Low-Tunnel Strawberry Production: Comparison of Cultivars and Films

ABSTRACT

Growing repeat-fruiting strawberries (Fragaria ×ananassa Duchesne ex Rozier) under low tunnels can extend a strawberry harvest season from a few weeks to several months. Yields from multiple dayneutral cultivars have been compared under a single film, and yields from single cultivars have been compared under multiple films, but a study of possible cultivar and film interaction effects in low tunnels has not been reported. From 2014–2018, six cultivars were compared under low tunnels covered with a standard polyethylene film and three other films purported to improve growing conditions. Traits analyzed included total yield, percent rotted yield, berry weight, percent marketable yield, and average market score. ‘Monterey’ performed above average for all traits and better under TIII TES/IR 0.102 mm thick film (TES film) or Kool Lite Plus 0.152 mm thick polyethylene film (KL film). ‘Albion’ performed above average for all traits except yield and better under TES film or Clear TIII 0.102 mm thick polyethylene film (“standard clear” or SC film). ‘Portola’ performed above average for all traits except percent rotted yield, better under SC, KL, and TES films, and had lower percent rotted yield under TES film. Because TES film was helpful to the three better-performing cultivars evaluated, it was considered a good choice for growers to try in on-farm evaluations of available repeat-fruiting cultivars. Generally, yield, berry weight, market score, and percent marketable yield were correlated positively with each other.

KEYWORDS:

• Fragaria ×ananassa
• low tunnels
• dayneutral
• repeat fruiting
• marketable yield

Strawberries ranked as the fourth most available fresh-market fruit in the US (USDA-ERS, 2019a), with annual per capita consumption increasing from 2.1 kg in 2002 (USDA-ERS, 2016) to 3.2 kg in 2018 (Perez and Minor, 2019). Although most US production is on the West Coast, strawberries are grown in winter in Florida and in nearly every state (USDA-NASS, 2020) for local direct sale. Consumers purchase fresh strawberries all year long (USDA-ERS, 2019b), but, until recently, producing strawberries in the US for more than a month each year has been difficult outside California and Florida.

Growing strawberries in protected cultivation has been shown to increase the length of the local growing season in many countries. Significant season extension in the US and Canada has been achieved growing what the industry calls “dayneutral” strawberry cultivars in high tunnels (Gude et al., 2018; Rowley et al., 2011) or low tunnels (Anderson et al., 2019; Condori et al., 2017; Lewers et al., 2017; Orde et al., 2018; Petran et al., 2017; Pritts and McDermott, 2017; Van Sterthem et al., 2017). In New York, New Hampshire, and Maryland, the extra cost of establishing low tunnels was calculated to pay for itself in the first year from increased production from five California-bred cultivars (Lewers et al., 2017; Orde et al., 2018; Pritts and McDermott, 2017), and differences between cultivar yields were observed for cultivar and planting dates (Pritts and McDermott, 2017). In Maryland, growing strawberries in low-tunnels allowed harvesting up to nine months with less diseased fruit (Condori et al., 2017; Lewers et al., 2017), compared with a typical season of a few weeks using what the industry calls “short-day” cultivars in open fields. This growing season extension was partly a result of the capability of the film to maintain higher temperatures within the beds in cold months. Strawberry yield in low tunnels increased as temperature increased until an optimum temperature was reached. Production decreased at temperatures higher than the optimum, which varied by cultivar. The primary way to increase temperature in cold months was to lower the sides of the plastic film that covered the low tunnels to the ground to trap radiant heat generated as sunlight passed through the plastic film over the low tunnels. In warmer months, the sides of the film covering the low tunnels were raised to allow ambient air to pass through so that the air within the low tunnels was not substantially warmer than air in the open field. Although raising and lowering of the sides of the low-tunnel film was thought to be cumbersome in the New York study (Pritts and McDermott, 2017), this practice was found unnecessary in the Maryland study (Lewers et al., 2017). In Minnesota, yields from six dayneutral cultivars grown under low tunnels, covered with a single type of film, exceeded that of typical “June-bearing” cultivars, and significant yield differences between cultivars were observed in three of four testing environments (Petran et al., 2017).

There are several types of plastic film available that manufacturers market as being beneficial to crop production by modifying the light that passes through them and either warming or cooling the plants beneath them (Baeza and López, 2012). ‘Albion’ yields were similar between open beds and beds with low tunnels covered with two experimental plastics either blocking or transmitting ultraviolet light (Anderson et al., 2019). ‘Albion’ grown under the UV-transmitting plastic produced higher marketable yield than the UV-blocking in one of two years and higher marketable yield than ‘Albion’ grown without low-tunnels. Similar marketable yields were produced by ‘Seascape’ grown under clear and thermal films in Quebec, Canada, and marketable yields in low tunnels with either film were 1.7 to 3 times greater than from ‘Seascape’ grown without low tunnels (Van Sterthem et al., 2017). In New Hampshire, no yield differences were reported between cultivars grown under low tunnels covered with different thicknesses of the same type of plastic, though differences were reported for ease of physically manipulating films of different thicknesses (Orde et al., 2018).

Further research with multiple dayneutral cultivars is needed to evaluate different film materials for low tunnels. The objective of this study was to compare readily available repeat-fruiting strawberry cultivars and a few film types with widely differing light transmission qualities during each month from which a significant crop could be harvested from under low tunnels in Mid-Atlantic conditions. Within that objective was the goal of understanding if some cultivars would perform better under some films or if some films would be more beneficial during some months.

Materials and Methods

Field Preparation

The experiment fields were established on the North Farm of the USDA-ARS Henry A. Wallace Beltsville Agricultural Research Center at Beltsville, MD (39°01ʹ48.42”N, 76°56ʹ07.99”W, 49.4 m elevation), on Downer-Hammonton complex loamy sand, and Russet-Christiana complex fine sandy loam soils (USDA-NRCS, 2020), supplemented each year with potassium, sulfur, boron, and calcitic lime to correct deficiencies reported by annual soil tests. Daylengths ranged from 9 h, 25 min to 14 h, 54 min. Field preparation was as described by Lewers et al. (2017). Briefly, soil amendments were applied in late winter. The rape cover crop (Brassica napus L. var. napus cv. Dwarf Essex) was tilled under using a chisel plow as soon as the ground was thawed, and followed by a tandem offset disc. The fields were not fumigated prior to making beds.

Experiment Structure

The “main plots” were the individual beds to which film treatments were applied. The six beds were divided into two groups of three beds each, and three films were randomly assigned to the beds in each group. Each bed was divided into two “sub-plots”, or “half beds”, into which a “complete block” of all cultivars was planted. Six-plant plots of each cultivar were planted as the smallest experimental unit.

Treatments

Treatments included cultivars, films over the low tunnels, and months where “month” denotes a repeated monthly measurement observed (total for yield, average for other traits) on each six-plant plot. Four tests were established, one test in each of four years (2014, 2015, 2016, and 2017). All four tests compared five repeat-fruiting strawberry cultivars developed in California: Albion (PP16,228), Monterey (PP19,767), Portola (PP20,552), San Andreas (PP19,975), and Seascape (PP7,614). The cultivar Sweet Ann (PP22,472) was included in the first three tests. All four tests compared Clear TIII 0.102 mm thick polyethylene film (“standard clear” or SC film) and TIII TES/IR 0.102 mm thick polyethylene film (TES film) (Berry Plastic Corporation, Greenville, SC). The first two tests (2014 and 2015) also included TempCool™ 0.102 mm thick polyethylene film (TC film) (Berry Plastic Corporation, Greenville, SC). The second two tests (2016 and 2017) also included Kool Lite Plus 0.152 mm thick polyethylene film (KL film) (Klerks Hyplast Inc., Chester, SC).

Establishment

In March of each test year, eight raised beds with trickle irrigation were covered with white-on-black embossed plastic mulch, 0.032 mm thick (Rain-Flo Irrigation, East Earl, PA). Beds were 15.24 cm high at the sides, and 1.52 m wide with a 7.62 cm crown. Two lines of 0.381 mm thick drip tape with 30-cm in-line spacing, 30 cm apart were laid 7 cm below the plastic mulch using a 2600 Raised Bed Plastic Mulch Layer (Rainflow Irrigation, East Earl, PA). Six beds were for the experiment, and the two outer beds, planted with ‘Albion’ and ‘Monterey’ served as buffers. A border plot of ‘Albion’ was planted at each end of each bed in the six experiment beds to serve as a buffer. Low tunnels were erected over the beds after planting as described by Lewers et al. (2017). Briefly, stainless steel rods, 5 mm in diameter × 366 cm long, spaced every 122 cm, provided hoops as vertical support for film the length of each bed. The film was secured to a T-post at each end and also to each hoop. The height of the hoops was 61 cm above the bed centers. An individual film stayed on the low tunnels from their assembly in March or April until their disassembly in June the year after assembly, for a total of 15–16 months.

Measuring Total Transmittance of Films

Samples of all four types of film were acquired from new rolls on the same day and taken to the lab for measurement. Total transmittance of the plastic sheeting was measured with a 50-mm integrating sphere accessory (RSA-PE-20, Labsphere Inc. North Sutton, NH) for a spectrometer (PerkinElmer Lambda 40) across the 190 to 1100 nm wavelength range at 1 nm intervals. Multiple locations on each sample piece of the film were measured, and transmittance factors were calculated (Daughtry et al., 1989).

Plant Care

Planting and plant care was as described by Lewers et al. (2017) with some modifications. Briefly, dormant bare-root plants of the cultivars (Lassen Canyon Nursery, Redding, CA) were hand-planted to the six-plant plots in pre-moistened soil in late March or early April. Two planting rows in each bed were 30.5 cm apart. Planting holes within row were 38.1 cm apart and staggered relative to the planting holes in the adjacent row. There was a 76.2 cm linear gap between plants in adjacent plots. After planting, the film was pushed up and secured until fall. For about six weeks after planting, flowers and stolons were removed, and the field was fertigated weekly with nitrogen at 5.6 kg∙ha^–1. At six weeks, flower removal ceased and the nitrogen fertigation rate was reduced to 3.4 kg∙ha^–1. No fungicides were used to control any disease. In fall, the film was lowered on both sides of each tunnel to protect plants from increasing cold weather. Flowers and fruit were protected from frost using water from impact sprinklers over the top of the low tunnels so that the plants, flowers, and fruit stayed dry. The water was applied through an automated system. Six individual microclimate monitoring stations were installed, one per bed or main plot, and measured air temperatures at 10 cm above the bed (type K thermocouples, Omega Engineering, Inc., Stamford, Conn.). All sensors were wired to a data logger and multiplexors (CR1000-ST; AM16/32B-St, Campbell Scientific, Logan, Utah) for 15-min increments of data acquisition as in Condori et al. (2017). The system checked the temperature inside all the low tunnels every 15 min. If the temperature was below 0.6°C in any low tunnel, the water was turned on for 5 min. Harvesting for a test ceased when the weather was expected to be cold enough to damage the plumbing. When frost protection was no longer possible, the water was turned off, and the sides of the low tunnels were lifted to their original position, allowing the plants to enter a dormant state through the winter. In February, the sides of the low tunnels were again lowered, and water was turned on again for irrigation, fertigation, and frost protection. In April, the sides were raised again, and the frost protection continued as before, keeping the film raised. Even though the water never touched the flowers, this system protected them from frost damage in April and May the year after planting.

Harvest Data Collection

The 2014 test was harvested eight months total: June through November 2014, then stopped for winter and harvested again May 2015, and June 2015. The 2015 test was harvested nine months: July through December 2015, May 2016, and June 2016. The 2016 test was harvested 9 months: June through November 2016, then April through June 2017. The 2017 test was harvested 9 months: June through December 2017, May 2018, and June 2018.

Harvest data were collected as described in Lewers et al. (2017). Briefly, individual ripe fruits were hand-harvested from each six-plant plot twice weekly, or once weekly in fall (from some time each September until late December). Fruits showing no rot were harvested into one bucket for the plot, and fruits showing any kind of rot were placed into a separate bucket for that plot. As the buckets of fruit were weighed, all the non-rotted fruit were counted to calculate average berry weight for that plot and harvest.

Non-rotted fruit also were given a subjective “market score” from 2.0 (worst) to 9.0 (best). Size, shape, uniformity, glossiness, and coloration (bronzing, bruising, white “shoulders”) were key features in determining the market score. A market score of 7.0 was the lowest score considered acceptable by consumers for fresh consumption and 6.5 was the lowest score acceptable for processing. Faults and diseases were identified and noted for each plot at every harvest. Data and notes were collected at each harvest in Microsoft Office Excel software (Microsoft Corporation, Redmond, WA, USA).

Additional Calculations

Additional columns were inserted into the Excel data-collection sheets to calculate meaningful estimates based on the raw data so that trends could easily be monitored through the seasons. Calculations were made to estimate, for each plot’s harvest, yield per plant (total, non-rotted, and rotted) and average non-rotted fruit size. If the market score for the plot was over 6.5, the non-rotted yield was considered to contribute to the estimate of marketable yield. The monthly sums across all harvests of rotted fruit, non-rotted fruit, marketable fruit, and total fruit harvested from each plot were obtained using Excel’s Pivot Table function. From these, percent marketable yield and percent rotted yield also were calculated for each month.

Statistical Analyses

These plot sums for total yield (g/plant), and the plot averages for percent marketable yield, berry weight, market score, and percent rotted yield were used in analyses of variance and correlation determinations using SAS software, version 9.4, 2016 (SAS Institute, Inc., Cary, NC, USA). Pearson correlations among the five primary traits of interest were calculated using observed cultivar plot averages, standardized to zero mean and unit standard deviation for each year and trait. Separately for each year and trait, a film-by-cultivar-by-month split-split-plot analysis of variance model was fit, with film replications nested in film, and cultivar replications nested in film-by-film-replication-by-cultivar specified, respectively, as the film-plot- and cultivar-plot-error terms (i.e., random effects). Distribution properties of each trait determined the distribution specified for each ANOVA model: a general linear (i.e., normal distribution) model for market score and berry weight; a generalized linear (i.e., negative binomial with log link) model for yield; and a generalized linear [i.e., negative binomial with log link and log_e (yield) offset] model for percent marketable yield and percent rotted yield. In some years, and for some traits, when the model produced zero estimates for the cultivar replication error variance components, the cultivar replication random effect was removed from the model, effectively changing it to a split-plot model, basing the film-by-cultivar effect F-test on the model’s residual error. Similarly, for percent marketable yield in all four years, the magnitude of both the film replication and cultivar replication variance components (i.e., random effects) was negligible, indicating F-tests for all model effects were based on the model’s residual error. All models were fit using SAS v9.4 PROC MIXED or PROC GLIMMIX; modeling correlations among months (repeated measurements) using a compound symmetric covariance structure, with homogeneous or heterogeneous within-month variances, as determined by AICC-fit statistic for each trait. Sidak-adjusted means comparisons were generated using the pdmix800.sas macro (Saxton, 1998). April data were sparse and observed only for some cultivars in 2015 and 2016. To obtain estimates of means (except film-by-cultivar-by-month) the April data had to be omitted.

When interaction effects were significant, interaction means were evaluated to identify trends. If the interaction effect means were contradictory rather than a matter of degree, the analyses of variance were not used as a primary source to understand the main effects of cultivar, film, or cultivar-by-film means. Instead, above-average interaction means were given a value of “1” and summed across traits and tests to identify cultivars, films, or cultivar-by-film combinations that performed above average for multiple traits and test years. Similarly, Lewers et al. (2012b) used above-average performance to make dispositions of breeding selections evaluated for postharvest rot when compared with other selections and cultivars in the same harvest season.

Results and Discussion

Plastic Transmittance

TC and KL films transmitted 10–15% less photosynthetically active radiation (PAR, 400–700 nm) and near-infrared radiation (NIR, 700–1100 nm) than the SC and TES films (Figure 1). TC and KL films also transmitted less ultraviolet radiation (UV, 200–400 nm) than SC and TES films. At the North Farm, incoming solar radiation was approximately 4% ultraviolet (<400 nm), 45% visible (400–700 nm), and 51% infrared (<700 nm).

Figure 1. Percentage transmittance of radiation through clear plastic films used to cover low tunnels for strawberry production. Transmittance through the plastic film was measured with an integrating sphere coupled with a single-mode fiber optic probe to a spectroradiometer across the 350 to 2500 nm wavelength range at 1 nm intervals. Films used were TempCool™ 0.102 mm thick film (TC film) (Berry Plastic Corporation, Greenville, SC), Clear TIII 0.102 mm thick film (“standard clear” or SC film), TIII TES/IR 0.102 mm thick film (TES film) (Berry Plastic Corporation, Greenville, SC), and Kool Lite Plus 0.152 mm thick Poly film (KL film) (Klerks Hyplast Inc., Chester, SC)

Total Yield

For all four tests, the sources of variance that were significant for total yield per month were cultivar, month, and cultivar-by-month. Film as a main effect was not a significant source of variance for yield. This of particular interest, because film is one of the two sources of variance that a grower can make a decision about, the other being cultivar choice. The film-by-cultivar interaction effect was significant for the 2014 and 2015 planting year tests comparing TC film with SC and TES films. The film-by-month interaction effect was significant for all tests except for the 2016 planting year comparison of KL film with SC and TES films.

As mentioned, besides film, cultivar is the other source of variance about which a grower can make a choice. Cultivar yield ranks varied from test to test (Table 1). ‘Portola’ was the only cultivar in the highest yielding group in all four tests. ‘Sweet Ann’ had the highest yield in the 2015 test, but the lowest yield in the 2014 and 2016 tests and was not included in the 2017 test. ‘Albion’ was among the lowest yielding cultivars in three of the four tests.

Table 1. Strawberry cultivars compared using analyses of variance and simple average performance relevant to overall cultivar mean performance. Statistically significant differences between cultivar trait means within a year are designated by lower-case letters. Above-average cultivar trait means within and across years are designated with bold text and a grayed field. An index for cultivar performance across all traits used average performance across years to identify “above average” cultivars. Above-average means were given a value of “1” and summed across traits and tests to identify cultivars that performed above average for multiple traits and test years

Index for all traits	Yld	PR	BW	PM	MS		Total
Albion		1	1	1	1		4
Monterey	1	1	1	1	1		5
Portola	1		1	1	1		4
San Andreas			1	1			2
Seascape		1					1
Sweet Ann			1				1
Average							3
Yield (g/plant) (Yld)	2014		2015		2016		2017		Ave.
Albion	35	bc	101	c	66	b	64	b	67
Monterey	49	a	114	b	69	b	73	ab	76
Portola	39	ab	126	ab	119	a	82	a	91
San Andreas	39	ab	119	b	66	b	70	b	74
Seascape	42	ab	123	ab	70	b	64	b	75
Sweet Ann	29	c	134	a	57	c			73
Average	39		119		75		71		76
Percent rotted yield (PR)	2014		2015		2016		2017		Ave.
Albion	0.02	d	0.02	b	0.02	a	0.02	b	0.02
Monterey	0.05	a	0.01	c	0.01	b	0.02	b	0.02
Portola	0.03	bc	0.03	a	0.02	a	0.03	a	0.03
San Andreas	0.05	a	0.02	b	0.02	a	0.04	a	0.03
Seascape	0.02	cd	0.02	b	0.01	c	0.01	c	0.01
Sweet Ann	0.04	ab	0.03	a	0.02	a			0.03
Average	0.03		0.02		0.02		0.03		0.02
Berry weight (g) (BW)	2014		2015		2016		2017		Ave.
Albion	10	b	13	a	12	a	13	a	12
Monterey	9	c	12	b	10	bc	13	b	11
Portola	8	c	13	a	11	b	12	bc	11
San Andreas	11	ab	12	b	10	c	12	c	11
Seascape	7	d	9	c	7	d	8	d	8
Sweet Ann	11	a	12	b	11	b			11
Average	9		12		10		12		11
Percent marketable yield (PM)	2014		2015		2016		2017		Ave.
Albion	0.26	ab	0.50	ab	0.43	a	0.54	a	0.43
Monterey	0.31	a	0.40	bc	0.30	ab	0.26	b	0.32
Portola	0.23	ab	0.59	a	0.36	a	0.25	b	0.36
San Andreas	0.33	a	0.45	b	0.23	b	0.19	c	0.30
Seascape	0.15	c	0.34	c	0.03	d	0.09	d	0.15
Sweet Ann	0.19	bc	0.40	bc	0.07	c			0.22
Average	0.24		0.45		0.24		0.27		0.30
Market score (MS)	2014		2015		2016		2017		Ave.
Albion	5.8	abc	7.6	a	5.9	b	7.1	a	6.6
Monterey	6.0	a	7.4	b	5.8	b	6.9	b	6.5
Portola	5.9	ab	7.4	b	6.4	a	6.7	c	6.6
San Andreas	5.6	cd	7.3	b	5.4	c	6.5	d	6.2
Seascape	5.6	bcd	6.7	d	5.4	c	6.0	e	5.9
Sweet Ann	5.3	d	6.9	c	5.3	c			5.9
Average	5.7		7.2		5.7		6.7		6.3

The month in which the greatest yield was harvested, by far, was the May after the year of planting, for all four tests. The second highest was either August or September (Figure 2), depending on the test. The months with the lowest yields were June of the planting year, while the plants were getting established, and November or December of the planting year, as winter approached. The yield from June the year of planting was relatively small, but when combined with the yield from June the year following planting, was similar to the yield from October. With plantings from both the current year and the previous year, a grower could expect a modest but substantial yield in June.

Figure 2a. Monthly yield, percent rotted yield, berry weight, percent marketable yield, and market score from strawberries grown under low tunnels. Work was done in five years through four test plantings (2014, 2015, 2016, 2017). Each test was harvested from June or July of the year of planting through November or December, and then again the following April or May through June

short-legendFigure 2b.

The significant cultivar-by-month interaction effect was most obvious in varying cultivar rankings from month to month, and these were not consistent from test to test. In months of low yield, when the plants were less vigorous, separation of cultivar means was less evident and sometimes nonexistent. In practical use, yield variations from month to month are less important than a comparison between production in the year of planting, and production in the year after planting. A grower needs to decide which production pattern is easier to market. Higher yield in the year of planting could result in a faster return on investment. Yields from ‘Albion’ and ‘Monterey’ were greater the year of planting than the year after planting (Figure 3). Yields for ‘Portola’ were generally greater the year of planting, except for the 2015 planting. Yields from ‘San Andreas’, ‘Seascape’, and ‘Sweet Ann’ were greater the year after planting. These latter cultivars were sometimes fruited heavily a month before the usual strawberry season and may be considered valuable for an early market.

Figure 3. Yield from six strawberry cultivars grown under low tunnels. Work was done in five years through four test plantings (2014, 2015, 2016, 2017). Each test was harvested from June or July of the year of planting through November or December, and then again the following April or May through June. Cultivar yields were averaged across all four tests for each month, then monthly harvests were totaled for the months in the year of planting, and totaled separately for the months following the year of planting. Film yields were averaged within test then totaled. Films tested were TempCool™ 0.102 mm thick film (TC film) (Berry Plastic Corporation, Greenville, SC), Clear TIII 0.102 mm thick film (“standard clear” or SC film), TIII TES/IR 0.102 mm thick film (TES film) (Berry Plastic Corporation, Greenville, SC), and Kool Lite Plus 0.152 mm thick Poly film (KL film) (Klerks Hyplast Inc., Chester, SC)

Since there were no significant differences for film as a main effect, it is interesting to determine if the film-by-cultivar or film-by-month interaction effects are significant. In other words, did any cultivar seem to perform significantly better or worse under a particular film? Or did any film seem to provide a significant yield advantage in any month? Lack of consistency could indicate that the weather is more important to film effects than film effects per se.

Only the data from the 2014 and 2015 plantings comparing TC film with SC and TES films showed significant cultivar-by-film interaction effects, and the effects were not consistent (Table 2). Significant film-by-month interaction effects were observed from the 2014, 2015, and 2017 plantings (Figure 4). No film provided a statistically significant yield advantage in any month in 2016. In the 2014 planting, higher yields were produced under SC film from June through November of the year of planting, and this advantage was significant August through November. In the 2015 planting, higher yields were produced under TES film from November the year of planting through June the year after planting, and the advantage over TC film was significant in December and May. Yields produced under SC film were similar to those produced under TES. Yields produced under TC film also were similar to those under the other two films in July and August, but then were lower in all the other months. So, in the two tests comparing TC film with TES and SC films, yields under TC film were generally lower, and yields from SC film and TES films were generally similar, except for yields under SC film being significantly higher August through October in 2014. In the 2017 planting, the highest yield was produced under KL film throughout the year of planting, and the higher yield was statistically significant in August. A similar pattern was observed in 2016, but, again, differences were not significant. So, in the two tests comparing KL film with SC and TES films, the highest yields were produced under KL film, and the advantage was statistically significant in the hot month of August 2017. Comparing SC and TES films all four tests, yields were similar in the year of planting with no statistical differences except in the 2014 test, when yields under SC film were significantly higher from June through November. In the year after planting, the highest yields were produced under TES film, but the yields were not significantly higher than under SC film. Under TES film the greatest yield was consistently produced the year after planting, compared with the year of planting (Figure 3).

Table 2. Performance of repeat-fruiting strawberry cultivars when grown in low tunnels covered with different films. Statistically significant differences between film means of cultivar performance under each film, by trait each year, are designated by lower-case letters. Above-average trait means of specific cultivars under specific films, within and across years, are designated with bold text and a grayed field. An index of performance of each cultivar under each film, across all traits, used counts of above-average performance. Above-average cultivar-by-film interaction means were given a value of “1” and summed across traits and tests to identify cultivar-by-film combinations that performed above average for multiple traits and test years

2014, 2015	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Index
TC^a	3		3		1		2		2		5
SC	6		5		6		8		5		4
TES	8		7		5		4		6		6
2016, 2017
KL	4		6		6		5		6
SC	3		3		4		3		2
TES	6		6		5		5		8
2014	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Yield (g/plant)
TC	37	a	38	b	28	b	27	b	32	b	28	a
SC	35	a	69	a	57	a	54	a	65	a	26	a
TES	33	a	44	ab	37	ab	40	ab	35	b	34	a
2015
TC	76	b	91	b	125	a	103	a	126	a	116	a
SC	126	a	119	ab	119	a	135	a	108	a	141	a
TES	107	ab	138	a	135	a	122	a	136	a	145	a
2016
KL	64	a	62	a	106	b	63	a	61	a	54	a
SC	66	a	68	a	122	ab	66	a	72	a	58	a
TES	70	a	77	a	129	a	69	a	79	a	58	a
2017
KL	74	a	86	a	84	a	78	a	72	a
SC	52	a	61	a	78	a	65	a	55	a
TES	67	a	75	a	84	a	68	a	65	a
2014	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Percent rotted yield
TC	2	a	5	a	3	a	2	b	2	a	2	b
SC	2	a	5	a	4	a	7	a	3	a	8	a
TES	1	a	4	a	2	a	7	a	1	a	3	ab
2015
TC	2	a	1	a	3	a	2	a	2	a	4	a
SC	2	a	2	a	4	a	2	a	1	a	3	a
TES	1	a	1	a	3	a	2	a	2	a	3	a
2016
KL	2	a	1	ab	2	a	2	a	1	a	2	ab
SC	2	a	1	b	2	a	3	a	1	a	3	a
TES	3	a	2	a	2	a	2	a	1	a	1	b
2017
KL	2	ab	3	a	4	a	5	a	1	ab
SC	3	a	2	ab	5	a	3	b	2	a
TES	2	b	2	b	2	b	3	b	1	b
2014	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Berry weight (g)
TC	11	a	9	a	8	a	10	a	7	a	12	a
SC	10	a	9	a	9	a	11	a	8	a	11	a
TES	10	a	8	a	8	a	10	a	7	a	11	a
2015
TC	13	a	12	ab	13	a	12	a	9	a	13	a
SC	14	a	12	b	14	a	13	a	9	a	12	a
TES	14	a	13	a	13	a	12	a	9	a	12	a
2016
KL	12	a	10	a	11	a	10	a	7	a	10	a
SC	12	a	11	a	11	a	10	a	7	a	11	a
TES	12	a	10	a	11	a	10	a	7	a	11	a
2017
KL	14	a	13	a	13	a	12	a	9	a
SC	12	a	12	a	11	b	12	a	8	a
TES	14	a	13	a	12	ab	12	a	9	a
2014	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Percent marketable yield
TC	29	a	35	a	21	a	26	b	12	b	25	a
SC	21	a	34	a	28	a	56	a	32	a	13	a
TES	30	a	25	a	21	a	24	b	10	b	19	a
2015
TC	37	b	34	a	58	a	32	b	38	a	43	a
SC	63	a	38	a	59	a	60	a	22	b	37	a
TES	54	ab	48	a	59	a	49	ab	48	a	39	a
2016
KL	38	a	28	a	55	a	14	b	4	a	7	a
SC	55	a	23	a	19	b	35	a	2	b	7	a
TES	37	a	41	a	47	a	25	ab	4	a	8	a
2017
KL	69	a	29	a	26	a	17	a	13	a
SC	46	a	23	a	29	a	20	a	5	b
TES	51	a	28	a	21	a	20	a	13	a
2014	Albion		Monterey		Portola		San Andreas		Seascape		Sweet Ann
Market score
TC	5.7	a	5.7	b	5.4	b	5.1	b	5.3	b	5.2	a
SC	5.9	a	6.3	a	6.4	a	6.3	a	5.9	a	5.1	a
TES	5.9	a	6.1	ab	5.9	ab	5.4	b	5.7	ab	5.7	a
TC	7.3	b	7.2	b	7.4	a	7.2	b	6.6	a	6.9	a
2015
SC	7.8	a	7.3	b	7.4	a	7.5	a	6.6	a	6.9	a
TES	7.7	a	7.6	a	7.5	a	7.4	ab	6.8	a	7.0	a
KL	5.9	a	6.0	a	6.3	a	5.6	a	5.6	a	5.3	a
2016
SC	5.9	a	5.7	a	6.4	a	5.4	a	5.2	a	5.3	a
TES	6.0	a	5.8	a	6.3	a	5.4	a	5.4	a	5.4	a
KL	7.1	ab	7.1	a	6.8	a	6.6	a	5.9	a
2017
SC	6.9	b	6.7	b	6.7	a	6.5	a	6.0	a
TES	7.4	a	6.9	ab	6.7	a	6.5	a	6.1	a

^aTempCool™ 0.102 mm thick film (TC film) (Berry Plastic Corporation, Greenville, SC); Clear TIII 0.102 mm thick film (“standard clear” or SC film); TIII TES/IR 0.102 mm thick film (TES film) (Berry Plastic Corporation, Greenville, SC); Kool Lite Plus 0.152 mm thick Poly film (KL film) (Klerks Hyplast Inc., Chester, SC)

Figure 4. Total yield per month of repeat-fruiting strawberries when grown in low tunnels covered with different films. Statistically significant differences between film means each month, are designated by lower-case letters for each trait. Four tests were established, one test in each of four years (2014, 2015, 2016, and 2017). All four tests compared Clear TIII 0.102 mm thick polyethylene film (“standard clear” or SC film) and TIII TES/IR 0.102 mm thick polyethylene film (TES film) (Berry Plastic Corporation, Greenville, SC). The first two tests (2014 and 2015) also included TempCool™ 0.102 mm thick polyethylene film (TC film) (Berry Plastic Corporation, Greenville, SC). The second two tests (2016 and 2017) also included Kool Lite Plus 0.152 mm thick polyethylene film (KL film) (Klerks Hyplast Inc., Chester, SC)

Percent Rotted Berries

Percentage rotted yield for a plot ranged from 3% to 26% for planting year 2014, from 2% to 18% for planting year 2015, from 6% to 32% for planting year 2016, and from 10% to 45% for planting year 2017. For all four tests, the sources of variance that were significant for percent rotted berries by weight were cultivar, month, and cultivar-by-month. Film as a main effect was not a significant source of variance. Percent rotted yield was not correlated with total yield except for 2017 when there was a positive correlation of 0.44 (P < .0001) (Table 3).

Table 3. Pearson’s pairwise correlation coefficients, calculated from plot averages, for yield (YLD), percent marketable yield (PM), percent rotted yield (PR), berry weight (BW), and market score (MS) of repeat-fruiting strawberries grown under low-tunnels. The number (n) of (x,y) data points used to calculate each correlation estimate, and the probability of the calculated estimate is not zero (test of null hypothesis)

	Pairwise Pearson correlation					Probability of correlation equal to zero.					n
	YLD	PM	PR	BW	MS	YLD	PM	PR	BW	MS	YLD	PM	PR	BW	MS
2014
YLD	1	0.35	−0.05	0.18	0.26	–	≤0.001	0.2066	≤0.001	≤0.001	566	566	566	562	563
PM	0.35	1	−0.26	0.61	0.41	≤0.001	–	≤0.001	≤0.001	≤0.001	566	566	566	562	563
PR	−0.05	−0.26	1	−0.10	−0.23	0.2066	≤0.001	–	0.0168	≤0.001	566	566	566	562	563
BW	0.18	0.61	−0.10	1	0.25	≤0.001	≤0.001	0.0168	–	≤0.001	562	562	562	562	562
MS	0.26	0.41	−0.23	0.25	1	≤0.001	≤0.001	≤0.001	≤0.001	–	563	563	563	562	563
2015
YLD	1	0.15	0.03	0.14	0.10	–	0.0001	0.4364	0.0004	0.0093	625	625	625	613	624
PM	0.15	1	−0.32	0.38	0.71	0.0001	–	≤0.001	≤0.001	≤0.001	625	625	625	613	624
PR	0.03	−0.32	1	0.23	0.03	0.4364	≤0.001	–	≤0.001	0.4137	625	625	625	613	624
BW	0.14	0.38	0.23	1	0.68	0.0004	≤0.001	≤0.001	–	≤0.001	613	613	613	613	613
MS	0.10	0.71	0.03	0.68	1	0.0093	≤0.001	0.4137	≤0.001	–	624	624	624	613	624
2016
YLD	1	0.19	0.01	0.04	0.36	–	≤0.001	0.7225	0.3741	≤0.001	618	618	618	611	614
PM	0.19	1	−0.35	0.55	0.22	≤0.001	–	≤0.001	≤0.001	≤0.001	618	618	618	611	614
PR	0.01	−0.35	1	−0.14	0.16	0.7225	≤0.001	–	0.0006	≤0.001	618	618	618	611	614
BW	0.04	0.55	−0.14	1	0.13	0.3741	≤0.001	0.0006	–	0.0012	611	611	611	611	611
MS	0.36	0.22	0.16	0.13	1	≤0.001	≤0.001	≤0.001	0.0012	–	614	614	614	611	614
2017
YLD	1	0.12	0.44	0.32	0.29	–	0.0052	≤0.001	≤0.001	≤0.001	529	529	529	527	527
PM	0.12	1	−0.22	0.50	0.69	0.0052	–	≤0.001	≤0.001	≤0.001	529	529	529	527	527
PR	0.44	−0.22	1	0.28	0.01	≤0.001	≤0.001	–	≤0.001	0.7634	529	529	529	527	527
BW	0.32	0.50	0.28	1	0.50	≤0.001	≤0.001	≤0.001	–	≤0.001	527	527	527	527	527
MS	0.29	0.69	0.01	0.50	1	≤0.001	≤0.001	0.7634	≤0.001	–	527	527	527	527	527

Although significant differences in percent rotted yield were observed between cultivars in each test, the lowest and highest ranking cultivar differed each year (Table 1). ‘Portola’, ‘San Andreas’, and ‘Sweet Ann’ were among the cultivars with the highest percent rot in three of the four tests, and ‘Seascape’ was among cultivars with the lowest percent rot in three of the four tests.

If there were greater rot in some months than others, fungicide applications could be planned to maximize efficiency. Percent rot started at a low level each planting and increased in subsequent months. Although there were significant differences in percent rot from month to month, there was no month that consistently had the greatest rot in all four tests. Percent rot peaked in September in the 2014 and 2016 tests, December in 2015, and October in 2017 (Figure 2). In the year after planting, percent rotted yield increased from May to June in 2014 and 2015, and decreased from May to June in 2016 and 2017. Depending on the disease, it may be useful to start a preventative fungicide treatment starting as early as August and continue it again the following April.

However, the most prevalent berry disease noted each year until November was mucor fruit rot (Mucor spp.), as was observed previously (Lewers et al., 2017). Mucor was easily controlled by watering only immediately after harvest so that all berries being watered were immature and very firm. Since mucor was the primary problem until November, fungicide sprays for botrytis fruit rot (Botrytis cinereal Pers.) could possibly be delayed until October. Generally, percent rot was low under the low tunnels, compared with open beds (Lewers et al., 2017), and could be managed to a significant extent with cultural practices.

In November 2014 and 2015, “GG-40” Gro-Guard UV® 1.2 oz row covers (Atmore Industries, Atmore, AL) were used with impact sprinklers for frost protection. Botrytis fruit rot developed in November and was present again the following May and June. Previously, botrytis fruit rot had not been common in low tunnels at this location (Lewers et al., 2017). Row covers were not used in 2016 and 2017, and botrytis fruit rot did not develop as a frequent cause of fruit rot until the year after planting. Row covers combined with water for fall frost control was detrimental and not recommended in future. Percent rot was consistently greater the year after planting than the year of planting and was primarily due to botrytis. Fungicide treatment in fall the year of planting could reduce botrytis the year after planting but could be followed by additional fungicide applications in April. Given the potential length of the fruiting period for a low-tunnel production system, any recommendations for fungicide applications should consider the cost of the fungicide(s) and effect on developing pathogen resistance to fungicides.

Cultivar-by-month interaction effects for percent rotted yield were significant in all four tests, yet there was little consistency of trends. The only month providing significant differences between cultivars from the 2014 planting was July, when ‘Sweet Ann’ and ‘Monterey’ developed mucor and significantly higher percent rotted yield. The 2015 planting showed significant differences between cultivars in July (‘Portola’ developed significant mucor), October, and November (with ‘Sweet Ann’ having the most mucor and botrytis, and ‘Monterey’ having the least) the year of planting, and both months the year after planting (with ‘Sweet Ann’ having the most botrytis and mucor, and ‘Seascape’ having the least). The 2016 planting showed significant cultivar differences September through November (with ‘Seascape’ and ‘Sweet Ann’ having the least mucor) and again in June (with ‘Sweet Ann’ having the most botrytis). The 2017 planting showed significant differences between cultivars only in November and December (mucor).

Film-by-month interaction effects for percent rot were significant for all tests but 2015. Most months when differences were observed between films were in the year of planting, when mucor was prevalent. In the 2014 planting, the SC film allowed the highest percent rotted yield in July (3%), September (23%), and November (5%). The lowest percent rotted yield was produced under TC film in July (1%) and September (8%), but under TES film in November (1%). In the 2016 planting, the TES film allowed the highest percent rot in September (14%) and November (1%), and the following June (2%). In the 2017 planting, the lowest percent rotted yield was produced under the TES film in August (1%), November (3%), and December (0%). Significant cultivar-by-film interaction effects for percent rot were present for all but the 2015 test, and there was no consistency observed through analyses of variance (Table 2).

Berry Weight

The sources of variance that were significant for average berry weight in all four tests were cultivar, month, and cultivar-by-month. Film as a main effect was a significant source of variance only in the 2017 test, and film interaction effects were not significant for berry weight in any of the tests.

The cultivar with the largest berries across all months three out of four tests was ‘Albion’, and ‘Seascape’ berries were smallest in all four tests (Table 1). Separation of cultivar means was possible most months in all tests. Most cultivars produced fruit that, from month to month, was similar in weight relative to that of the other cultivars, but occasionally increased or decreased in comparison, resulting in significant cultivar-by-month interaction effects.

Across cultivars, berry weight was generally greater in July in the year of planting than in the following months, reaching a low in either August or September, and increasing again in the cooler months before winter (Figure 2). The lowest weight berries were harvested in August and September, the months with the highest yield in the year of planting. High-weight berries also were harvested in spring the following year, when total yield also was highest. Over the life of each planting or test, berry weight was positively correlated with total yield except for the test planted in 2016 (Table 3).

Percent Marketable Berries and Average Market Score

Percentage marketable yield for a plot ranged from 31% to 90% for planting year 2014, from 43% to 92% for planting year 2015, from 22% to 85% for planting year 2016, and from 21% to 88% for planting year 2017. For all four tests, the sources of variance that were significant for percent marketable berries and market score were cultivar, month, and cultivar-by-month. Film, as a main effect, was a significant source of variance for percent marketable berries and market score in the 2014 and 2015 tests of TC film compared with SC and TES films, in addition to being a significant source of variance for percent marketable berries in the 2017 test comparing KL film with SC and TES films. Film-by-cultivar and film-by month interaction effects were significant for all four tests for percent marketable berries. The film-by-cultivar interaction effect also was significant for market score in the 2014 and 2015 tests that included TC film.

Percent marketable yield was significantly positively correlated in all four tests with average market score, berry weight, and total yield (Table 3). Percent marketable yield was significantly negatively correlated with percent rotted yield. Non-rotted berries from plots with many rotted berries may not have shown obvious signs of rot, but harvest notes often indicated reduced gloss, discoloration, poor shape, or symmetry.

‘Albion’ had among the highest percent marketable berries in all four tests and among the highest average market scores in three of the four tests (Table 1). ‘Albion’ also had among the greatest berry weights. In all four tests, ‘Seascape’ had the lowest percent marketable berries, lowest average market score, and lowest berry weight. ‘Sweet Ann’ also had relatively low percent marketable yield and market scores in these tests. With all the cultivars tested, there were harvests of very high-quality berries, indicating that reduced market scores likely resulted from being grown in an environment very different from where they were bred and selected in California.

The monthly average market score was lowest in the first harvests from each planting, either June or July, and then the average market score generally increased each month of the planting year (Figure 2). Yet, in each test, the average market score decreased to a level that was statistically significant for about a month before resuming an increasing trend for the year of planting, and that month differed from test to test; it was most obvious in the 2014 test. Likewise, there was substantial variation, and no clear trend among test years, for percent marketable berries from month to month through the harvest period. Therefore, variations in weather each year likely affected market scores more than simple plant development or daylength.

The year 2014 had a significant dip in percent marketable yield in September, as did total yield, average market score, and berry size, while percent rotted yield increased significantly (Figure 2). Poor size, symmetry, and mucor were mentioned frequently in the September harvest notes. In 2015, percent marketable yield stayed somewhat low in the year of planting until it increased significantly in October, decreasing slightly through November to December (Figure 2). Berry weight also increased in October and continued to increase through the year of planting. Average market score was relatively unchanged through the year of planting. Total yield dropped significantly in November then increased in December, while percent rotted berries increased in November and significantly so in December. The 2016 percent marketable yield decreased once in September, with a decrease in berry weight and an increase in percent rotted yield, and decreased again in November as market score and total yield also decreased (Figure 2). The 2017 percent marketable yield increased steadily from June through August when it remained constant through the remainder of the year. Total yield, average berry weight, and market score also increased from June through August (Figure 2).

In the year after planting, for all four tests, when harvest resumed in May, the percent marketable yield was at or significantly above the average from the last harvest month before winter (Figure 2). In 2015 and 2016, percent marketable yield remained high through the next month, but in 2014 and 2016, the harvest score decreased significantly in the second month. In 2014, berry weight and average market score also decreased from May to June while percent rotted yield increased. In 2016, average market score also decreased, but percent rot decreased, and berry weight increased; harvest notes often indicated berry symmetry decreased market score.

Although, in all four tests, there were significant differences between cultivars for percent marketable yield and average market score for the test year, those differences were not significant in every month of each test year. Cultivar differences for percent marketable yield and average market score were significant in 13 and 23 of 33 months, respectively, across the four tests. ‘Albion’, ‘Monterey’, ‘Portola’, and ‘San Andreas’ regularly were among cultivars with the highest average market score and the highest percent marketable yield, though for each trait there were one to three months of the total 33 months of this study when each of these four cultivars were not included among the top.

In the 2014 and 2015 tests comparing TC film to SC and TES films, strawberries produced under TC film had the lowest average market scores, and these differences were significant (Table 4). There also were significant differences among films for percent marketable yield in all four tests. The highest percent marketable yield was under SC film in 2014, TES film in 2015 and 2016, and KL in 2017 (Table 4). The lowest percent marketable yield was under TES film in 2014, TC film in 2015, and SC film in 2016 and 2017.

Table 4. Plastic films covering low tunnels compared using analyses of variance and simple average performance relevant to overall mean performance under all films. Films used were TempCool™ 0.102 mm thick film (TC film) (Berry Plastic Corporation, Greenville, SC), Clear TIII 0.102 mm thick film (“standard clear” or SC film), TIII TES/IR 0.102 mm thick film (TES film) (Berry Plastic Corporation, Greenville, SC), and Kool Lite Plus 0.152 mm thick Poly film (KL film) (Klerks Hyplast Inc., Chester, SC). Statistically significant differences between film means by trait each year are designated by lower-case letters. Above-average trait means under films within and across years are designated with bold text and a grayed field. An index of performance under each film across all traits used average performance across years to identify “above average” performance under films. Above-average interaction means were given a value of “1” and summed across traits and tests to identify films that performed above average for multiple traits and test years

Index (2 yr)	Yld	PR	BW	PM	MS	Total
TC		1	1			2
SC	1		1	1	1	4
TES	1	1		1	1	4
Average						3
Index (2 yr)	Yld	PR	BW	PM	MS	Total
KL	1		1	1	1	4
SC						0
TES	1	1	1	1	1	5
Average						3
Index (4 yr)	Yld	PR	BW	PM	MS	Total
SC						0
TES	1	1	1	1	1	5
Average						3
Yield (g/plant/month) (YLD)	2014		2015		Ave. (2 yr)			2016		2017		Ave. (2 yr)	Ave. (4 yr)
TC	31	a	104	a	68		KL	72	a	79	a	76
SC	48	a	124	a	86		SC	70	a	64	a	67	75
TES	37	a	130	a	83		TES	74	a	69	a	72	76
Average	39		119		79			72		71		71
Percent rotted yield (PR)	2014		2015		Ave. (2 yr)			2016		2017		Ave. (2 yr)	Ave. (4 yr)
TC	3	a	2	a	2		KL	1	a	3	a	2
SC	4	a	2	a	3		SC	2	a	3	ab	2	3
TES	3	a	2	a	2		TES	2	a	2	b	2	2
Average	3		2		3			2		3		2
Berry weight (g) (BW)	2014		2015		Ave. (2 yr)			2016		2017		Ave. (2 yr)	Ave. (4 yr)
TC	10	a	12	a	11		KL	10	a	12	a	11
SC	10	a	12	a	11		SC	10	a	11	b	11	11
TES	9	a	12	a	11		TES	10	a	12	a	11	11
Average	9		12		11			10		12		11
Percent marketable yield (PM)	2014		2015		Ave. (2 yr)			2016		2017		Ave. (2 yr)	Ave. (4 yr)
TC	23	ab	40	b	31		KL	17	ab	26	a	22
SC	28	a	44	ab	36		SC	15	b	20	b	17	28
TES	20	b	49	a	35		TES	20	a	24	ab	22	29
Average	24		44		34			17		23		20
Market score (MS)	2014		2015		Ave. (2 yr)			2016		2017		Ave. (2 yr)	Ave. (4 yr)
TC	5.4	b	7.1	b	6.2		KL	5.8	a	6.7	a	6.2
SC	6.0	a	7.3	ab	6.6		SC	5.6	a	6.5	a	6.1	6.6
TES	5.8	ab	7.3	a	6.5		TES	5.7	a	6.7	a	6.2	6.6
Average	5.7		7.2		6.5			5.7		6.7		6.2

In the 2014 and 2015 tests, there also were significant cultivar-by-film interaction effects for average market score and percent marketable yield (Table 2). In the 2014 test, ‘Monterey’, ‘Portola’, ‘San Andreas’, and ‘Seascape’ produced fruit with higher average market scores under SC film and lower market scores under TC film. ‘San Andreas’ and ‘Seascape’ produced significantly greater percent marketable yield under SC film in the 2014 test. In the 2015 test, ‘Albion’, ‘Monterey’, and ‘San Andreas’ produced fruit with lower market scores under the TC film. ‘Albion’ also produced the lowest percent marketable yield under TC film and the highest under SC film in the 2015 test. Both test years, ‘Albion’ produced fruit with lower market scores under TC film, and the difference was significant in 2015. In the 2017 test, ‘Seascape’ produced significantly lower percent marketable fruit under the SC film.

Film-by-month interaction effects were significant for percent marketable yield in all four tests. The only noticeable trend was that significantly higher percent marketable yield was produced under TES film in three of four Novembers and higher percent marketable yield in both Decembers, though the difference was significant in only December 2017.

Cultivar Comparison

Although analyses of variance consistently showed significant differences between cultivars, interaction effects with film and month often were significant too. There was no cultivar that consistently performed significantly better than the others for all traits in all tests. Therefore, above-average means across traits and tests were summed for each cultivar. Some cultivars consistently performed above average in most traits (Table 1). ‘Monterey’ was above average for all traits. ‘Albion’ was above average for all traits except yield. ‘Portola’ was above average for all traits except percent rotted yield. The other cultivars were above average for only one or two traits out of five.

Film Comparison

Film, as a main effect, was a significant source of variance only for berry weight (2014, 2015, 2016), percent marketable yield (2016), and market score (2016, 2017). Film interaction effects with cultivar and month were often significant and contradictory. Therefore, above-average means across traits and tests were summed for each film (Table 4). In the 2014 and 2015 tests, when TC film was compared with SC film and TES film, performance of strawberries grown under SC film was above average for all traits except percent rotted yield. Strawberry performance under TES film was above average for all traits except berry weight. Strawberry performance under TC film was above average but not significantly different from strawberries grown under the other films for percent rotted yield and berry weight, so there was no advantage to using TC film either year, and performance of strawberries grown under TC film was generally poor in comparison. In the 2016 and 2017 tests, when KL film was compared with SC film and TES film, strawberry plants grown under KL film performed above average for all traits except percent rotted yield. Percent rotted yield was lower under KL film in 2016, but the difference was not significant; percent rotted yield was significantly lower under TES film in 2017. Overall performance under TES film was above average for all traits, and performance under SC film was below average. Comparing just SC and TES films, but across all four tests, performance of strawberry plants grown under TES film was superior to those grown under SC film for all traits.

Cultivar and Film Combinations

Cultivar-by-film interaction effects were often significant but the rankings of interaction means were not consistent across tests. Therefore, above-average means across traits and tests were summed for each cultivar-by-film combination (Table 2). In the 2014 and 2015 tests, ‘Monterey’, the cultivar that performed above the cultivar average for all traits, had above-average yield and market score under SC or TES films, below average percent rotted yield under TC or TES films, similar berry weight and percent marketable yield under all films. In the 2016 and 2017 tests, ‘Monterey’ had above-average yield and percent marketable yield under KL and TES films, below-average rotted yield and above-average berry weight under all three films, and higher market score under the KL film. Overall, ‘Monterey’ performed above average under TES or KL films. In the 2014 and 2015 tests, ‘Albion’, the cultivar that performed above average for all traits except yield, had above-average yield, berry weight, and percent marketable yield under all three films, below-average percent rotted yield under TES film, and above-average market scores under SC and TES films. In the 2016 and 2017 tests, ‘Albion’ had above-average yield under TES and KL films, below-average percent rotted yield and above-average berry weight under all three films, above-average percent marketable yield under SC and KL films, and above-average market scores under TES film. Overall, ‘Albion’ performed above average under TES or SC films. In the 2014 and 2015 tests, ‘Portola’, the cultivar that performed above average for all traits except percent rotted yield, had below-average percent rotted yield under TES and TC films, above-average yield, percent marketable yield, and market score under TES and SC films, above-average berry weight under SC film. In the 2016 and 2017 tests, ‘Portola’ had below-average percent rotted yield under TES film, above-average yield, berry weight, and percent marketable yield under all three films, and above-average market scores under KL and SC films. Overall, ‘Portola’ performed above average under SC, KL, and especially TES film. The TES film was the film under which ‘Portola’ had below-average percent rotted yield. This was the one trait that ‘Portola’ was generally below average for, so it was interesting that the TES film may have compensated for ‘Portola’s weakness.

Month Comparison

Yield, berry weight, market score, and percent marketable yield all generally increased from the first harvests until some point following summer heat when all these traits decreased (Figure 2), and all generally were positively correlated (Table 3). As summer heat abated, all these traits increased again and remained high in the following May and June harvests until they decreased again in late June. Growers producing low-tunnel fields in succession will have June yields that, when combined, are similar to the monthly yields of September or October, and have good market scores.

There seemed to be a significant division in yield and percent marketable yield the year of planting compared with the year after planting (Figure 3). Yields from ‘Albion’ and ‘Monterey’ were greater the year of planting than the year after planting, but yields from ‘San Andreas’, ‘Seascape’, and ‘Sweet Ann’ were greater the year after planting. Yields for ‘Portola’ were generally greater the year of planting, except for the 2015 planting. Yields were generally greater in the year of planting for strawberries grown under SC, TC, and KL films, but higher in late fall and the following year for those grown under TES film. Significantly higher percent marketable yield was produced under TES film in three of four Novembers and higher percent marketable yield in both Decembers, though the difference was significant in only December 2017. Therefore, TES film may provide a more productive environment in and following colder months.

Percent rotted yield negatively impacted percent marketable yield (Table 3) and was more affected by weather events and management practices than the month of the year. Mucor can be managed by watering only immediately after harvest. Botrytis was made worse by the use of both row covers and overhead sprinklers for frost protection. Lewers et al. (2012a) found humidity in the field (without low-tunnels) a few days before harvest to play a role in botrytis development in postharvest storage, and it may be that humidity facilitates botrytis development in the low-tunnel environment to a greater degree than previously recognized in open-field studies which focused on the effect of rainfall. It may be possible to manage the row covers in a way that does not exacerbate botrytis development. For example, they may be relied on as the only frost protection method, rather than in conjunction with overhead sprinklers, so they don’t freeze in place and may be removed each morning to reduce humidity.

Summary

This study evaluated yield, percent rotted yield, berry weight, percent marketable yield, and market score of six repeat-fruiting strawberry cultivars grown in low tunnels covered with four different plastic films. The objective was to identify cultivars and films that would result in superior yield and fruit quality with less disease. Work for the study was done in five years through four test plantings. Each test was harvested from June or July of the year of planting through November or December, and then again the following April or May through June. Results from analyses of variance found significant effects from cultivars and months of harvests for all traits and also found significant two-way interaction effects for most traits most test years. Combining the results from analyses of variance with summing the number of times a cultivar, film, or cultivar and film combination produced above-average results was helpful in identifying above-average cultivar and film combinations, rather than relying on only the analyses of variance.

‘Monterey’ produced better than average results for all five traits; ‘Albion’ produced better than average results for all traits except yield. ‘Portola’ produced better than average results for all traits except percent rotted yield. These three produced greater yield in the year of planting than in the year after planting and should be good choices for growers in the Mid-Atlantic to try.

There was no advantage to using TC film, and performance of strawberries grown under TC film was generally poor in comparison. Performance under TES film was above average for all traits, and performance under KL film was above average for all traits except percent rotted yield. The KL film appeared to provide a yield advantage in the months of the year of planting, when weather was hot, KL film might also be worth trying for a grower in a hot region who plans to market the bulk of the harvest in the year of planting. TES film provided an advantage the year after planting, and some years, each of these advantages were statistically significant. Yield under TES film was consistently greater the year after planting than in the year of planting, which could be an advantage or a disadvantage depending on marketing needs. ‘Monterey’ performed above average under TES or KL films. ‘Albion’ performed above average under SC or TES films, neither of which provided an advantage over the other in compensating for ‘Albion’s low yield. ‘Portola’ performed above average under SC, KL, and TES films with the TES film providing an advantage over the others in compensating for ‘Portola’s above-average percent rotted yield. TES film supported above-average performance for all three above-average cultivars. Although these three cultivars consistently produced greater yield in the year of planting than in the year after planting, TES film compensated some by supporting increased yield in the year after planting. Because each of these cultivars performed above average under TES film, it’s possible TES film has properties that allow a wider range of genotypes to perform well, though that generalization requires further testing.

Additional information

Funding

This project was funded by USDA-ARS Projects 8042-21220-257-00D, 8042-11660-001-00D, and 8042-66000-001-00-D. The authors wish to thank John Enns, Philip Edmonds, Jackson Fisher, Emily Morris, Andrew Russ, and the BARC Research Farm Services, for their contributions to this research. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Dept. of Agriculture, and does not imply its approval to the exclusion of other products or vendors that also may be suitable.