Northern Highbush Blueberry Cultivars Differed in Yield and Fruit Quality in Two Organic Production Systems from Planting to Maturity

in HortScience

Northern highbush blueberry (Vaccinium corymbosum L.) cultivars were evaluated from planting (Oct. 2006) through 2014 in a certified organic research site in Aurora, OR. The treatments included cultivar (Duke, Bluecrop, Reka, Bluejay, Bluegold, Draper, Legacy, Liberty, Ozarkblue, and Aurora), amendment-mulch [“compost + sawdust” (included preplant amendment and a surface mulch of either an agricultural on-farm crop waste compost or yard-debris compost and sawdust), and “weed mat” (no preplant amendments but with a sawdust mulch topped with weed mat)]. Adding on-farm compost as a preplant amendment and as part of the mulching program increased soil pH from 4.9 to 6.9, organic matter content (OM), and calcium (Ca), magnesium (Mg), and potassium (K) levels compared with the weed mat treatment. The reduced plant growth and yield in some cultivars grown in the compost + sawdust treatment was likely due to the higher soil pH. ‘Bluegold’ and ‘Draper’ were among the cultivars with consistently high flower bud set (40% to 57%), whereas others had consistently low values (e.g., 22% to 45% in ‘Bluecrop’). The number of flowers per bud was affected only by cultivar. There was no effect of year or amendment-mulch treatment on percent fruit set which averaged 93% during the study; however, ‘Ozarkblue’ had a significantly lower fruit set (88%) than only ‘Aurora’ (96%). Berry weight was affected by year (plant age), cultivar, and amendment-mulch treatment. ‘Ozarkblue’ produced the largest berries. Type of amendment-mulch had little effect on berry weight, except in ‘Ozarkblue’, ‘Aurora’, and ‘Reka’ where plants grown with weed mat produced larger fruit than those grown with compost + sawdust. On average, ‘Bluejay’, ‘Draper’, and ‘Liberty’ fruit had the highest percent soluble solids (TSS) and ‘Ozarkblue’ the lowest. Fruit harvested from plants grown with weed mat were firmer than when compost + sawdust was used. ‘Draper’ fruit were much firmer than those of the other cultivars in all years of the study. The number of flower buds per plant multiplied by the number of flowers/bud and berry weight (cultivar specific) and average fruit set was a good predictor of yield in young plants. Yield per plant increased from the second through seventh growing seasons as plants matured in all cultivars except for ‘Duke’ which had the greatest yield in 2014. Cumulative yield was highest in ‘Legacy’ and lowest in ‘Bluejay’ and in ‘Draper’, which had relatively low yield when plants were young. Most cultivars had greater yield when grown with weed mat, whereas ‘Bluegold’ and ‘Liberty’ were unaffected by amendment-mulch treatment. Because weeds were managed in all plots, the cultivar response to amendment-mulch was likely a reflection of sensitivity to preplant amendment with on-farm compost and the resulting higher soil pH. It is possible that the cultivars differed in their adaptability to the various fertility regimes caused by the amendment-mulch treatments and fertilizers used in our study.

Abstract

Northern highbush blueberry (Vaccinium corymbosum L.) cultivars were evaluated from planting (Oct. 2006) through 2014 in a certified organic research site in Aurora, OR. The treatments included cultivar (Duke, Bluecrop, Reka, Bluejay, Bluegold, Draper, Legacy, Liberty, Ozarkblue, and Aurora), amendment-mulch [“compost + sawdust” (included preplant amendment and a surface mulch of either an agricultural on-farm crop waste compost or yard-debris compost and sawdust), and “weed mat” (no preplant amendments but with a sawdust mulch topped with weed mat)]. Adding on-farm compost as a preplant amendment and as part of the mulching program increased soil pH from 4.9 to 6.9, organic matter content (OM), and calcium (Ca), magnesium (Mg), and potassium (K) levels compared with the weed mat treatment. The reduced plant growth and yield in some cultivars grown in the compost + sawdust treatment was likely due to the higher soil pH. ‘Bluegold’ and ‘Draper’ were among the cultivars with consistently high flower bud set (40% to 57%), whereas others had consistently low values (e.g., 22% to 45% in ‘Bluecrop’). The number of flowers per bud was affected only by cultivar. There was no effect of year or amendment-mulch treatment on percent fruit set which averaged 93% during the study; however, ‘Ozarkblue’ had a significantly lower fruit set (88%) than only ‘Aurora’ (96%). Berry weight was affected by year (plant age), cultivar, and amendment-mulch treatment. ‘Ozarkblue’ produced the largest berries. Type of amendment-mulch had little effect on berry weight, except in ‘Ozarkblue’, ‘Aurora’, and ‘Reka’ where plants grown with weed mat produced larger fruit than those grown with compost + sawdust. On average, ‘Bluejay’, ‘Draper’, and ‘Liberty’ fruit had the highest percent soluble solids (TSS) and ‘Ozarkblue’ the lowest. Fruit harvested from plants grown with weed mat were firmer than when compost + sawdust was used. ‘Draper’ fruit were much firmer than those of the other cultivars in all years of the study. The number of flower buds per plant multiplied by the number of flowers/bud and berry weight (cultivar specific) and average fruit set was a good predictor of yield in young plants. Yield per plant increased from the second through seventh growing seasons as plants matured in all cultivars except for ‘Duke’ which had the greatest yield in 2014. Cumulative yield was highest in ‘Legacy’ and lowest in ‘Bluejay’ and in ‘Draper’, which had relatively low yield when plants were young. Most cultivars had greater yield when grown with weed mat, whereas ‘Bluegold’ and ‘Liberty’ were unaffected by amendment-mulch treatment. Because weeds were managed in all plots, the cultivar response to amendment-mulch was likely a reflection of sensitivity to preplant amendment with on-farm compost and the resulting higher soil pH. It is possible that the cultivars differed in their adaptability to the various fertility regimes caused by the amendment-mulch treatments and fertilizers used in our study.

The Pacific northwestern United States is an important region for production of cultivated blueberry (Vaccinium spp.) (U.S. Department of Agriculture, 2014). The proportion of total U.S. blueberry production grown on certified organic and exempt organic (less than $5000/year gross income and not requiring certification) farms was relatively small (3%) in 2008 (U.S. Department of Agriculture, 2010). However, the Pacific northwestern region accounted for 49% of the total blueberry organic area planted in the United States in 2008, when the last survey was conducted. The certified organic area has grown considerably since the last survey, increasing to an estimated 915 ha in Oregon and Washington in 2011, 55% of total U.S. organic blueberry area (Strik, 2014). By 2014, the organic blueberry area in Oregon and Washington accounted for about 20% of total blueberry area planted. Growth in organic production continues as consumer demand for organic products remains strong, and this region offers substantial advantages for organic production (DeVetter et al., 2015; Strik, 2016; Strik et al., 2016; Strik and Yarborough, 2005).

A wide range of cultivars are grown in this region for the fresh and processed markets, offering a range in fruiting seasons from the earliest (‘Duke’) to the latest (‘Aurora’) (Strik et al., 2014). The development of yield and fruit quality of the range of cultivars grown has not been compared in research studies, likely because of the relatively long time from planting to maturity (about 8 years). Differences in the performance of cultivars have been found in organic production of ‘Duke’ and ‘Liberty’ blueberry (Larco et al., 2013a, 2013b; Strik, 2016; Strik et al., 2016) and various blackberry cultivars (Fernandez-Salvador et al., 2015).

Blueberry plants are adapted to soils with low pH (4.5–5.5) and high OM (>4%) (Hart et al., 2006). Organic and conventional growers are interested in using composts because of hypothesized OM, nutrient, and microbiological benefits (e.g., Forge et al., 2003) on soil properties and nutrients. Because organic sources of nitrogen (N) are expensive and often laborious to apply, the potential benefit from a slow-release N from compost is also of great interest to growers. However, use of plant- and animal-based composts as a preplant amendment may be problematic in this crop as these materials have a high pH and often a high salt content (animal-based) (Sullivan et al., 2014).

Our objective was to characterize yield and associated plant and fruit quality traits of important highbush blueberry cultivars in the region from planting to maturity and evaluate their adaptation to common organic amendments and mulches used in certified organic production.

Materials and Methods

Study site.

The study was established in Oct. 2006 at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR (NWREC; lat. 45°28′ N, long. 122°76′ W). Weather data for this site are available from an AgriMet weather station (U.S. Deptartment Interior, 2014). The planting was certified as meeting the USDA organic criteria before the first fruit harvest year (2008) by a USDA-accredited agency (Oregon Tilth, Corvallis, OR). The soil at the site is a Willamette silt loam (fine-silty mixed superactive mesic Pachic Ultic Argixeroll) and had a pH of 4.9 and 3.7% OM. Details on preplant site preparation can be found in Larco et al. (2013a).

After adding any preplant amendments, if used (see below), raised beds were constructed using a bed shaper; beds were 0.3-m high and 0.4-m wide at the top and 1.5-m wide at the base when established, but settled to a height of ≈0.25 m by Autumn 2007. Plants were irrigated using a single line of polyethylene drip tubing (Netafim, Fresno, CA) with 2 L·h−1 pressure-compensating, inline emitters spaced every 0.3 m. The line was located along the row near the base of plants, under the mulch. Irrigation was controlled by electric solenoid valves and an automatic timer set weekly and scheduled to maintain a soil water content suitable for highbush blueberry production [25% to 30% soil water content from the soil surface to 0.3 m, based on time domain reflectometry measurements (SoilMoisture Equip. Corp., Santa Barbara, CA)] (L. Valenzuela-Estrada, unpublished data).

Amendment and mulch.

Two preplant amendment-mulch treatments were evaluated. The “compost + sawdust” treatment included a preplant amendment and a mulch of compost and sawdust. Commercially available agricultural crop waste compost (Wilt Farms, Corvallis, OR) made on farm (≈2-cm deep centered on the row; 76 m3·ha−1) and fresh douglas fir sawdust (Pseudotsuga menziesii M.; ≈5-cm deep; 200 m3·ha−1) were incorporated before forming the raised beds in Sept.–Oct. 2006. The on-farm compost consisted of well-composted rye grass clippings and other agricultural crop waste products and had lime (107 kg·t−1 dry weight) added during the composting process based on the carbonate content of the raw material (note Ca content in Table 1). Immediately after planting, the beds were mulched with additional on-farm compost (≈2-cm deep; 76 m3·ha−1), and then, the compost was topped with sawdust (≈7.5-cm deep; 300 m3·ha−1) with a goal of creating a barrier to weed establishment. The mulches were spread mechanically in 0.75 m wide strips under and on each side of the plant rows. On-farm compost was used when the mulch needed replenishing in Autumn 2007, but in Jan.–Feb. 2011 and 2013, compost made with municipal yard waste clippings was used (Rexius, Eugene, OR). The composts and sawdust were analyzed by Soil Control Laboratory (Watsonville, CA) and total nutrient application was calculated (Table 1).

Table 1.

Total nutrients applied as an amendment-mulch (“compost + sawdust”) and as a mulch (“weed mat”) at establishment (2006) and when the mulch needed replenishment (2011 and 2013, compost + sawdust treatment only) in an organic blueberry field at Oregon State University’s North Willamette Research and Extension Center (Aurora, OR), 2006–14. Sawdust replenishment was not required in the weed mat treatment after establishment. Composts and sawdust were analyzed by Soil Control Laboratory (Watsonville, CA).

Table 1.

The second amendment-mulch treatment, “weed mat,” involved no preplant amendments, but included a mulch of fresh douglas fir sawdust (≈7.5-cm deep; 300 m3·ha−1) topped with black, woven polyethylene groundcover (water flow rate of 6.8 L·h·m−2 and a density of 0.11 kg·m−2 as measured by the manufacturer; TenCate Protective Fabrics, OBC Northwest, Canby, OR). Weed mat was placed over a sawdust mulch layer, not currently a commercial practice, with a goal of offsetting the reduction in soil OM observed under weed mat mulch in long-lived perennial crops such as apple (Malus ×domestica Borkh.) (Choi et al., 2011). The weed mat was 1.5-m wide and was centered over the planting beds before securing it in place with landscape staples. A 20-cm diameter hole was cut in the weed mat for each plant and the area covered with 5 cm of douglas fir sawdust mulch (1.4 m3·ha−1) after planting. The weed mat was replaced in Dec. 2010 with a similar product and was installed as a “zippered” system (overlapping and secured with landscape staples) such that the weed mat could be opened to apply any needed granular fertilizers. The sawdust mulch under the weed mat was still at an adequate depth (5–10 cm) and did not require replenishment in 2010.

Cultivars.

Ten cultivars (Duke, Bluecrop, Reka, Bluejay, Bluegold, Draper, Legacy, Liberty, Ozarkblue, and Aurora) were selected to represent a range in fruiting or harvest season and are important commercial cultivars in the Pacific northwestern United States (Strik et al., 2014). Standard, 18-month-old container stock (3.8 L), with two to four whips per plant, was purchased from a commercial nursery and transplanted into the field at an in-row spacing of 0.75 m and a between-row spacing of 3.0 m (4385 plants/ha) in early Oct. 2006.

Experimental design.

Each experimental unit consisted of a 5-m plot containing seven plants. The experimental design was a split-plot with amendment-mulch treatment as the main effect and cultivar as the subplot effect. The main effect (amendment-mulch) was limited to two replications because of the size and duration (cost) of the study and because the study was conducted in the guard rows of a 0.5-ha certified organic research planting established concurrently.

Planting management.

The rate of fertilizer nutrients and the products applied to each amendment-mulch treatment in 2007–14 are presented in Table 2. Two fertilizer sources were used to achieve the total rate of N applied. Half of the N was applied as fish emulsion by hand as a drench around the base of the plants in 2007–09, sidedressed with a sprayer on each side of the row in 2010, and injected through the drip system (fertigated) in 2011–14 in seven equal applications every 2 weeks from mid-April to early July. The remaining N was applied using a granular soybean [Glycine max (L.) Merr.] meal applied in early March on top of the compost + sawdust mulch or under the weed mat. In addition, 258 kg·ha−1 of Ca (Ca sulfate applied as gypsum), 5.5 kg·ha−1 of Mg, and 7.3 kg·ha−1 of sulfur (S, as Mg sulfate) were applied to all plots in late Winter 2013. Elemental S was applied to the surface of the compost + sawdust mulch plots on May 2012 (335 kg·ha−1) when soil pH had increased to 6.7 and in Feb. 2013 (112 kg·ha−1) when pH was 5.8. No S was applied to the weed mat treatment as soil pH was within the recommended range of 4.5 to 5.5 (Hart et al., 2006).

Table 2.

Targeted fertilizer rates and actual nutrients applied, based on nutrient analysis (Brookside Laboratories Inc., New Bremen, OH) in an organic blueberry field at Oregon State University’s North Willamette Research and Extension Center (Aurora, OR), 2007–14.

Table 2.

The permanent grass cover crop (certified organic Festulolium braunii K. Richt.) in the alleys was mowed during the growing season as required. Weeds were managed using OMRI-approved (Organic Materials Review Institute) postemergent acetic acid (20%; WeedPharm®, Pharm Solutions, Inc., Port Townsend, WA), lemon grass oil or d-limonene (GreenMatch EX® and Avenger®, respectively; Cutting Edge Formulations, Inc., Buford, GA), and by hand weeding, as required in all treatment plots.

In 2011–14, the planting was sprayed weekly with a spinosad (metabolites of Saccharopolyspora spinosa Mertz & Yao) insecticide (EntrustSC; Dow Agro Science, Indianapolis, IN) or pyrethrins (PyganicEC 1.4; Valent, Walnut Creek, CA) from the time when ‘Liberty’ fruit first turned blue through harvest, to help control spotted wing drosophila (Drosophila suzukii Matsumura) per recommendations (DeFrancesco et al., 2014); applications to the earlier-season cultivars (e.g., Duke) were not required as insect populations were very low. In general, management for other pests was not required although scouting for any presence was done regularly. Some cultivars were affected by Blueberry shock virus (BlShV); the proportion of the plot showing symptoms (necrotic flowers) and thus not producing fruit was estimated annually. This pollen-borne virus was unaffected by amendment-mulch treatment, but cultivars do vary in their sensitivity (Pscheidt and Harper, 2014).

Plants were pruned each winter to maintain a balance of vegetative growth and fruit production (Strik and Buller, 2005; Strik et al., 1990, 1993).

Data collection.

From 2008 to 2013, counts of vegetative and flower buds were taken on four lateral shoots (0.15–0.45 m long originating from 2-year-old wood) per plot and percent flower bud set was calculated (number of flower buds/total buds). In blueberry, buds are simple and flower buds may be distinguished from vegetative buds because of their relatively large size. In 2009, the number of flower buds was counted on two plants per plot after pruning and average flower buds/plant calculated. Four flower buds per plot (the third bud distal of the tip of the lateral) were randomly selected and labeled; the number of flowers/bud (per cluster) was counted at the late pink to 5% bloom stage and then the number of berries/cluster was counted in mid to late May and percent fruit set calculated. In 2009, yield/plot was estimated as follows: Estimated yield (kg/plant) = [no. flower buds/plant × no. flowers/bud × percent fruit set × average berry weight (g)/1000]; values were compared with actual harvested yield.

Ripe fruit were harvested by hand every 7–14 d from the second (2008; except for ‘Aurora’ that was first harvested in 2009 when plants were more vigorous) through the eighth (2014) growing seasons from the entire plot of each experimental unit and yield measured; yield per plant was calculated. A 25-berry subsample was taken per plot per harvest to determine average berry weight (and a weighted seasonal average mass was then calculated) and berry firmness (FirmTech II; BioWorks, Inc.; Wamego, KS). The fruit were then macerated by hand, in a zippered plastic bag and TSS measured on a temperature-compensated digital refractometer (Atago, Bellevue, WA).

Soil samples were collected in 2006, before planting and annually in late Oct. to early Nov. 2010–14 from all ‘Duke’ plots. Samples were pooled in 2006 and in 2014 (not replicated). Soil samples were collected using a 2.4-cm diam., 0.5-m long, slotted, open-side, chrome-plated steel soil probe (Soil Sampler Model Hoffer; JBK Manufacturing, Dayton, OH). Mulch was removed from the soil surface before sampling to a depth of 0.2 m at the center of the row, between plants and within the water emitter drip zone or fertilization area. Soil samples were analyzed for macro- and micronutrient content, OM, and pH by Brookside Laboratories (New Bremen, OH).

Data analysis.

Analysis of amendment-mulch and cultivar effects on plant traits and yield was done for a split plot design by year and for cumulative yield (during the length of the study) using the PROC MIXED procedure in SAS version 9.3 (SAS Institute, Cary, NC). The effect of year was analyzed using a split-split plot design [year as the main effect (n = 7 or 8, depending on the trait), amendment-mulch as the subplot effect (n = 2), and cultivar as the sub-subplot effect (n = 10)] for all plant and fruit traits except yield per plant, which was expected to change as plants matured. We did analyze whether yield significantly increased from year 7 to 8 (2013–14) for all cultivars. Mean comparison was performed using least-square means. The relationship between estimated yield and actual yield in 2009 was analyzed by linear regression, using a best fit as well as forcing the intercept through zero. Soil analysis results were analyzed for the effect of amendment-mulch, by year (2010–13) using PROC MIXED in SAS.

Results and Discussion

Soil properties.

A pooled soil sample taken before planting indicated that soil pH, OM, and all nutrients except for Ca (lower than recommended) were at appropriate levels (Table 3) for blueberry (Hart et al., 2006). No fertilizers or amendments were applied to the whole field before establishing the treatments and planting. We analyzed the composts and sawdust used in our amendment-mulch treatments for pH and nutrient content and calculated the total nutrients applied throughout the study (Table 1). The slow-release N provided by the compost was estimated at roughly 25 kg·ha−1·yr−1 (3% of total compost-N applied), based on experience with similar composts applied before planting in grass (Sullivan et al., 2003) and sweet corn (Gale et al., 2006). However, the proportion or rate of availability of the nutrients applied in the composts or sawdust (Table 1) is not known. Although we have not yet determined impacts of mulches on soil microbiology, we noted differences in the impact of the amendment-mulch treatments on soil properties and nutrients (Table 3) (Larco et al., 2013b, 2014). The on-farm compost used as a preplant amendment was prepared using lime and thus had a high Ca content (Table 1) and a high pH (7.5). We chose this product because it was being commonly sold to commercial blueberry growers for use in new and established fields. In our study, incorporating this product before planting increased soil pH from 4.9 to 6.9 in 4 years (2010) such that the compost + sawdust treatment had a significantly higher soil pH than the weed mat (Table 3). Application of S in 2011–12 (Table 2) reduced the soil pH from 6.8 to 5.9 the following year. Whereas there was no significant amendment-mulch effect on soil pH in 2012–13, there were only two replications. It is important to note that soil pH in the compost + sawdust treatment remained above the desirable range for blueberry (4.5–5.5; Hart et al., 2006), whereas it was within this range in the weed mat treatment throughout the study (Table 3). Larco et al. (2013a) found lower soil pH under weed mat mulch than under a yard-debris compost + sawdust mulch in organic blueberry. However, use of a yard-debris compost only as part of a mulching program had benefits for mitigating the decline in soil pH that occurs with fertilization over time, while maintaining soil pH within the desired range for blueberry (Larco et al., 2013a; Strik, 2016). In our study, the on-farm compost likely had a rapid effect on increasing soil pH because of the lime effect. Composts without added lime may not have a similar effect when used as a preplant amendment.

Table 3.

Soil nutrient properties by amendment-mulch treatment in an organic blueberry field at Oregon State University’s North Willamette Research and Extension Center (Aurora, OR), 2006–14.

Table 3.

The amendment-mulch treatments led to differences in soil pH, OM, and particularly levels of soil Ca, Mg, and K (Table 3). Despite these differences in soil properties, Strik and Vance (2015) noted no effect of amendment-mulch on leaf nutrient concentrations in this same research planting other than for manganese (Mn) and aluminum (Al) whose availability is decreased at high soil pH. Therefore, although compost supplied a high rate of total N (Table 1), and an estimated 25 kg·ha−1 N released per year of the study, it apparently supplied low amounts of plant-available N, or the added N was not required by the plants. Larco et al. (2013b, 2014) also found that yard-debris compost provided little to no plant-available N when used as part of a mulching program in organic blueberry production. In our study, the nutrients applied via fertilization (Table 2) were thus considered sufficient for plant growth based on plant tissue analysis (Strik and Vance, 2015) and observed growth. The high pH in the organic amendment-mulch treatment, however, likely reduced plant growth and yield in some cultivars (see below).

Plant traits.

The effects of year, mulch, and cultivar on the proportion of flower buds per lateral (flower bud set), flowers per cluster, and percent fruit set are shown in Table 4. There was no main effect of year on any of these variables, but there was a year × cultivar interaction on flower bud set. While some cultivars had consistently high flower bud set from 2008 through 2013 such as ‘Bluegold’ (46% to 57%) and ‘Draper’ (40% to 51%) or consistently low values such as for ‘Bluecrop’ (22% to 45%), ‘Duke’ had high flower bud set in 2008 (57%) followed by a particularly low value in 2009 (40%) and was again among the cultivars with the highest flower bud set in 2012–13 (48% to 52%; data not shown). ‘Bluecrop’ was also reported to have lower flower bud set than other cultivars in establishing and conventionally managed mature blueberry plantings/trials (Strik and Buller, 2005; Strik et al., 2003). On average, ‘Bluegold’ and ‘Draper’ had the highest percentage of flower bud set, whereas ‘Legacy’, ‘Ozarkblue’, and ‘Bluecrop’ had the lowest (Table 4). There was no effect of amendment-mulch treatment on flower bud set.

Table 4.

Effect of amendment-mulch treatment and cultivar on percentage of flower buds per lateral, flowers/cluster, fruit set, fruit quality and yield from 2008 to 2014 in an organic blueberry field at Oregon State University’s North Willamette Research and Extension Center (Aurora, OR).

Table 4.

There was a strong cultivar effect on the number of flowers per bud (flowers/cluster) with no effect of year or amendment-mulch treatment (Table 4). In Michigan, there was some variability in flowers/bud across years (Hancock et al., 2000). Some of the variability in flowers/bud within a cultivar among years and studies may be caused by climate (location), sampling location in the bush, and location of the bud on the lateral (Almutairi, 2016; Hancock et al., 2000). In our study, the consistency of bud sampling likely reduced variation among years. ‘Ozarkblue’ and ‘Bluecrop’ had the highest number of flowers/bud, whereas ‘Bluegold’, ‘Duke’, and ‘Legacy’ had among the fewest. In Michigan, ‘Bluecrop’ and ‘Bluejay’ did not differ in flowers/bud (7.1–7.8), but these cultivars had among the lowest values of six cultivars studied (Hancock, 1989).

Fruit set among the cultivars was similar, averaging 93% during the study (Table 4), except for ‘Aurora’ and ‘Ozarkblue’, which averaged 96% and 88%, respectively. It is possible that ‘Ozarkblue’ had a particularly low fruit set, because it also had a high number of flowers/cluster that may limit resources. Fruit set in our study was higher than the 78% to 90% and 72% to 87% reported by Strik et al. (2003) and Hancock et al. (2000) in ‘Bluecrop’ grown in Oregon and Michigan, respectively. It is possible that we counted berries per cluster and calculated fruit set before any berry drop that might have occurred; in some cultivars, such as ‘Liberty’, small berries may fall from the cluster reducing final fruit set (Almutairi, 2016). Percent fruit set is typically high in the Willamette Valley, OR, as compared with other production regions (e.g., Hancock, 1989) because of consistently favorable weather during bloom.

Yield.

In 2009, when the plants were going into their third growing season, the total number of flower buds was counted to determine if this could be a good predictor of yield on young plants and be used as a tool to adjust severity of pruning; this method would likely be impractical on larger plants. The number of flower buds per plant was affected by cultivar (P < 0.001), but not amendment-mulch (data not shown). ‘Draper’ had the fewest average number of buds per plant (95), likely because these plants were short in stature because of their propensity toward a compact growth and/or an immature plant response to propagation by tissue culture. ‘Aurora’, ‘Duke’, ‘Bluejay’, and ‘Bluecrop’ had a moderate level of flower buds per plant (127–165), whereas ‘Reka’, ‘Liberty’, and ‘Legacy’ had the highest number (213–279). Too many plants of ‘Bluegold’ and ‘Ozarkblue’ were affected by BlShV or a somatic mutation (small leaves and berries) in 2009, respectively, to be able to get an accurate count of flower buds, so they were not included.

Although there was a significant linear relationship between predicted yield (based on the number of buds/plant) and actual yield, the strength of this relationship was reduced when the line was forced to have no intercept (Fig. 1). Yield was overestimated (slope <1) using either formula, particularly at higher bud numbers per plant, as has been found by others (Salvo et al., 2012). However, some yield loss would be expected because of a proportion of fruit being dropped during picking and pests (e.g., birds and possibly disease). In our study, the relationship between the number of flower buds/plant and yield, when considering the flowers/bud and berry weight (cultivar dependent) and typical fruit set for the region shows promise as a tool to adjust pruning and predict yield in young plants (Fig. 1). This relationship would be improved with the development of cultivar-specific formulae if more data points were available.

Fig. 1.
Fig. 1.

Scatterplot of estimated yield/plant {estimated yield = [flower buds/plant × flowers/bud × percent fruit set × average berry weight (g)]/1000} and actual measured yield/plant for eight cultivars grown in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR. There were insufficient data to include ‘Ozarkblue’ and ‘Bluejay’. Linear relationships shown for best fit and when forcing a 0 intercept.

Citation: HortScience horts 52, 6; 10.21273/HORTSCI11972-17

Yield per plant increased from the second through seventh growing seasons as plants matured (Fig. 2). Cultivar significantly affected yield in each year of the study. ‘Aurora’ did not produce its first crop until 2009, because plants were pruned to produce fruit in their second growing season only if they were vigorous enough; cropping plants in the second year can reduce plant growth and subsequent yield (Strik and Buller, 2005). The other cultivars were considered vigorous enough and were pruned to produce a limited crop in 2008. Plantings in this region are considered mature in year 7 or 8; our data agree with these findings as there was no significant increase in yield from 2013 to 2014, with the exception of ‘Duke’. Yield can certainly differ among years for many reasons. For example, Nemeth et al. (2017) reported increased plant growth (biomass) and yield from year 9 to 10 in ‘Elliott’ blueberry. Based on our experience, once plants reach 8–10 years of age, yield tends to fluctuate about a mean typical for the cultivar with pruning having the largest influence on yield in our region.

Fig. 2.
Fig. 2.

Effect of cultivar on harvested yield per plant from 2008 to 2014 (note ‘Aurora’ was not harvested in 2008) in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR.

Citation: HortScience horts 52, 6; 10.21273/HORTSCI11972-17

Cumulative yield, on average, was highest in ‘Legacy’ (Table 4), a cultivar that is widely grown in Oregon’s Willamette Valley. Yields of 35–40 t·ha−1 are common in mature commercial ‘Legacy’ fields that are well-managed (conventional or organic; B.C. Strik, personal observation); these yields are equivalent to what we harvested in 2013 and 2014 (Fig. 2). ‘Ozarkblue’ had the next highest cumulative yield. However, yield per plant in this cultivar was higher than what would be expected at an in-row spacing of 0.75 m because several plants per plot had a somatic mutation and had to be cut back to just above crown height, so they would not be harvested; this mutation likely occurred during tissue culture propagation and is the main reason this cultivar is no longer readily available from commercial nurseries. The remaining ‘Ozarkblue’ plants had more space to grow within the plots and thus yield per plant was higher. The lowest cumulative yield, on average, was measured in ‘Draper’, which had relatively low yield when plants were young (small bushes that needed hard pruning), and ‘Bluejay’. The mature yield (2013 and 2014) of most of the cultivars we studied was similar to what is typically harvested in mature conventionally managed fields (B.C. Strik, personal observation). In a cultivar evaluation trial in Missouri, ‘Legacy’ and ‘Reka’ were among the highest yielding, ‘Ozarkblue’ and ‘Bluecrop’ were intermediate, and ‘Duke’ was in the lowest yielding group in conventional production (Kaps et al., 2010).

During the establishment years, amendment-mulch treatment only had a significant effect on yield in 2009 when those grown with weed mat averaged 2.1 compared with 1.6 kg/plant when compost + sawdust were used as the amendment-mulch (data not shown). Larco et al. (2013b) also reported a higher yield in the second growing season when ‘Duke’ and ‘Liberty’ were grown with a weed mat mulch (no sawdust present) as compared with a sawdust mulch alone, but found no difference between weed mat and a compost + sawdust mulch (with no preplant amendment). In our study, there was only a cultivar by amendment-mulch interaction on yield/plant in 2011 as all cultivars had greater yield with weed mat except for ‘Bluegold’ (data not shown).

There was a significant cultivar × amendment-mulch effect on cumulative yield/plant (Table 4; Fig. 3). Most cultivars had greater yield when grown with weed mat, whereas ‘Bluegold’ and ‘Liberty’ were unaffected by amendment-mulch treatment over the length of the study. The presence of weeds has been shown to reduce yield, and mulch often improves yield when compared with unweeded plots (Burkhard et al., 2009). In our study, weed management costs were considerably higher in the compost + sawdust treatment because of greater weed presence as compared with weed mat mulch (data not shown). We found similar results in another organic study (Julian et al., 2012) as have others (Tertuliano et al., 2012). Because weeds were managed in all plots in our study, the cultivar response to amendment-mulch type was likely a reflection of sensitivity to the preplant amendment and resulting higher soil pH (Table 3). For example, ‘Duke’ is known to perform poorly in high pH soils, whereas ‘Reka’ is much more tolerant (B.C. Strik, personal observation). While we observed symptoms of soil pH being too high in ‘Duke’ (lime-induced iron chlorosis), no symptoms were apparent in any of the other cultivars. It is possible that the cultivars differed in their adaptability to the various fertility regimes caused by the amendment-mulch treatments and fertilizers used in our study. Larco et al. (2013a, 2013b) and Strik (2016) observed differences in plant growth and yield between ‘Duke’ and ‘Liberty’ when fertilized with fish emulsion or feather meal in a certified organic production system.

Fig. 3.
Fig. 3.

Effect of amendment-mulch treatment (“compost + sawdust” = preplant amendment with on-farm crop waste compost and sawdust plus a mulch of on-farm and yard-debris compost, depending on year, topped with sawdust; “weed mat” = sawdust mulch topped with porous black polyethylene groundcover) and cultivar on cumulative yield/plant (2008–14; except for ‘Aurora’ which was not harvested in 2008) in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR. se bars provided. Cultivars are listed in approximate order of ripening.

Citation: HortScience horts 52, 6; 10.21273/HORTSCI11972-17

Incorporation of a preplant amendment, particularly douglas fir sawdust in our region, is common (Julian et al., 2011a, 2011b) and has been shown to improve growth and production over the long-term compared with no preplant amendment in conventionally managed blueberry fields (Nemeth et al., 2017). Our study confirms the importance of testing any amendments before use in blueberry and either not using those with a high pH or acidifying them before use (Costello, 2011).

Fruit traits.

Berry weight was affected by year (plant age), cultivar, amendment-mulch, and the interaction of year × cultivar and mulch × cultivar (Table 4). On average, ‘Ozarkblue’ had the largest berries (Table 4). ‘Bluegold’ and ‘Bluejay’ tended to have consistently large and small fruit, respectively, throughout the study (data not shown). In 2008, the first fruiting season, ‘Ozarkblue’ and ‘Liberty’ plants produced particularly large berries. ‘Draper’, ‘Ozarkblue’, and ‘Aurora’ produced relatively large berries in 2010. ‘Reka’ had particularly small berries in 2012, even though the plants did not appear to be overcropped (Fig. 2). Berry size in ‘Liberty’ fluctuated considerably from year to year (data not shown), even though plants were consistently pruned and there appeared to be little relationship between berry weight and yield, as is commonly found in blueberry (Strik et al., 2003). ‘Liberty’ tends to produce canes that look good at pruning time, but then proceeds to produce very small berries; growers call this “little berry,” and this trait could have reduced average berry weight in this cultivar in our study. Even though plants were pruned relatively consistently from year to year, with a goal of balancing vegetative growth with fruiting potential (Strik et al., 1990, 1993), variability in pruning within cultivars from year to year could have caused differences in berry weight (Hancock and Nelson, 1985; Siefker and Hancock, 1987; Strik et al., 2003). The berry weights we found throughout the study period were higher than those reported for these same cultivars in some other studies (Hancock, 1989; Hancock et al., 2000; Kaps et al., 2010).

Although there was a significant amendment-mulch × cultivar interaction for berry weight (Table 4), the effect was small; only ‘Ozarkblue’, ‘Aurora’, and ‘Reka’ produced heavier fruit (≈10%) when grown with weed mat as compared with the compost + sawdust amendment-mulch treatment (data not shown). Larco et al. (2013a) also noted cultivar differences in response to mulch type.

There was a year × cultivar interaction on TSS (Table 4). ‘Bluejay’ had the highest TSS in most years of the study (data not shown), likely because this cultivar is typically machine harvested for processing, and fruit were thus left to hang before harvest; the entire crop was harvested in two pickings, and the fruit were thus ripe to overripe. ‘Ozarkblue’ fruit had the lowest TSS in many years. Most cultivars had a higher fruit TSS when they were young and in their second growing season, except for ‘Duke’, ‘Reka’, ‘Aurora’, and ‘Ozarkblue’ (data not shown). On average, ‘Bluejay’, ‘Draper’, and ‘Liberty’ fruit had the highest TSS and ‘Ozarkblue’ the lowest (Table 4). There was little difference in TSS among the other cultivars.

Berry firmness was affected by year, cultivar, amendment-mulch, and the interaction of year and cultivar (Table 4). The fruit harvested from plants grown with weed mat were firmer than when the compost + sawdust amendment-mulch was used. ‘Draper’ fruit were much firmer than those of the other cultivars in all years of the study (data not shown). ‘Duke’ fruit were next firmest, except in 2012 when ‘Bluegold’ produced firmer fruit than ‘Duke’. ‘Reka’ and ‘Bluecrop’ produced the least firm fruit in 2011 and 2013, respectively. While fruit firmness was relatively similar in 2010 and 2011 in most cultivars, firmness increased in ‘Ozarkblue’ and ‘Aurora’. On average, ‘Draper’ fruit were the firmest and ‘Bluecrop’ and ‘Reka’ the softest (Table 4).

Conclusions

All of the cultivars studied grew and produced well under the amendment-mulch treatments used during the 8-year study. However, cultivars differed in their apparent tolerance to the higher soil pH resulting from using the on-farm compost, which contained lime, as a preplant amendment. Maintaining soil pH within the current recommended standard of 4.5–5.5 is thus important for many cultivars for optimal growth and production. Cultivars differed relatively consistently in percent flower bud set and the number of flowers per bud throughout the study, whereas percent fruit set was consistently high for all cultivars. Predicting yield in young plants is possible by counting the number of flower buds per plant and using cultivar-specific data on the number of flowers/bud and berry weight along with a typical fruit set value for the region; however, yield was overestimated using this method. Yield increased from the second to the seventh growing season in most cultivars, but yield continued to increase between year 7 and 8 in ‘Duke’ indicating these plants may not yet have been mature. Average cumulative yield was highest for ‘Legacy’ and lowest for ‘Draper’ and ‘Bluejay’. Most cultivars had lower yield when grown in plots amended and mulched with on-farm crop waste compost than when grown with weed mat; this was likely a negative response to the high pH of the on-farm compost, as weeds were controlled in all treatments. On average, plants grown with the weed mat mulch produced firmer and larger berries than those grown with the compost + sawdust amendment-mulch. The differences among cultivars in the measured fruit quality parameters were similar to what has been observed in conventional production systems.

Literature Cited

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    • Search Google Scholar
    • Export Citation
  • LarcoH.BrylaD.R.StrikB.C.SullivanD.M.2013bWeed and fertilizer management practices for organic production of highbush blueberries—II. Impact on plant and soil nutrients, yield, and fruit quality during establishmentHortScience4814841495

    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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  • StrikB.BullerG.2005The impact of early cropping on subsequent growth and yield of highbush blueberry in the establishment years at two planting densities is cultivar dependantHortScience4019982001

    • Search Google Scholar
    • Export Citation
  • StrikB.BullerG.HellmanE.2003Pruning Severity Affects Yield, Berry Weight, and Picking Efficiency of Highbush BlueberryHortScience38196199

    • Search Google Scholar
    • Export Citation
  • StrikB.C.FinnC.E.MooreP.P.2014Blueberry cultivars for the Pacific Northwest. Oregon. Ore. State Univ. Ext. Serv. PNW 656. 15 Dec. 2016. <http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/45871/pnw656.pdf>.

  • StrikB.C.VanceA.2015Seasonal variation in leaf nutrient concentration of northern highbush blueberry cultivars grown in conventional and organic production systemsHortScience5014531466

    • Search Google Scholar
    • Export Citation
  • StrikB.C.VanceA.BrylaD.2016Organic production systems research in blueberry and blackberry – A review of industry-driven studiesActa Hort.1117139148

    • Search Google Scholar
    • Export Citation
  • StrikB.C.YarboroughD.2005Blueberry production trends in North America, 1992 to 2003 and predictions for growthHortTechnology15391398

  • SullivanD.M.BaryA.I.NarteaT.J.MyrheE.A.CoggerC.G.FransenS.C.2003Nitrogen availability seven years after a high-rate food waste compost applicationCompost Sci. Util.11265275

    • Search Google Scholar
    • Export Citation
  • SullivanD.M.BrylaD.R.CostelloR.C.2014Chemical characteristics of custom compost for highbush blueberry p. 293–311. In: Z. He and H. Zhang (eds.). Applied Manure and Nutrient Chemistry for Sustainable Agriculture and Environment Springer-Verlag New York NY

  • TertulianoM.G.KrewerSmithJ.E.PlattnerK.ClarkJ.JacobsJ.AndrewsE.StanalandD.AndersenP.LiburdO.FonsahE.G.SchermH.2012Growing organic rabbiteye blueberries in Georgia, USA: Results of two multi-year field studiesIntl. J. Fruit Sci.12205215

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If the inline PDF is not rendering correctly, you can download the PDF file here.

Contributor Notes

The authors appreciate the valuable assistance of Gil Buller, former Senior Faculty Research Assistant and Emily Vollmer, former Faculty Research Assistant at the North Willamette Research and Extension Center, OSU. We appreciate the funding support provided by the Oregon Blueberry Commission.

Professor.

Faculty Research Assistant.

Research Geneticist.

Corresponding author. E-mail: bernadine.strik@oregonstate.edu.

  • View in gallery

    Scatterplot of estimated yield/plant {estimated yield = [flower buds/plant × flowers/bud × percent fruit set × average berry weight (g)]/1000} and actual measured yield/plant for eight cultivars grown in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR. There were insufficient data to include ‘Ozarkblue’ and ‘Bluejay’. Linear relationships shown for best fit and when forcing a 0 intercept.

  • View in gallery

    Effect of cultivar on harvested yield per plant from 2008 to 2014 (note ‘Aurora’ was not harvested in 2008) in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR.

  • View in gallery

    Effect of amendment-mulch treatment (“compost + sawdust” = preplant amendment with on-farm crop waste compost and sawdust plus a mulch of on-farm and yard-debris compost, depending on year, topped with sawdust; “weed mat” = sawdust mulch topped with porous black polyethylene groundcover) and cultivar on cumulative yield/plant (2008–14; except for ‘Aurora’ which was not harvested in 2008) in an organic planting at Oregon State University’s North Willamette Research and Extension Center, Aurora, OR. se bars provided. Cultivars are listed in approximate order of ripening.

  • AlmutairiK.2016Water and soil management practices to enhance plant growth berry development and fruit quality of northern highbush blueberry (Vaccinium corymbosum L.). Ore. St. Univ. Corvallis OR PhD Thesis. 13 June 2017. <http://ir.library.oregonstate.edu/xmlui/handle/1957/60028>.

  • BurkhardN.LynchD.PercivalD.SharifiM.2009Organic mulch impact on vegetation dynamics and productivity of highbush blueberry under organic productionHortScience44688696

    • Search Google Scholar
    • Export Citation
  • ChoiH.-S.RomC.R.GuM.2011Plant performance, and seasonal soil and foliar nutrient variations in an organic apple orchard under four ground cover management systemsJ. Amer. Pomol. Soc.65130146

    • Search Google Scholar
    • Export Citation
  • CostelloR.C.2011Compost acidification increases growth and nutrient uptake of highbush blueberry under a low N fertilizer regime. Ore. St. Univ. Corvallis OR MS Thesis. 20 May 2015. <http://hdl.handle.net/1957/26590>.

  • DeFrancescoJ.PscheidtJ.W.YangW.2014Blueberry pest management guide for the Willamette Valley. Oregon State Univ. Ext. Ser. Pub. EM 8538

  • DeVetterL.W.GranatsteinD.KirbyE.BradyM.2015Opportunities and challenges of organic highbush blueberry production in Washington StateHortTechnology25796804

    • Search Google Scholar
    • Export Citation
  • Fernandez-SalvadorJ.StrikB.C.ZhaoY.FinnC.E.2015Trailing blackberry genotypes differ in yield and post-harvest fruit quality during establishment in an organic production systemHortScience50240246

    • Search Google Scholar
    • Export Citation
  • ForgeT.A.HogueE.NeilsenG.NeilsenD.2003Effects of organic mulches on soil microfauna in the root zone of apple: Implication for nutrient fluxes and functional diversity of the soil food webAppl. Soil Ecol.223954

    • Search Google Scholar
    • Export Citation
  • GaleE.S.SullivanD.M.HemphillD.CoggerC.G.BaryA.I.MyhreE.A.2006Estimating plant-available nitrogen release from manures, composts, and specialty productsJ. Environ. Qual.3523212332

    • Search Google Scholar
    • Export Citation
  • HancockJ.F.1989Why is ‘Elliot’ so productive? A comparison of yield components in 6 highbush blueberry cultivarsFruit Var. J.43106109

    • Search Google Scholar
    • Export Citation
  • HancockJ.F.CallowP.KeeslerR.PrinceD.BordelonB.2000A crop estimation technique for highbush blueberryJ. Amer. Pomol. Soc.54123129

  • HancockJ.F.NelsonJ.W.1985Factors influencing yields of Vaccinium corymbosum L. in MichiganActa Hort.165107113

  • HartJ.StrikB.WhiteL.YangW.2006Nutrient management for blueberries in Oregon. Ore. State Univ. Ext. Serv. EM 8918. 15 Dec. 2016. <http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/20444/em8918.pdf>.

  • JulianJ.StrikB.YangW.2011aBlueberry economics: The costs of establishing and producing blueberries in the Willamette Valley Oregon. AEB 0022. 15 Dec. 2016. <http://arec.oregonstate.edu/oaeb/files/pdf/AEB0022.pdf>.

  • JulianJ.StrikB.PondE.YangW.2011bBlueberry economics: The costs of establishing and producing organic blueberries in the Willamette Valley Oregon. Oregon State Univ. Pub. AEB 0023. 15 Dec. 2016. <http://arec.oregonstate.edu/oaeb/files/pdf/AEB0023.pdf>.

  • JulianJ.W.StrikB.C.LarcoH.O.BrylaD.R.SullivanD.M.2012Costs of establishing organic northern highbush blueberry: Impacts of planting method, fertilization, and mulch typeHortScience4718

    • Search Google Scholar
    • Export Citation
  • KapsM.L.ByersP.L.OdnealM.B.2010Productivity comparison of fourteen highbush blueberry cultivars in Missouri, 2000-2008J. Amer. Pom. Soc.64218225

    • Search Google Scholar
    • Export Citation
  • LarcoH.SullivanD.M.StrikB.BrylaD.2014Mulch effects on highbush blueberry under organic managementActa Hort.1018375382

  • LarcoH.StrikB.C.BrylaD.R.SullivanD.M.2013aWeed and fertilizer management practices for organic production of highbush blueberries—I. Early plant growth and biomass allocationHortScience4812501261

    • Search Google Scholar
    • Export Citation
  • LarcoH.BrylaD.R.StrikB.C.SullivanD.M.2013bWeed and fertilizer management practices for organic production of highbush blueberries—II. Impact on plant and soil nutrients, yield, and fruit quality during establishmentHortScience4814841495

    • Search Google Scholar
    • Export Citation
  • NemethD.LambrinosJ.G.StrikB.C.2017The effects of long-term management on patterns of carbon storage in a northern highbush blueberry production systemSci. Total Environ.57910841093

    • Search Google Scholar
    • Export Citation
  • PscheidtJ.W.HarperN.2014Progression of symptoms on blueberry infected with Blueberry shock virusPlant Health Prog.doi: 10.1094/PHP-RS-13-01221

    • Search Google Scholar
    • Export Citation
  • SalvoS.MuñozC.ÁvilaJ.BustosJ.Ramírez-ValdiviaM.SilvaC.VivalloG.2012An estimate of potential blueberry yield using regression models that relate the number of fruits to the number of flower buds and to climatic variablesSci. Hort.1335663

    • Search Google Scholar
    • Export Citation
  • SiefkerJ.A.HancockJ.F.1987Pruning effects on productivity and vegetative growth in the highbush blueberryHortScience22210211

  • StrikB.C.2014Organic blueberry production systems – advances in research and industryActa Hort.1017257267

  • StrikB.C.2016A review of optimal systems for organic production of blueberry and blackberry for fresh and processed markets in the northwestern United StatesSci. Hort.20892103

    • Search Google Scholar
    • Export Citation
  • StrikB.BrazeltonD.PenhallegonR.1990Grower’s guide to pruning highbush blueberries. Oregon State University Extension Service Video VTP002 Corvallis OR

  • StrikB.BrunC.AhmedullahM.AntonelliA.AskhamL.BarneyD.BristowP.FisherG.HartJ.HavensD.InghamR.KaufmanD.PenhallegonR.PscheidtJ.ScheerB.ShanksC.WilliamsR.1993Highbush blueberry production. Oregon State University Extension Service Publication PNW215 Corvallis OR

  • StrikB.BullerG.2005The impact of early cropping on subsequent growth and yield of highbush blueberry in the establishment years at two planting densities is cultivar dependantHortScience4019982001

    • Search Google Scholar
    • Export Citation
  • StrikB.BullerG.HellmanE.2003Pruning Severity Affects Yield, Berry Weight, and Picking Efficiency of Highbush BlueberryHortScience38196199

    • Search Google Scholar
    • Export Citation
  • StrikB.C.FinnC.E.MooreP.P.2014Blueberry cultivars for the Pacific Northwest. Oregon. Ore. State Univ. Ext. Serv. PNW 656. 15 Dec. 2016. <http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/45871/pnw656.pdf>.

  • StrikB.C.VanceA.2015Seasonal variation in leaf nutrient concentration of northern highbush blueberry cultivars grown in conventional and organic production systemsHortScience5014531466

    • Search Google Scholar
    • Export Citation
  • StrikB.C.VanceA.BrylaD.2016Organic production systems research in blueberry and blackberry – A review of industry-driven studiesActa Hort.1117139148

    • Search Google Scholar
    • Export Citation
  • StrikB.C.YarboroughD.2005Blueberry production trends in North America, 1992 to 2003 and predictions for growthHortTechnology15391398

  • SullivanD.M.BaryA.I.NarteaT.J.MyrheE.A.CoggerC.G.FransenS.C.2003Nitrogen availability seven years after a high-rate food waste compost applicationCompost Sci. Util.11265275

    • Search Google Scholar
    • Export Citation
  • SullivanD.M.BrylaD.R.CostelloR.C.2014Chemical characteristics of custom compost for highbush blueberry p. 293–311. In: Z. He and H. Zhang (eds.). Applied Manure and Nutrient Chemistry for Sustainable Agriculture and Environment Springer-Verlag New York NY

  • TertulianoM.G.KrewerSmithJ.E.PlattnerK.ClarkJ.JacobsJ.AndrewsE.StanalandD.AndersenP.LiburdO.FonsahE.G.SchermH.2012Growing organic rabbiteye blueberries in Georgia, USA: Results of two multi-year field studiesIntl. J. Fruit Sci.12205215

    • Search Google Scholar
    • Export Citation
  • U.S. Department of Agriculture2014Table 33: Berries: 2012 and 2007. In: 2012 Census of Agriculture. U.S. Dept. Agr. Natl. Agr. Statistical Serv. Washington DC

  • U.S. Department of Agriculture2010Table 6: Organic berries harvested from certified and exempt organic farms: 2008. In: Organic production survey (2008) 2007 Census of agriculture. U.S. Dept. Agr. Natl. Agr. Statistical Serv. Washington DC

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