Abstract
The potential for table-top production in Scandinavia is currently unknown, but may be considered more limited than in central Europe. In this study, the performance of six strawberry cultivars was studied in a table-top system using peat bags as substrate inside or outside an open polythene tunnel. The table-top system resulted in high-quality fruit production, with easy-to-pick fruit and a high percentage of marketable produce. The tunnel reduced average solar radiation and increased temperature slightly and simultaneously protected the plants from rain, this did reduce yield or fruit quality parameters compared with uncovered plots. Partly because of an increased incidence of powdery mildew (Sphaerotheca macularis) inside the tunnel, fruit brightness was reduced in the second harvest year. Of the cultivars included, ‘Elsanta’, ‘Sonata’ and ‘Alice’ may be considered well adapted to the system, whereas ‘Honeoye’, ‘Florence’ and ‘Babette’ are inferior.
Introduction
Production of strawberries in Europe has changed considerably over the past decades, due to market demands for high quality fruit over a prolonged season. Systems have been developed to meet such demands, including protected cultivation and soil-less cropping.
Protected cultivation includes various tunnel systems as well as glasshouse cultivation, which minimize the influence of bad weather conditions such as rain and wind damage. In Europe, protected cultivation was originally put into practice by progressive growers and based on trials performed in private organisations or by advisers with no obligation to publish their results. Therefore, only a limited number of studies have been published on the subject. The present situation for protected cropping of strawberries in Europe has recently been reviewed by Lieten (Citation2001). Protected cropping in central Europe generally covers 10–80% of the strawberry production area (Lieten et al., Citation2004).
Soil-less cultivation using peat, perlite, rockwool, etc. initially was used by growers on soils contaminated with fungi or pests, but has recently also gained interest in intensive protected strawberry production (Lieten, Citation1991). Soil-less cropping includes a variety of systems in which strawberry plants are grown in bags or containers, lifted from soil level, and drip irrigated. Systems may be closed, leaving drainage water to be recycled (Lieten et al., Citation2004). Apart from the obvious advantage of avoiding soil-borne pests and diseases, soil-less cultivation also gives several other benefits: the use of pesticides can be minimized, fruit rot and other fruit damage is limited, and strawberries are clean and well ripened as they do not come into contact with soil at any time. Also, the better working conditions with soil-less cultivation attract labour more easily and increase picking capacity (Lieten et al., Citation2004).
Soil-less cultivation has been a topic of research over the last few decades. In Europe, soil-less cultivation was developed mainly in The Netherlands and Belgium, where there is a tradition of year-round production. A number of different substrates have been tested, although peat is still the most widely used (Lieten et al., Citation2004). Peat bag culture offers several advantages: good working conditions, easy and fast picking, clean and well ripened fruit, prevention of soil-borne diseases, and efficient nutrition and spraying (Lieten, Citation1991; Daugaard, Citation2006). Recently, the use of peat bags has decreased due to the problems posed by plastic waste at the end of cultivation, and this has prompted renewed interest in container culture (Lieten et al., Citation2004).
Unfortunately, all of the systems mentioned require high investments, as well as more skill and technical knowledge from growers. The more sophisticated cultivation systems become, the higher the requirements become not only for investment and skills, but also for yield and optimization of cultivation.
In Scandinavia, there has so far been only a limited interest in protected cultivation and soil-less cropping (Davik et al., Citation2000; Lieten et al., Citation2004). In these countries there is enough land available for crop rotation, and the fruiting season is relatively short due to climatic conditions. Soil-less cultivation is also more susceptible to frost. Therefore, when tunnel systems are used, strawberry plants are cultivated in the soil. The use of soil-less systems in Scandinavia may therefore be limited to one-year cropping. There is thus a need for testing and developing low-investment systems in which some of the advantages mentioned above can be achieved, but without the need for very large investments. One alternative is table-top, which was evaluated in this project. The yield and quality potential of the table-top system was investigated by including a number of cultivars with assumed contrasting adaptability to the system.
Materials and methods
A+ frigo plants of six strawberry cultivars [‘Honeoye’ (USA), ‘Elsanta’ and ‘Sonata’ (The Netherlands), ‘Alice’ and ‘Florence’ (England) and ‘Babette’ (Norway)] were planted on May 4, 2005, at the Institute of Horticulture Aarslev, situated at 10° 30′ E longitude and 55° 20′ N latitude. Plants were A+ plants (crown diameter 15–18 mm) and were delivered from Bioforsk, Norway (cv. Babette) and Goosens Flevo Plant Denmark (all other cultivars). They were planted in strawberry peat bags, 25×50 cm from Delta Green, with 6 plants per bag. The distance between table rows was 95 cm and the table height was 120 cm, resulting in an overall plant density of 7.5 plants per m2. Each plot consisted of 1 peat bag containing 6 plants, and there were 12 replications per cultivar, 6 of which inside the tunnel and 6 outside the tunnel. The plastic tunnel used for this experiment was an open type (Viking Tunnel, 8 metres wide and 30 metres long, covered with plastic), which provided rain shelter but had no plastic at the ends or sides. The system thus consisted of 6 table rows, the outer ones serving as guard rows leaving the four 4 rows for the trial.
During the growing season, plants were cultivated according to good agricultural practice including conventional pest and disease control. In the first trial year four applications of fungicides (Kresoxim 0.2%, Tolylfluanide 0.2%, Pyrimethanil 0.1% and Fenhexamide 0.5%) and one application of insecticide (Alpha-cypermethrin 0.012%) were made, and in the second year there were four applications of fungicides only (Kresoxim 0.03% + Fenhexamide +.075%, Tolylfluanide 0.2%, Azoxystrobin 0.05% + Pyrimethanil 0.1%, and Azoxystrobin 0.1%). All runners were removed continuously.
Fertilization was supplied according to . Drip irrigation was managed according to outdoor temperature and tensiometer recordings, but generally 4–6 times a day till drainage. The content of nutrients in drainage water was controlled weekly. Leaf analyses were carried out as a means of control of nutrient levels. Data concerning climatic conditions (temperature, relative humidity, solar radiation) were recorded continuously both inside and outside the tunnel. Relative humidity and solar radiation were, however, recorded only in the first year of trial. The experiment was extended to two cropping years in order to follow the development of plant growth and fruit quality as well as the long term effects of the production system. During the winter period (from November to April the following year) the bags were moved to the tunnel floor and covered with synthetic mulch (agryl) as winter protection. In April the following year the bags were replaced in the table-top system.
Table I. The amounts of each nutrition element supplied to the trial before flowering and from flowering to harvest (µg per ml).
During the first year, plant growth and the number of flower clusters were rated at 7 and 11 weeks after planting, respectively, in both cases using a 5-point rating (0–5, 5 best). Each year, fruit size and yield were recorded on each plot. Fruits were picked twice a week from all plants, including the guard plants. The total weight of marketable fruit and that of discarded fruit were recorded separately. Fruit size was recorded for 25 fruits representative for each picking. Fruit quality was rated each year as well, using a 9-point rating (1–9, 9 best) for the following: fruit colour, brightness and firmness, using the protocol from COST 836 (Navatel & Krüger, Citation2003; Navatel & Krüger, Citation2004. CTIFL colour chart can be purchased from the authors). According to this protocol, rating of fruit colour was made using the CTIFL colour chart (1–9, 9 darkest), whereas the other quality parameters were rated according to the method used by The European Network for Strawberry Cultivar Evaluation (Navatel & Krüger, Citation2003). In the first year, fruit was rated twice and in the second year three times during the picking period, and every single plot of the trial was rated each time.
All data were subjected to statistical analysis using the General Linear Model of SAS (SAS Institute, Inc., 1999–2001, Cary, NC). The least significant differences between means were determined at P<0.05 using Duncan's test.
Results
Climate conditions inside and outside the tunnel
The climate in Denmark is temperate but with considerable annual variation. This was also the case during this experiment (). During the first cropping year, April was relatively mild and dry, whereas May, June, and August were normal and July was warmer and more humid than normal. During the second year, April–May temperatures were normal but precipitation was above normal, whereas June and July were warm and dry. As plants were cultivated inside and outside an open polythene tunnel, precipitation had an impact on the growth and yield of only those plants outside the tunnel. Despite the tunnel being open, climatic data were different under cover compared with open field; the biggest difference was seen in solar radiation, which was reduced by 26% under cover, whereas relative humidity was slightly higher and temperature slightly lower outside than under cover ().
Table II. Monthly mean climatic records for the experimental site 2005 and 2006, compared with the 30-year normal from a climatic station near the experimental field (DMI).
Table III. Average values of solar radiation (MJ/m2/day), relative humidity (%) and temperature (°C) inside and outside the tunnel in 2005 and average values for temperature (°C) inside and outside the tunnel in 2006.
Plant growth and number of flower clusters for the first year
The vegetative growth of frigo plants 7 and 11 weeks after planting differed according to cultivar, time of rating and cultivation (). Total average growth inside and outside the tunnel reveals a significant growth increase inside the tunnel at both times compared with outside. Cultivars differed in regard to growth habit. For example, ‘Elsanta’ showed the fastest initial growth whilst ‘Florence’ was much slower (data not shown). However, cultivar differences were more or less eliminated 11 weeks after planting.
Figure 1. Growth of cultivars in the first year of trial (2005) according to a 5-point rating (0–5, 5 best), 11 weeks after planting. Comparison between tunnel and outside tunnel is based on means of all cultivars. Comparison between cultivars is based on cultivar means of tunnel and outside tunnel growth. Columns with the same letter (comparisons tunnel-outside or within cultivars) are not significantly different.
The number of flower clusters was unaffected by the presence of the tunnel (data not shown), which was expected as flower formation had already taken place long before planting. However, there were small differences in the number of flower clusters among cultivars, with ‘Elsanta’ showing the highest number and ‘Florence’ and ‘Babette’ the lowest (data not shown).
Marketable and total yield
The highest yielding cultivars in the first cropping year were ‘Elsanta’ and ‘Sonata’, whereas ‘Florence’ and ‘Honeoye’ were the lowest yielding, possibly because of poor adaptation to table-top production ().
Table IV. Marketable yield (kg/m2 and % of total yield) during the first and second year of harvest. Results with the same letter within years are not significantly different.
In the second cropping year, ‘Sonata’ had the highest marketable yield, followed by ‘Elsanta’ and ‘Alice’. The lowest yielding cultivars were ‘Honeoye’ and ‘Babette’. In both years the differences between marketable yield and total yield were very small, indicating that yield losses are very low with the table-top system. However, there were some differences among cultivars with respect to the percentage of marketable yield. For example, cv. ‘Babette’ during the second cropping year both had very low yield in general and a relatively low percentage of marketable yield as well. For ‘Alice’ the situation was the opposite, as both total yield and the percentage of marketable yield improved during the second year of harvest. The lower percentage of marketable yield in ‘Alice’ during the first cropping year was mainly seen in plots outside the tunnel (data not shown).
A comparison between average marketable or total yield outside and inside the open tunnel revealed no significant differences during the first year of harvest, whereas in the second year average yields were slightly higher outside the tunnel. A moderate mildew attack in the tunnel during the last part of the harvest period did not affect the total yield but may have caused the fruit brightness.
Fruit size and quality characteristics
Fruit size varied according to cultivar, but with only limited variation within years, and no significant differences inside or outside the tunnel eres observed in either year (). However, there was a significant difference between years, as in the second cropping year there was a significant reduction in fruit size compared with the first year. Furthermore, some cultivars (‘Sonata’ and ‘Alice’) maintained a relatively better fruit size than others (‘Elsanta’ and ‘Florence’), indicating a variation in suitability for perennial production.
Table V. Fruit size in the first and second years of harvest (g/fruit). Results with the same letter within years are not significantly different.
Three other characteristics of fruit quality were recorded as well: colour, brightness and firmness (). Differences between cultivars were evident for all characteristics. Thus it appears that ‘Babette’, ‘Honeoye’ and partly ‘Florence’ were relatively dark-coloured, whereas ‘Elsanta’, ‘Sonata’ and ‘Alice’ had a lighter, bright red colour. As to brightness, ‘Alice’ and ‘Elsanta’ generally had the highest score, but with relatively small differences between cultivars. Furthermore, a mildew attack during the last part of the harvest period in the second year clearly affected fruit brightness negatively (data not shown). Fruit firmness, however, was significantly different between cultivars. ‘Alice’, ‘Florence’ and ‘Elsanta’ generally were given the highest rates for firmness.
Table VI. Fruit quality characteristics (colour, brightness and firmness) during the first and second years of harvest. Rating 1–9, 9 darkest (colour), most bright or most firm. Results with the same letter within years are not significantly different.
Apart from cultivar differences in fruit quality parameters, the trial also revealed some differences among years and between systems of cultivation (with or without polythene tunnel). Compared with the first year, fruit yield in the second year was characterized by relatively darker fruit colour and higher fruit firmness. When fruit from cultivation under cover was compared with fruit grown outside, there were significant differences in fruit colour as well as brightness, but the results were contradictory among years. As to firmness trends were more uniform among years (i.e. highest firmness outside), and differences were not significant.
Discussion
A major concern about soil-less culture has been that of maintaining high fruit quality. Research results on this matter are few and contradictory. In a comparison between fruit grown in the soil under polythene tunnel and in substrate culture in a greenhouse, soil cultivation resulted in a better fruit quality in terms of firmness and sugar content (Wozniak et al., Citation1997). Fernández et al. (Citation2004) made a comprehensive study, comparing field-grown fruit with fruit from soil-less culture, using different substrates. Variables included fruit contents of a number of sugars, acids, and nutrients, and the results showed no significant differences in any variable. In our study fruit quality was compared in a table-top system with peats bags, with or without the use of a polythene tunnel. Small differences were seen only for fruit colour and brightness, but with contradictory results between years; for all other quality characteristics no significant differences were seen. However, it must be taken into account that during the second year, fruit quality under cover suffered from a severe mildew attack. Other differences in research results between this and other studies may be ascribed to differences in experimental design.
The aim of this experiment was to study the potential of the table-top system, including the adaptation of various different cultivars to this system. As new cultivars are generally selected on the basis on their performance in conventional field-grown production, it may not automatically be assumed that they are well adapted to protected cropping as well. In our study it was evident that cultivars responded differently to table-top production in a number of respects, among which the level of marketable yield and fruit quality may be considered the most important. It appeared that, for varying reasons, ‘Honeoye’, ‘Babette’ and ‘Florence’ were poorly adapted to the table-top system, whereas ‘Elsanta’, ‘Sonata’, and ‘Alice’ seemed to be well adapted. Elsanta is the main cultivar in protected cropping systems of central Europe (Lieten, Citation2001). In the Scandinavian countries ‘Elsanta’ has generally not performed well in open-field production (Navatel & Krüger, Citation2003; Navatel & Krüger, Citation2004; Hietaranta et al., Citation2004), but our results indicate that table-top production of this cultivar may be feasible even in our climate. The poor results for ‘Honeoye’ are in accordance with those of a previous study in Norway (Nestby et al., Citation2003).
The potential for table-top production in Scandinavia is currently unknown, but may be considered more limited than in central Europe. From this study, however, it may be concluded that it is a feasible alternative, resulting in high-quality fruit production, with easy to pick fruit and with a high percentage of marketable produce. An open polythene tunnel, although protecting from rain, did not cause any further improvement in fruit quality, possibly due to an attack of powdery mildew (Sphaerotheca macularis), especially during the second year of harvest. The most suitable cultivars for table-top production are considered to be ‘Elsanta’, ‘Alice’, and ‘Sonata’.
Acknowledgements
The technical assistance of J. Eriksen, E. Rosenstr⊘m, and K. Nielsen is greatly appreciated.
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