Cultivars and cultivation systems for organic strawberry production in Norway

ABSTRACT

Cultivars and cultivating methods for organic strawberry production were studied in experiments in open fields and high plastic tunnels during four cropping seasons in southern Norway. In open fields, flowers and fruits were attacked by grey mould when the flowering and harvest seasons were wet, and marketable yield was reduced by up to 20%. Production in high tunnels showed a potential of high yields of fruits of good quality when strawberry powdery mildew was controlled. Berry size varied significantly among the cultivars. ‘Frida’ had the largest fruits followed by ‘Sonata’ and ‘Florence’, while ‘Polka’, ‘Korona’ and ‘Iris’ had the smallest fruits. All cultivars yielded well, but due to fruit decay caused by grey mould the marketable yield was significantly reduced, especially in open field. Grey mould was the most important factor influencing marketable yields. Fruits from matted rows were largest, while the highest yield was obtained on woven polyethylene. There were no effects of mulching methods on marketable yield or the amount of fruits with grey mould. High tunnels with good control of pests and diseases showed a potential of high and stable yields of good quality.

KEYWORDS:

• Berry size
• diseases
• Fragaria × ananassa
• marketable yield
• pests
• sustainable production
• yield

Introduction

The cultivated strawberry, Fragaria × ananassa Duch, is commonly grown as a perennial crop thoughout the cool temperate regions of the world. First harvest is carried out one year after the strawberry plants are established, and the fields are kept for several years. There is an increasing demand for organically produced food; however, the plants and fruits of most strawberry cultivars have low resistance to important pests and diseases, and this is the most important reason for the limited organic production of this crop. Organically grown strawberry may have better fruit quality and even better shelf life than conventionally grown strawberry (Cayuela et al. 1997; Reganold et al. 2010), and profits may also be better (Brzozowski & Zmarlicki 2012); however, others contradict these conclusions (Hargreaves et al. 2008). In Norway, ‘Polka’ and ‘Honeoye’ gave high yield and good fruit size and appeared to be most suitable for a perennial, organic cropping system in open fields (Sønsteby et al. 2005). In Finland, ‘Bounty’ and ‘Polka’ were the most productive cultivars in organic production, but since the content of both soluble solids and ascorbic acid was highest in ‘Bounty’, that cultivar was recommended as the most suitable for organic production (Kahru et al. 2010). Rhainds et al. (2002) found that due to high productivity and low susceptibility to the tarnished plant bug (Lygus lineolaris), ‘Honeoye’ was the most profitable cultivar for organic strawberry production in New York.

The effects of mulching are reported in several studies. In conventional and organic perennial berry cropping systems in Norway (Sønsteby 1998) and Sweden (Larsson 1997), respectively, mulching with black coloured polyethylene provided good weed control, increased soil temperature and soil moisture. Mulching may also improve plant establishment and plant growth and thus provide earlier and higher yield and runner production (Berglund et al. 2007). During three harvest seasons, plants established in polyethylene mulch yielded much more than plants in bark or straw mulch (Berglund et al. 2007). Similar effects of black polyethylene mulch were found when compared to wood chip mulches from deciduous and coniferous trees in perennial strawberry production in Central Finland (Prokkola et al. 2003). In California, all coloured plastic mulches, but not clear mulch, provided satisfactory weed control with no use of herbicides (Johnson & Fennimore 2005). Clear and black mulches provided the highest soil temperatures and highest marketable yields. Organically grown strawberry plants in Norway established in polyethylene mulch performed better and gave higher yield than those in straw mulch or bark mulch in the first three cropping years, while plants in bark mulch yielded most in the fourth year (Birkeland et al. 2002). In Sweden, the strawberry cultivars ‘Bounty’ and ‘Kent’ grown in degradeable plastic mulch yielded better than those grown in green mulch with fresh red clover and timothy (Svensson 2002). The incidence of grey mould in organic strawberry production in Finland was more severe in buckwheat husk mulch than in black plastic or other organic mulches (Prokkola et al. 2003). Furthermore, the same authors reported more severe attacks by grey mould in green mulch of young cut timothy grass than in wood chip mulches. In Turkey, strawberry plants grown in polyethylene mulch had lower yield than those grown in a floating sheet mulch (Balci & Demirsoy 2008).

New cultivars are continually introduced into the strawberry industry and need to be evaluated in various growing conditions and systems. To improve fruit quality and make strawberry production less dependent on changing weather conditions, the use of high plastic tunnels has increased over the recent decades. There are few studies of organic strawberry production in high tunnel systems, and thus the objectives of the present investigations were to find cultivars and the best cultivation methods suitable for organic production of strawberry in Norway.

Materials and methods

The studies were carried out in three field experiments in organic strawberry farms in Vest-Agder County in southern Norway. Experiment 1 was established at Lindesnes (58° 04′ N; 07° 17′ E) in early June 2007. Half the experimental field was covered with standard high plastic tunnels (Haygrove Ltd., Ledbury, England) the year after planting, and the other half was open field. There were no recording of weather conditions in the experimental fields, and weather data from time of first flowering throughout harvest are thus from weather stations (Norwegian Meteorological Institute) 25–40 kilometres from the sites. Variations in time of flowering and ripening between the cultivars were recorded only during the first cropping season, but dates of 50% yield were calculated for three cultivars in all experiments. Assessments were made from seven cultivars both in open fields and in high plastic tunnels. Three of the cultivars were removed after the first season, and the following year data were taken from the four remaining cultivars. Experiments 2 and 3 were established at a farm in Søgne (58° 05′ N; 07° 49′ E) in mid-May 2010. Experiment 2 took place in an open field, and experiment 3 in high plastic tunnels. Experiment 2 was finished after the first harvesting season, and experiment 3 continued for one more year. In all three experiments, planting took place during early summer the year before the first assessments. The high tunnels were covered with polyethylene just after flowering. No crown removal was carried out. Old leaves and runners were removed by brushing as soon as the fields were dry in the spring, and runners were removed three times during the growing season along with weeding.

The soil at both farms was sandy with moderate amounts of organic matter. Before planting, and based on soil analyses, both fields were supplied with about 30 t ha⁻¹ of organically approved types of farmyard manure (90 kg N, 15.5 kg P and 18 kg K). The soil was low in potassium, and thus both fields were fertilized with about 30 t ha⁻¹ of a K-rich sand (Røynebergsand, Norstone AS, Sandnes, Norway). In the first cropping year, the field at Lindesnes was fertilized with 0.8 t ha⁻¹ of Binadan 5-2-4 (Binadan AS, Nørre Snede, Denmark) in the spring, followed by 1.0 t ha⁻¹ of Vadheim Groplex 2-0-5 (Vadheim Groplex Import AS, Bergen, Norway) one month later. In early September, the field was supplied with 1.0 t ha⁻¹ dried chicken manure of Marihøne 4-1-2 (Norsk Naturgjødsel AS, Stavanger, Norway). In the second cropping year, the field was supplied with in total 0.65 t ha⁻¹ of Marihøne 4-1-2 in early spring, followed by 1.0 t ha⁻¹ of Vadheim Groplex 2-0-5 in early summer.

In the first year of experiments 2 and 3 at Søgne, 0.5 t ha⁻¹ of Marihøne Pluss 8-4-5 was used in spring, followed by 0.25 and 0.4 t ha⁻¹ of the same fertilizer in early autumn and the following spring, respectively. Because of the low content of B and Zn, all plants were fertilized with foliar sprays at recommended rates of Yara Vita Bortrac and Yara Vita Zintrac (Yara, Norway) late in the autumn of the first cropping year.

All cultivars were June-bearers, and the cultivation was perennial. The experiments were all factorial in a split-plot design. Experiment 1 had two splits; high plastic tunnels and open fields in the main plots and the cultivars (country where the cultivars originate from in parentheses) ‘Korona’ and ‘Polka’ (the Netherlands), ‘Florence’ (UK), ‘Honeoye’ (USA) and ‘Gudleif’, ‘Frida’ and ‘Iris’ (Norway) in a randomized block design with two replicated plots of 30 plants of each cultivar in the subplots. Experiments 2 and 3 had cultivating methods in the main plots and a randomized block design with four cultivars in two replicates on the subplots (30 plants) (Table 5). The plants were established in double rows on 10–15-cm raised beds, and the cultivating methods were (i) matted rows, (ii) mulching with 0.05-mm-thick black polyethylene (Norfolier, Notodden, Norway) or (iii) mulching with woven black polyethylene (100 gsm Mypex, Juta, Dvur Käiovë, Czech Republic). The cultivars were ‘Sonata’ (the Netherlands), ‘Polka’, ‘Korona’ and ‘Florence’.

Certified plug plants (0.053 L plugs, 96 plugs per tray) were used in all fields. The plants of the different cultivars were of approximately the same size at the time of planting. Planting dates were 6 June for experiment 1 and 15 May for experiments 2 and 3. The distance between the midpoints of the ridges was 150 cm and the planting distance in the rows was 36 cm, giving approximately 37,000 plants per hectare. All fields were fitted with a pressure compensated trickle irrigation system with an emitter spacing of 30 cm and a capacity of 1.6 l per hour between the double rows (Dryppvanning AS, Lier, Norway). Weeds were removed by hand. To control strawberry powdery mildew (Podosphaera aphanis) in high tunnels, one-minute application of overhead misting was used every second day at 20-minute intervals in the middle of the day, modified from Asalf et al. (2015), keeping the plants nearly constantly wet during warm and dry days from the end of flowering throughout harvest. There was no application of chemicals against pests and diseases in experiments 1 and 2. In the last year of experiment 3, the field was treated six times against powdery mildew and spider mites from early spring until one week before harvest with sulphur (Thiovit Jet, Syngenta Crop Protection AG, Basel, Switzerland) at recommended rates. Application was done with a motorized backpack sprayer. For insect prevention, the fields were covered with Novagryl P18 fleece (Cropsolutions, Perth, Scotland) with mask size 0.94 × 1.54 mm from early spring until flowering. The plants in the tunnel were frequently supplied with predatory mites (Phytoseiulus persimilis and Amblyseius cucumeris) to control two-spotted spider mite (Tetranychus urticae) and strawberry mite (Phytonemus pallidus var. fragariae). The plastic (polyethylene) was mounted on the tunnel frames just before flowering and removed in late October. To reduce winter damage, the fields were covered with one layer of Novagryl P18 fleece from late November to early April each year, and no frost damage was observed in the fields.

The fields were picked three times a week, 16–18 times per season, and total and saleable yield (fruits with diameter >28 mm), fruit size and number of fruits with grey mould were recorded at all pickings. Discarded fruits and small fruits (diameter <28 mm) were sorted out from the total yield to make the marketable yield. At all harvests, fruit size was calculated from the weight of 50 arbitrarily picked fruits from each plot.

Due to the lack of replicates of high plastic tunnels and open fields, there were no statistical comparison between them, and assessments from high tunnels and open fields were analysed and presented separately each year. Analysis of variance was carried out using the MiniTab® Statistical Software package (Release 16; Minitab Inc., State College, PA, USA), and Tukey’s test at p ≤ .05 was used to separate means.

Results

Experiment 1

In the first cropping season, weather conditions were rather dry and warm during flowering, but the harvesting season was wet and chilly (Table 1). The following year was wet with low temperature during flowering, followed by a wet but rather warm cropping season.

Table 1. Precipitation (mm) and mean temperature (°C) from the time of first flowering to the beginning of harvest (Flowering) and from the beginning of harvest throughout the harvest period (Harvest) of the experiments.

Experiment/site/year	Precipitation		Mean temperature
	Flowering	Harvest	Flowering	Harvest
1/Lindesnes/2008	18.6	149.2	13.4	13.9
1/Lindesnes/2009	74.1	127.7	11.2	16.4
2 and 3/Søgne/2011	217.6	138.4	12.6	16.2
3/Søgne/2012	47.3	101.4	12.9	15.9

‘Honeoye’ flowered ca. one week before the latest cultivar ‘Florence’ (Table 2). Most of the cultivars were early flowering, with ‘Honeoye’ and ‘Korona’ as the earliest. ‘Honeoye’ and ‘Korona’ also ripened earlier than the other cultivars, and ‘Korona’ had the shortest period between flowering and ripe fruits, followed by ‘Honeoye’. Although ‘Florence’ was the latest cultivar, the period from flowering to the first pick was about the same as for the other cultivars. Number of days from flowering until 50% harvest varied greatly among the cultivars, with ‘Honeoye’ and ‘Frida’ having the shortest period. For all trials and years reported here, data of 50% harvest are shown for ‘Korona’, ‘Polka’ and ‘Florence’ (Tables 2 and 3), and the latter cultivar reached 50% harvest 6–11 days later than the other two.

Table 2. Phenological records from the first year of harvest (2008) of seven strawberry cultivars in a perennial open field system in experiment 1.

Cultivars	Dates of first		Dates of 50% harvest
	Flower	Harvest
‘Florence’	15 May	16 June	4 July
‘Frida’	12 May	13 June	2 July
‘Gudleif’	9 May	10 June	25 June
‘Honeoye’	8 May	6 June	20 June
‘Iris’	9 May	10 June	28 June
‘Korona’	10 May	6 June	26 June
‘Polka’	9 May	10 June	29 June

Table 3. Dates of 50% harvest of three cultivars in the experiments^a.

Experiment/site/year	Cultivars
	Korona	Polka	Florence
1/Lindesnes/2009	27 June	29 June	6 July
2 and 3/Søgne/2011	6 July	8 July	17 July
3/Søgne/2012	6 July	8 July	14 July

^aData for 2008 included in Table 2.

Both total and marketable yield were high for all cultivars (Table 4). ‘Iris’ had the highest total yield followed by ‘Polka’ and ‘Frida’, while ‘Honeoye’ had the lowest yield, medium fruit size and rather poor quality for the fresh market. ‘Iris’ and ‘Polka’ also had the highest marketable yield. ‘Frida’ had significantly larger fruits than any of the other cultivars, followed by ‘Florence’, ‘Honeoye’, ‘Korona’, ‘Polka’, ‘Gudleif’ and ‘Iris’. Because of the heavy attack of strawberry powdery mildew both in high tunnels and in the open field (data not presented), ‘Frida’ had the lowest percentage of saleable yield. ‘Frida’ and ‘Iris’ developed more grey mould than the other cultivars.

Table 4. Total and marketable yield (tons/hectare), fruit size (g/fruit) and fruits with grey mould (tons/hectare) of seven strawberry cultivars in open field and in high plastic tunnel in the first cropping season of experiment 1.

Cultivars	Total yield^a		Marketable yield		Fruit size		Grey mould
	Open	Tunnel	Open	Tunnel	Open	Tunnel	Open	Tunnel
‘Iris’	40.7a^b	42.2a	31.4a	29.5a	24.2a	20.7a	3.4a	2.4a
‘Polka’	29.9b	37.2a	25.7a	28.7a	22.1a	22.1a	1.3b	0.9b
‘Frida’	23.3b	25.7b	16.3b	11.3c	11.4b	5.0d	3.9a	2.4a
Gudleif	17.1bc	20.1bc	14.6bc	18.3b	12.4b	16.7b	0.7c	0.3bc
‘Florence’	19.5bc	16.5c	18.2b	14.3bc	16.9b	12.4c	0.8bc	0.5bc
‘Korona’	13.8c	16.6c	11.2c	13.3bc	9.1bc	10.6cd	0.8bc	0.2c
‘Honeoye’	8.0d	12.9d	3.4d	5.1d	5.8c	8.1cd	0.2c	0.1c
p value	<.001	<.001	<.001	<.001	<.001	<.001	<.001	<.001

^aThe cultivars are sorted from high to low total yield.

^bNumbers followed by different letters are significantly different; Tukey’s test at p ≤ .05 used to separate means.

Following an evaluation of cultivar performance in the first cropping season, ‘Frida’, ‘Honeoye’ and ‘Iris’ were removed from the experiments. Also, in the second year yields were high for all cultivars, and there were no significant differences among them for total yield or fruit size (Table 5). ‘Polka’ had lower marketable yield than the other cultivars, and fruits of ‘Polka’ and ‘Gudleif’ were smaller than the other two cultivars.

Table 5. Total and marketable yield (tons/hectare), fruit size (g/fruit) and fruits with grey mould (tons/hectare) of four strawberry cultivars in open field and in high plastic tunnel in the second cropping season of experiment 1.

Cultivars	Total yield^a		Marketable yield		Fruit size		Grey mould
	Open	Tunnel	Open	Tunnel	Open	Tunnel	Open	Tunnel
‘Gudleif’	29.9	26.1	12.6ab^b	13.5a	23.3	15.8	1.5b	5.2ab
‘Florence’	23.5	22.8	13.0a	13.6a	16.9	14.6	3.3a	5.5a
‘Korona’	29.0	20.1	14.1a	14.4a	22.3	13.5	2.0ab	3.6b
‘Polka’	28.4	21.5	11.8b	11.8b	20.7	13.1	2.6a	5.8a
Mean	27.7	22.6	13.1	13.3	20.8	14.3	2.4	5.0
p value	n.s.	n.s.	<.001	<.001	n.s.	n.s.	<.001	<.001

^aThe cultivars are sorted from high to low total yield.

^bNumbers followed by different letters are significantly different; Tukey’s test at p ≤ .05 used to separate means.

Experiments 2 and 3

In the open field experiment (experiment 2), no significant interactions were found between cultivating methods and cultivars for any factors recorded. The highest total yield was achieved from plants on woven polyethylene (Mypex, Table 6). Fruits from matted rows were significantly larger than those in the other treatments. ‘Polka’ had a higher total yield than ‘Korona’ and ‘Sonata’, but there were no significant differences in marketable yield among ‘Polka’, ‘Korona’ and ‘Sonata’ (Table 6). ‘Florence’ had both lower total and marketable yield compared to the other cultivars. ‘Sonata’ had significantly larger fruits than the other cultivars. All cultivars were heavily attacked by grey mould (some nearly 40%), and the open field experiment was not continued after the first year. ‘Polka’ and ‘Sonata’ had the highest yield losses due to the disease, while the yield of ‘Florence’ was less affected. Fruits from matted rows had less discarded fruits due to grey mould in percent of total yield than the other cultivating methods.

Table 6. Total and marketable yield (tons/hectare), fruit size (g/fruit) and fruits with grey mould (tons/hectare) in the first year of harvest of four strawberry cultivars in open field with different mulching in experiment 2.

Cultivation methods^a	Cultivars	Total yield	Marketable yield	Fruit size	Grey mould
Polyethylene	Mean of	11.7b^b	5.8	16.8b	5.0
Mypex	the four	15.0a	7.0	16.4b	6.3
Matted row	cultivars	12.9b	7.0	18.1a	4.4
p value		<.005	n.s.	<.001	n.s.
Mean of the three	‘Polka’	16.0a	8.1a	16.3b	6.5a
cultivating methods	‘Korona’	14.2b	7.2a	16.1b	5.2b
	‘Sonata’	15.0ab	7.1a	19.3a	6.9a
	‘Florence’	7.6c	4.2b	16.9b	2.4c
p value		<.001	<.001	.003	.002

^aThere were no significant interaction of cultivating methods and cultivars.

^bNumbers followed by different letters are significantly different; Tukey’s test at p ≤ .05 used to separate means.

Data from the two years of experiment 3 are presented in Tables 7 and 8. In the first cropping season, the highest total yield was obtained on woven polyethylene (Mypex, Table 7). There were no significant differences in marketable yield among the cultivation methods. The fruit size was highest on polyethylene mulch and had most grey mould on woven polyethylene. ‘Polka’ and ‘Korona’ yielded more than ‘Sonata’, and ‘Florence’ had the lowest total yield. ‘Polka’ and ‘Sonata’ had higher marketable yield than the other two cultivars. ‘Florence’ had a low level of grey mould. ‘Polka’, ‘Korona’ and ‘Sonata’ were heavily attacked by powdery mildew (data not shown).

Table 7. Total and marketable yield (tons/hectare), fruit size (g/fruit) and fruits with grey mould (tons/hectare) in the first year of harvest of four strawberry cultivars in a high plastic tunnel with different mulching in experiment 3.

Cultivation methods	Cultivars	Total yield	Marketable yield	Fruit size	Grey mould
Polyethylene	Mean of	19.8b^a	11.2	17.3a	1.5b
Mypex	the four	24.3a	12.2	15.3b	2.6a
Matted row	cultivars	21.5ab	12.0	16.0b	1.3b
p value		<.001	n.s.	.004	<.001
Mean of the three	‘Polka’	26.5a	18.1a	15.1b	2.0ab
cultivating methods	‘Korona’	23.4ab	7.2b	15.7b	1.7b
	‘Sonata’	21.5b	16.3a	18.8a	2.4a
	‘Florence’	16.0c	5.5b	15.2b	0.9c
p value		<.001	<.001	.004	.002

^aNumbers followed by different letters are significantly different; Tukey’s test at p ≤ .05 used to separate means.

Table 8. Total and marketable yield (tons/hectare), fruit size (g/ fruit) and fruits with grey mould (tons/hectare) in the second year of harvest of four strawberry cultivars in a high plastic tunnel with different mulching in experiment 3.

Cultivation methods	Cultivars	Total yield	Marketable yield	Fruit size	Grey mould
Polyethylene	Mean of	28.0a^a	22.0a	14.6	1.7
Mypex	the four	29.4a	19.8b	13.7	3.6
Matted row	cultivars	25.2b	18.4b	13.5	2.6
p value		<.005	<.005	n.s.	n.s.
Mean of the three	‘Polka’	22.9c	15.2c	11.6c	1.6b
cultivating methods	‘Korona’	27.8b	19.8b	14.4b	1.8b
	‘Sonata’	33.7a	27.3a	15.5a	3.0a
	‘Florence’	25.8bc	17.9bc	14.2b	3.9a
p value		<.001	.003	.004	<.001

^aNumbers followed by different letters are significantly different; Tukey’s test at p ≤ .05 used to separate means.

In the second year of experiment 3, both total and marketable yield were influenced by the cultivation methods (Table 8), but no effects of cultivating methods were found on fruit size or on yield reduction caused by grey mould. Plants on polyethylene mulch gave a high marketable yield. There were no significant differences in marketable yield between matted row and woven polyethylene. Total yield was lowest on matted row, but for marketable yield there was no significant difference compared to woven polyethylene. ‘Sonata’ gave the highest yields and ‘Polka’ the lowest in all cultivation methods, but there was no difference compared to ‘Florence’ for the two cultivars. Loss of yield due to grey mould was most significant in ‘Florence’, but also ‘Sonata’ lost more than ‘Polka’ and ‘Korona’.

Fruit size showed a significant interaction between cultivating methods and cultivars in both years. In the first year, plants on polyethylene gave the largest fruits (Table 7), but no effect on fruit size of cultivating methods was found the following year (Table 8). ‘Sonata’ had the largest fruits in both years. There was no significant difference in fruit size between the other three cultivars in the first year, but ‘Polka’ had the smallest fruits in the second year.

Discussion

Based on these studies and the works of others, it may be concluded that the use of high yielding cultivars grown in high plastic tunnels to reduce grey mould and supplied with overhead water sprinkling and the use of sulphur to control powdery mildew and spider mites is the most profitable in perennial organic strawberry production in Norway. The best suited cultivars were ‘Polka’ and ‘Sonata’, both showing high and stable yield of good quality (Tables 4–8). Polyethylene mulch gave the highest marketable yield in both cultivars (Tables 6–8).

Wet weather during flowering and harvest affects the incidence of grey mould on fruits (Bulger et al. 1987). In accordance with Prokkola et al. (2003) and Rhainds et al. (2002), the amount of decayed fruits and subsequent yield losses in the present experiments were lower when the flowering period was dry. Marketable yield was significantly higher in plastic tunnels than in open field. The cultivars showed significant differences in resistance to grey mould, as well as differences in fruit size and yield (Tables 4–8). In previous reports, ‘Honeoye’ was one of the best cultivars for organic strawberry production (Rhainds et al. 2002; Prokkola et al. 2003; Sønsteby et al. 2005), but in the present study, ‘Honeoye’ had medium fruit size and total and marketable yields were low (Table 4), and it was thus removed after the first harvest season of experiment 1. ‘Polka’ yielded most in experiments 1 and 2 (Tables 4–6) and confirmed the results of Kahru et al. (2010). The fruit size of ‘Polka’ was medium, and in experiments 2 and 3, ‘Sonata’ had significantly larger fruits (Tables 6–8).

In open field, significant differences in natural resistance to strawberry powdery mildew among cultivars have been demonstrated (Sønsteby et al. 2005), and in the first year of experiment 1, more than 50% of the fruits of ‘Frida’ were attacked. The environment in high plastic tunnels reduced the amount of fruits damaged by grey mould, but the potential of damage by strawberry powdery mildew was high. However, the disease was well controlled by overhead sprinkling. In experiments with overhead sprinkling irrigation as a means to control strawberry powdery mildew in high tunnels in Norway, one-minute application of water four times per day was as effective as the application of fungicides (Asalf et al. 2015).

Total yields were much higher in experiment 1 (Tables 4 and 5) than in experiment 2 (Table 6). The experiments were carried out at different sites and in different years. Variation in temperature in September and October as well as in April the following spring has proved to affect fruit production of June-bearing strawberry plants (Ljones 1978; Sønsteby & Heide 2008). There might also have been small differences in the quality of the transplants used. The amount of fruits damaged by grey mould was also higher in experiment 2, so the difference in fruit loss due to early stage Botrytis infections might also have affected total yield. The effects on yield and fruit quality of different mulching are well documented (Larsson 1997; Sønsteby 1998; Prokkola et al. 2003; Berglund et al. 2007; Balci & Demirsoy 2008). Polyethylene mulch increases the soil temperature (Sønsteby 1998; Johnson & Fennimore 2005), leading to faster plant growth (Berglund et al. 2007), while other organic mulches may have the opposite effect on soil temperatures and plant growth (Larsson 1997). However, most mulching materials reduce evaporation, providing high and stable soil moisture. Experiments 2 (Table 6) and 3 (Tables 7 and 8) showed that different mulching affected yield and quality, confirming earlier studies which obtained the highest marketable yield on black polyethylene mulch and showed that grey mould was controlled successfully by low plant density and good weed control (Birkeland et al. 2002; Prokkola et al. 2003). The effects of mulching methods on fruit size, yield and incidence of grey mould were not consistent in the study and did not confirm earlier studies in Sweden (Svensson 2002) and Finland (Prokkola et al. 2003). In the high tunnel in the present study, plants on woven black polyethylene (Mypex) gave not only the highest total yield, but also most discarded fruits due to grey mould, and thus the highest marketable yield was obtained from plants grown on polyethylene mulch. The positive effect of mulching on grey mould confirms results from earlier studies (Prokkola et al. 2003; Balci & Demirsoy 2008).

The present study has clearly shown that it is possible to have a high yielding and profitable organic strawberry production in a perennial system under Nordic growing conditions. The successful use of overhead sprinkling to reduce powdery mildew and biological control against mite pests are important means to control these important disease and pest problems in high tunnel production. This knowledge may as well reduce the demand for synthetic pesticides in integrated strawberry production in high tunnels.

Notes on contributors

Dr. Arnfinn Nes is a research professor emeritus at the Norwegian Institute of Bioeconomy Research. His main research interest is applied research in berry crop cultivars and cultivation.

Jan Karstein Henriksen is an extension officer at the Norwegian Agricultural Extension Service, with a special interest in berry crops and agricultural technique.

Grete Lene Serikstad is a researcher at the Norwegian Centre for Organic Agriculture. Her special interest is legumes and biological nitrogen fixation, environmental effects of organic agriculture and organic standards.

Dr. Arne Stensvand is a research professor at the Norwegian Institute of Bioeconomy Research and a professor at the Norwegian University of Life Sciences. He is a specialist on fungal plant pathogens and their control in fruit and berry crops.

Acknowledgements

The authors acknowledge the strawberry growers who hosted the experiments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was supported financially by the Norwegian Agricultural Agency, the Norwegian Extension Service, the County Governor of Vest-Agder and the Norwegian Institute of Bioeconomy Research (NIBIO).