Strawberry nursery trials
Two field trials were conducted in the nursery sector at Toolangi, Victoria, Australia, in 2014/2015 with the strawberry cultivars ‘Fortuna’ and ‘Albion’. Soils at the trial sites were brown ferrosols, with a clay–loam texture, which is the main soil type occurring in the district. Soils were rotary-hoed to a depth of 25 cm and fumigated with a mixture of methyl bromide and chloropicrin (50:50), 500 kg ha−1 in May 2014. In October 2014, beds were marked out (1.73 m apart) and a single row of strawberry mother plants were planted by hand in the beds (spaced 100 cm apart in the Fortuna trial and 50 cm apart in the Albion trial). Before planting, strawberry mother plants were soaked overnight in a 1:400 solution (dilution recommended by the manufacturer) of the seaweed extract or water (untreated control), which is a common industry practice. Two bands of a fertiliser blend (Pivot 800; Incitec Pivot, Portland, Victoria, Australia) were chiselled to a depth of 10 cm on either side of the transplant row at a rate of 900 kg ha−1. Top dress applications of fertilisers (Pivot 800 and Calcium Nitrate GHG; Campbells, Laverton, Victoria, Australia) were applied twice through the season (at rates of 900 and 50 kg ha−1, respectively). Water was applied by overhead sprinklers using the growers’ irrigation schedules. Runner plants were regularly de-flowered and daughter plants pinned into soil to encourage root production (approximately monthly), and plants sprayed with a rotation of registered pesticides to control mites, aphids, leaf hoppers, powdery mildew, grey mould and leaf blotch.
The trials were conducted as randomised complete block designs, with 16 blocks. Treatments consisted of a seaweed extract and an untreated control. The seaweed extract used in the trials was an alkaline hydrolysis product from D. potatorum and A. nodosum (Seasol®; Seasol International, Bayswater, Victoria, Australia). The product contains > 50% extract from D. potatorum with a soluble solid level of 16% (w/w) (Arioli et al. 2015). The seaweed extract was applied as a monthly drench (1:400 concentration) over the experimental plot and strawberry plants (including the foliage) at a rate of 10 L ha−1. Equivalent volumes of water were applied as the control. The first application was at planting and the last application was 2 weeks before harvest (March 2015 in the Fortuna trial and June 2015 in the Albion trial). In total, there were six applications of the seaweed extract in the Fortuna field trial (i.e. a total 0 (control) and 60 L of the seaweed extract ha−1) and eight applications in the Albion trial (i.e. a total of 0 (control) and 80 L of the seaweed extract ha−1). Individual plots were 5 m in length (an area of 8.65 m2) with 1-m guards between plots.
At final harvest (March 2015 and June 2015 for the Festival and Albion trials, respectively), all runners were dug out from a 1-m linear section of row from each plot. Runners were graded into firsts (runners with a crown diameter > 7.5 mm and a well-formed root system) and rejects as per commercial practice. Runners were harvested with leaves-on in the Fortuna trial and leaves-off in the Albion trial, which is the commercial standard for these cultivars. Following harvest, Albion runners were placed in cold storage (− 2 °C) until planting in a separate trial in the strawberry fruit sector (see below).
Parameters of runner quality (leaf number, area of the most expanded leaf, petiole length, crown diameter, root length, feeder roots score and density of feeder roots) were measured on a random subsample of ten harvested runners per plot. Petiole length, crown diameter and root length were measured with digital callipers, while the area of the most expanded leaf was determined with an area meter (Li-Cor LI300C with transparent belt accessory, USA). To determine the density of feeder roots (secondary roots), five randomly selected structural roots (primary roots) were cut from each runner. A 5 cm length, subsection of root was cut from the middle of each structural root. The number of feeder roots on each subsection of structural root was counted using a dissector microscope. Feeder roots scores were assessed using the scale described by Wing et al. (1995).
Soil samples were taken from each plot at planting and harvest, and analysed for soil nutrients through a commercial laboratory (Nutrient Advantage, Werribee, Victoria, Australia). In this procedure, 20 soil cores (2.5 cm diameter) were taken to a depth of 10 cm from each plot. Soil was thoroughly mixed and a subsample (500 g) taken for analysis.
Strawberry fruit trial
A field trial was conducted in the strawberry fruit sector at Coldstream, Victoria, Australia, in 2015/2016 with cold-stored runners (cultivar Albion) sourced from the strawberry nursery trial (see above). Soil at the site was a sodosol, with a clay–loam texture. Soil was prepared by rotary hoeing to depth of 25 cm and raising beds. In November 2015, soil was fumigated with chloropicrin (340 kg ha−1) and beds covered with black plastic (low-density polyethylene). During soil fumigation, two bands of a fertiliser blend (Pivot 800) were chiselled into soil to a depth of 10 cm at a rate of 800 kg ha−1 and trickle-irrigation tape laid under the plastic (two lines per bed).
Prior to planting, half the runners were soaked overnight in a 1:400 concentration of the seaweed extract and half in water (control). In December 2015, runners were planted by hand into plots through holes in the plastic (ca. 10-cm diameter). Beds were 1.02 m wide and there were four rows of strawberries per bed. Individual plants were spaced 40 cm apart. Plants were watered by regular overhead irrigation during establishment (ca. 1 month), after which plants were mostly irrigated by drip irrigation through the trickle tape. All other agronomical procedures in the trial followed standard industry practices.
Half of the plots and plants in the trial were treated with the seaweed extract (1:400 concentration) as a monthly drench (10 L ha−1) over the plant and soil in the planting holes. The other half of plots and plants were treated with equivalent volumes of water (control). The first application was at planting and the last application was 2 weeks (June 2016) before final harvest. In total, there were seven applications of the seaweed extract in the field trial (i.e. a total 0 (control) and 70 L of the extract ha−1). The trial was conducted as a randomised factorial (2 × 2 = 4 treatments) design with three blocks (20 plants per plot). Individual plots were 2 m in length (an area of 2.04 m2), with an 80-cm guard between plots. Treatments included (1) application of the seaweed extract in the nursery industry (two levels—the seaweed extract applied monthly in the nursery (2014/2015) or untreated (water)) and (2) application of the seaweed extract in the fruit industry (two levels—the seaweed extract applied monthly in the fruit sector (2015/2016) or untreated (water)).
Total and marketable fruit were counted and weighed one to three times per week from January to June 2016. Revenue from strawberry fruit for each pick was calculated from national wholesale prices for strawberry fruit (FreshLogic, Hawthorn, Victoria, Australia). At final harvest, soil cores (100 cm3) containing strawberry roots from one plant (to a soil depth of 20 cm) were taken (five per plot) using the method described by Yuen et al. (1991). All roots contained in the core samples were washed free from soil and placed into zip-lock polyethylene bags, quarter filled with water and sealed. Images of the roots in the bag were taken with a digital scanner. The total root length in the samples was calculated from the images using Sigmascan Pro 5.0 (Systat Software Inc., USA). Root samples were then oven dried at 80 °C for 4 days and their mass measured. Results were expressed as root length density (root length per volume of soil) and specific root length (root length per root dry weight).
Soil samples were taken from each plot at planting and at final harvest, as described previously, and analysed for soil nutrients through a commercial laboratory (Nutrient Advantage, Werribee, Victoria, Australia).
Data from the trials were statistically analysed using ANOVA on Genstat 18th ed. (VSN International). Homogeneity of variance was determined by examining plots of fitted values versus residuals, while histograms of residuals were examined for normality of distribution. Where variance was heterogeneous across treatments, appropriate data transformations were made to restore homogeneity. Fisher’s least significant difference (LSD) test was used to identify differences between treatment means. The level of significance used was p ≤ 0.05. Linear regression was used to test the relationships between root growth and strawberry runner and fruit yields.