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A long-term trial was established in Oct. 2006 in western Oregon to identify organic production systems for maximum yield and quality in highbush blueberry (Vaccinium corymbosum L.). The planting was transitional during the first year after planting and was certified organic during fruit production (2008–16). Treatments included plantingmethod (on raised beds or flat ground), fertilizer source (granular feather meal or fish solubles), and rate (‘‘low’’ and ‘‘high’’ rates of 29 and 57 kg·ha N during establishment, increased incrementally as the planting matured to 73 and 140 kg·ha N, respectively), mulch [sawdust, yard debris compost topped with sawdust (compost + sawdust), or black, woven polyethylene groundcover (weed mat)], and cultivar (‘Duke’ and ‘Liberty’). Mulches were replenished, as needed, and weeds were controlled throughout the study. Raised beds resulted in greater yield than flat ground during the establishment years but had less effect on yield once the plants weremature. After 9 years, cumulative yield was 22% greater on raised beds than on flat ground in ‘Liberty’ but was unaffected by plantingmethod in ‘Duke’. Cumulative yield was also 10% greater with feather meal than with fish solubles, on average, and 4% greater with the low rate than with the high rate of fertilizer. ‘Duke’ was particularly sensitive to fertilizer source and produced 35% less yield overall with fish solubles than with feather meal. By contrast, there was relatively little effect of fertilizer source or rate on yield in ‘Liberty’. In five of 9 years, yield was 8% to 20% greater with weedmat than with sawdust or compost + sawdust. Mulch type had no effect on cumulative yield of ‘Duke’, but cumulative yield of ‘Liberty’ was 11% greater with weedmat than with sawdust or compost + sawdust. Soil temperature was warmer under weedmat than under sawdust, and plants on raised beds covered with weed mat required more irrigation than those grown on flat ground mulched with sawdust. ‘Duke’ produced heavier, larger, and firmer berries with lower total soluble solids (TSS) than ‘Liberty’. However, other treatment effects on berry quality were relatively small and inconsistent. For example, berry weight was greater on raised beds than on flat ground, on average, but only by 3% (0.06 g/berry). Plants on raised beds also produced berries with slightly lower TSS than those on flat ground (15.2%and 15.7%, respectively, in ‘Liberty’, and 13.1% and 13.3%, respectively in ‘Duke’). There was no effect of fertilizer source or rate on TSS in ‘Liberty’ (15.5% on average), whereas in ‘Duke’, TSS was highest when fertilized at the high (13.7%) or low (13.4%) rate of fish, and was lower when using feather meal (12.9% and 13.1% for low and high rate, respectively). Plants fertilized with fish produced firmer fruit than with feather meal in five of the 7 years in which the measurements were taken. Also, fertilization with the higher rate of either product increased berry firmness comparedwith the low rate in six of the 7 years. The impact ofmulch was inconsistent through the study period. On average, ‘Duke’ berries were softest when fertilized with the low (173 g·mm deflection) and high (176 g·mm) rates of feathermeal andwere the firmest with the high rate of fish (182 g·mm). In ‘Liberty’, the low rate of feather meal produced softer fruit (157 g·mm) than the other fertilizer treatments (162 g·mm on average).When this study was initiated in 2006, the most common organic production system in this region was raised beds with sawdust mulch and fertilizing with a high rate of fish solubles. For this production system, yield for mature plants in our study (2014L16) was the equivalent of 8.9L12.3 t·ha in ‘Duke’ and 11.8L23.7 t·ha in ‘Liberty’. However, when plants were grown on raised beds with weed mat and fertilized with the high rate of feather meal, yield increased to 10.2L19.3 t·ha, depending on year, in ‘Duke’. By contrast, there was little effect of production system on yield of mature ‘Liberty’ plants. These yields, particularly for the best-performing treatment combination in ‘Duke’, are similar to what are observed in commercial conventional fields or organic farms using similar management practices. Our results showed that choice of organic production system can have significant impact on yield and economic costs and returns. The Pacific northwestern United States is an important growing region for northern highbush blueberry (Vaccinium corymbosum L.). According to recent U.S. surveys, this region accounted for 20% of the total conventional area of highbush blueberry (U.S. Department of Agriculture, 2014) and 49% of the total organic area (U.S. Department of Agriculture, 2010). Organic blueberry production has increased in this region because of strong consumer demand, price premiums of 20% to 200% over conventional fruit, and a dry summer climate, which reduces the potential incidence for weeds, insect pests, and leaf and fruit diseases (DeVetter et al., 2015; FernandezSalvador et al., 2017; Strik, 2014). However, there were some challenges specific to organic production that needed to be addressed, including greater production costs and inputs (particularly for fertilization and weedmanagement), limited Organic Materials Review Institute (OMRI)-approved options for disease and insect control, and potential or perceived reduced yields of organic plantings and associated returns (Strik, 2014). In 2006, the production guides available for organic northern Received for publication 5 June 2017. Accepted for publication 1 July 2017. The authors value the assistance of Gil Buller and Emily Vollmer, former Faculty Research Assistants at the North Willamette Research and Extension Center, OSU and all of the members of our industry advisory board.We appreciate the funding support provided by the Oregon Blueberry Commission, Washington Blueberry Commission, Northwest Center for Small Fruits Research, and the USDA National Institute of Food and Agriculture (Formula Grant no. OREI 2008-04443). Corresponding author. E-mail: bernadine.strik@ oregonstate.edu. HORTSCIENCE VOL. 52(9) SEPTEMBER 2017 1201 highbush blueberry in North America were based on anecdotal information (Krewer and Walker, 2006; Kuepper and Diver, 2004). There was little research at that point comparing organic production strategies in blueberry, and most of the production research done in conventional blueberry systems was not applicable to organic systems. Highbush blueberry is typically planted on raised beds. Raised beds improve soil drainage (Strik, 2007), limit compaction (Magdoff and Van Es, 2010), reduce incidence of root pathogens such as Phytophthora (Bryla and Linderman, 2007), and improve efficiency of machine harvest (Strik and Buller, 2002). However, planting on flat ground can be beneficial to root growth in southern highbush blueberry (complex hybrids based largely on V. corymbosum and V. darrowiiCamp.) likely due to increased soil moisture and reduced soil temperature during the fruiting season (Spiers, 1995). Furthermore, mechanical weed-control methods are more effective on flat ground than on raised beds (Sciarappa et al., 2008; B. Strik, personal observation). Weed management is critical for successful production of blueberries (Pritts et al., 1992; Strik et al., 1993), but chemical herbicide options available for organic systems are expensive and limited and usually are not very effective on established perennial weeds (Julian et al., 2012; Larco et al., 2013a). Other forms of weed control such as organic mulches, propane flaming, string trimming, and hand weeding are often used instead (Burkhard et al., 2009; Granatstein and Mullinix, 2008; Krewer et al., 2009; Sciarappa et al., 2008). Organic mulches provide additional benefits in highbush blueberry, including increased yield and plant growth (Clark and Moore, 1991; Goulart et al., 1997; Karp et al., 2006; Kozinski, 2006; Krewer et al., 2009; Savage, 1942; White, 2006). Douglas fir (Pseudotsuga menziesii M.) sawdust is commonly used to mulch blueberry plantings in Oregon, Washington, and British Columbia, but sawdust has become expensive in the region (Julian et al., 2011a), and it tends to immobilize a considerable amount of the N applied from fertilizers (White, 2006). Some growers are using compost in addition to sawdust to provide additional nutrients and organic matter (Gale et al., 2006; Larco et al., 2014). Municipal yard debris compost is readily available in many production regions and may be suitable for commercial blueberry production (Sullivan et al., 2014). Weed mat (perforated landscape fabric) is approved for use as a weed barrier by the United States Department of Agriculture (USDA) Organic National Program (USDA-AMS-NOP, 2011), and because of economic advantages, it has been adopted by both organic and conventional blueberry growers (Julian et al., 2012; Strik and Vance, 2017). However, increased soil temperature under the weed mat can reduce plant growth (Neilsen et al., 2003;Williamson et al., 2006) and yield (Krewer et al., 2009). Larco et al. (2013a) reported that northern highbush blueberry grew less with weed mat than with sawdust mulch by the end of the first two growing seasons; however, the plants had greater yield with weed mat in the second year, which was the first season of fruit production. In addition to weed management, availability and affordability of fertilizers are of critical importance for the economical production of organic blueberry (Strik, 2014). Fish solubles and feather meal are common fertilizers used by organic blueberry growers. Fish solubles are typically applied by fertigation through the drip irrigation system, especially when weed mat is used and other application methods are less practical or more expensive. Feather meal is a granular product applied to the soil surface. Both fertilizers mineralize readily on application and quickly release N and other nutrients (Bar

hortscience

Organic Production Systems in Northern Highbush Blueberry: I. Impact of Planting Method, Cultivar, Fertilizer, and Mulch on Yield and Fruit Quality from Planting through Maturity