively more runners in fall, whereas ‘Strawberry Festival’ had more runners in spring (data not shown). Strawberry plants at Goldsboro generally developed more branch crowns and had higher fresh weight values of leaves and stems compared with plants at Goldsboro. This is mainly because Goldsboro has warmer climate than Greensboro.

Frost damage.

Severe frost events occurred from 18 to 20 Feb. at both locations (Fig. 1). The night temperatures under rowcovers in the high tunnels averaged 18.8 and 16.7 °F higher than temperatures outside the high tunnels at Greensboro and Goldsboro, respectively. The lowest temperature recorded outside during those 3 d was 6.5 °F at Greensboro, whereas the temperature inside the high tunnel was 29.6 °F during that same time. Strawberry plants may be permanently damaged when the temperature is lower than 22 °F (Poling et al., 2005). Therefore, severe cold damage was prevented for plants under rowcovers in high tunnels during these freeze events.

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Under protected culture, periods with temperatures lower than 32 °F lasted for 4.5 and 5 h on 19 and 20 Feb. at Greensboro and lasted for 4 h on 20 Feb. at Goldsboro. The freezing temperatures resulted in an average of 61.5% and 32.2% open blossom damage at Greensboro and Goldsboro, respectively (Table 5). Many of the damaged flowers were primary or secondary flowers that develop large and early maturing fruit and have been reported to contribute the most to marketable yield (Pollard et al., 1989). Using plastic-covered low tunnels inside high tunnels or adding supplemental heating has achieved success in frost protection and early season strawberry production in Utah (Maughan et al., 2015). These practices warrant future investigation to further early season production of strawberries in North Carolina.

Table 5.Percentages of frost damaged open blossoms of 10 organically managed strawberry cultivars grown under high tunnels in 2014–15 season at Greensboro and Goldsboro, NC.^z

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Fruit characteristics.

Fruit quality of the same cultivar differed remarkably during the season. Of all the evaluated strawberry cultivars, TSS declined as the season progressed to the peak harvest, and reached the lowest value in early April (Table 6). However, TSS recovered toward the end of the season. When averaging among cultivars, TSS was 10.1% in February, 5.7% in early April, and 10.1% in early May. These data support previous observations that TSS of strawberries is correlated with crop load and not determined by temperature during the production season (Agüero et al., 2015).

Table 6.Total soluble solids content of 10 organically managed strawberry cultivars grown under high tunnels in late February, March, April, and May in the 2014–15 season at Greensboro, NC.

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In early April, TSS content of all evaluated strawberry cultivars, except Sweet Charlie, was lower than 7%, the minimum requirement for U.S. no. 1 grade strawberry fruit (Mitcham, 2014). In May, ‘Sweet Charlie’ had the highest TSS, although it was relatively low in late February. Similar to ‘Sweet Charlie’, ‘Albion’ also exhibited high TSS late in the season but relatively low TSS early in the season. Among all evaluated strawberry cultivars, TSS of Strawberry Festival was above cultivar average in all harvest periods (11.4% in February, 6.8% in early April, and 10.8% in early May).

Continuous decreasing fruit weight was observed for all the evaluated strawberry cultivars at both locations (Table 7), which supports the statement that fruit weight can be a temperature-determined characteristic (Agüero et al., 2015). When temperatures rise, fruit become smaller (Chen, 2013). Fruit harvested late in the season are more likely developed from secondary or tertiary flowers in a flower cluster that may lead to smaller fruit size. With eight out of 10 cultivars, fruit weights in May were less than half of fruit weight in January. Of the evaluated strawberry cultivars, average fruit weight of Benicia was the greatest in January (46.2 g in Greensboro and 40.0 g in Goldsboro), significantly higher than day-neutral cultivars. However, fruit weight of ‘Benicia’ decreased dramatically in later months. In May, the average fruit weights of ‘Benicia’ were 8.6 and 9.9 g at Greensboro and Goldsboro, respectively. Throughout the season, ‘Chandler’ consistently had smaller fruit compared with other cultivars. In the peak season, the average fruit weights of ‘Chandler’ were 10.1 and 9.6 g at the Greensboro and Goldsboro sites, respectively, which are significantly lower than other cultivars except Strawberry Festival and Sweet Charlie. Although ‘Chandler’ is one of the most commonly grown, open-field cultivars in North Carolina, it may not be the best performing cultivar in high tunnels.

Table 7.Average fruit weight of 10 organically managed strawberry cultivars grown under high tunnels from November to May in the 2014–15 season at Greensboro and Goldsboro, NC.

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Plant health.

At Greensboro, younger leaves of ‘Benicia’ showed crinkling and stunned growth, accompanied by reddish lesions on leaves, petioles, and calyx during peak production season. The crinkling leaves became necrotic and dead and the whole plant declined in 2 weeks. These symptoms were only present on ‘Benicia’ at Greensboro, and about 80% of the plants were infected. Plant samples were analyzed by the North Carolina State University Plant Disease and Insect Clinic (PDIC) in Raleigh and no pathogens were found. Analysis of nutrient concentrations in leaves and petioles of strawberry cultivars indicated that ‘Benicia’ had the lowest magnesium concentration (data not shown). A magnesium deficiency may be part of the cause for the plants decline.

Two-spotted spider mites and crown and root rot (caused by Phytophthora cactorum) were the primary pest problems that limited plant growth and yield at Goldsboro (data not shown). ‘Florida Radiance’ is highly susceptible to crown and root rot (Whitaker et al., 2013), which caused stunted plant growth and plant decline. This disease was first identified on ‘Florida Radiance’ in Goldsboro in late March by PDIC, and then severely affected two of the three ‘Florida Radiance’ plots, which might partially explain the lower yield of ‘Florida Radiance’ at the Goldsboro site than that of the Greensboro site. The crown rot symptom was not noticeable on other cultivars at Goldsboro until the end of April, when leaves and petioles of Benicia, Camarosa, and Chandler started to wilt and became necrotic. ‘Camino Real’ seemed to have the least disease incidence (data not shown). This agrees with the result that Camino Real was the most resistant, whereas ‘Camarosa’ was the most susceptible cultivar to crown and root rot pathogens (Fang et al., 2012).

Two-spotted spider mites were found on all evaluated strawberry cultivars at the end of the season at Goldsboro. By evaluating two-spotted spider mite resistance using 85 strawberry cultivars, Shanks and Barritt (1975) found that there was no difference in mite populations on the cultivars although some cultivars showed fewer leaf symptoms. By evaluating development of two-spotted spider mites on leaf disks, Monteiro et al. (2014) noticed mortality of the two-spotted spider mite was higher on ‘Camarosa’, ‘Diamante’, and ‘Seascape’ than on other strawberry cultivars. Due to the highly skewed distribution of the population of two-spotted spider mites in the high tunnels, no significant differences in two-spotted spider mite damage were detected among cultivars in the present study (data not shown).

Interest in off-season and organically produced strawberries is increasing among local producers in North Carolina. In the present study, ‘Florida Radiance’, ‘Benicia’, ‘Winterstar’, and ‘Chandler’ showed high yield potentials. However, ‘Chandler’ produced small fruit, which can be a potential problem for fresh markets since it requires extra labor for harvesting small-sized berries. Day-neutral cultivars Albion and San Andreas and June-bearing cultivars Florida Radiance, Winterstar, and Camino Real are promising cultivars to target the early market. ‘Benicia’ produced large berries in the early season, which could be a highly attractive characteristic for early markets such as Thanksgiving, Christmas, and Valentine’s Day.

For future studies, increasing plant density of day-neutral cultivars might be an approach to increase total yield per square foot in high tunnels. Given the frost damage to open blossoms that occurred in the winter season, additional frost protection practices such as low tunnels warrant further investigation in North Carolina. Taking into consideration the variable year-to-year environmental conditions and the properties of high-tunnel and organic production systems, further research spanning multiple years is necessary to provide a thorough evaluation of strawberry cultivars.

Units

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