David R. Bryla

Are Mojave Desert annual species equal? Resource acquisition and allocation for the invasive grass Bromus madritensis subsp. rubens (Poaceae) and two native species.

Abundance of invasive plants is often attributed to their ability ot outcompete native species. We compared resource acquisition and allocation of the invasive annual grass Bromus madritensis subsp. rubens with that of two native Mojave Desert annuals, Vulpia octoflora and Descurainia pinnata, in a glasshouse experiment. Each species was grown in monoculture at two densities and two levels of N availability to compare how these annuals capture resources and to understand their relative sensitivities to environmental change. During >4 mo of growth, Bromus used water more rapidly and had greater biomass and N content than the natives, partly because of its greater root-surface area and its exploitation of deep soils. Bromus also had greater N uptake, net assimilation and transpiration rates, and canopy area than Vulpia. Resource use by Bromus was less sensitive to changes in N availability or density than were the natives. The two native species in this study produced numerous small seeds that tended to remain dormant, thus ensuring escape of offspring from unfavorable germination conditions; Bromus produced fewer but larger seeds that readily germinated. Collectively, these traits give Bromus the potential to rapidly establish in diverse habitats of the Mojave Desert, thereby gaining an advantage over coexisting native species.

Comparative Effects of Nitrogen Fertigation and Granular Fertilizer Application on Growth and Availability of Soil Nitrogen during Establishment of Highbush Blueberry

A 2-year study was done to compare the effects of nitrogen (N) fertigation and granular fertilizer application on growth and availability of soil N during establishment of highbush blueberry (Vaccinium corymbosum L. “Bluecrop”). Treatments included four methods of N application (weekly fertigation, split fertigation, and two non-fertigated controls) and four levels of N fertilizer (0, 50, 100, and 150 kg·ha−1 N). Fertigation treatments were irrigated by drip and injected with a liquid urea solution; weekly fertigation was applied once a week from leaf emergence to 60 d prior to the end of the season while split fertigation was applied as a triple-split from April to June. Non-fertigated controls were fertilized with granular ammonium sulfate, also applied as a triple-split, and irrigated by drip or microsprinklers. Weekly fertigation produced the smallest plants among the four fertilizer application methods at 50 kg·ha−1 N during the first year after planting but the largest plants at 150 kg·ha−1 N in both the first and second year. The other application methods required less N to maximize growth but were less responsive than weekly fertigation to additional N fertilizer applications. In fact, 44–50% of the plants died when granular fertilizer was applied at 150 kg·ha−1 N. By comparison, none of the plants died with weekly fertigation. Plant death with granular fertilizer was associated with high ammonium ion concentrations (up to 650 mg·L−1) and electrical conductivity (>3 dS·m−1) in the soil solution. Early results indicate that fertigation may be less efficient (i.e., less plant growth per unit of N applied) at lower N rates than granular fertilizer application but is also safer (i.e., less plant death) and promotes more growth when high amounts of N fertilizer is applied.

Effects of nitrogen rate and application method on early production and fruit quality in highbush blueberry

Ehret, D. L., Frey, B., Forge, T., Helmer, T., Bryla, D. R. and Zebarth, B. J. 2014. Effects of nitrogen rate and application method on early production and fruit quality in highbush blueberry. Can. J. Plant Sci. 94: 1165–1179. Proper nitrogen (N) fertilizer management of highbush blueberry (Vaccinium corymbosum L.) is of major importance in south-coastal British Columbia, but little information is available. A field study was conducted to examine the effects of N rate and method of application on growth, yield, and fruit quality in highbush blueberry during the first 4 yr after planting in south-coastal BC. Nitrogen was applied at 0–150% of current production guide rates either with three equal applications of broadcast granular ammonium sulphate each spring or by fertigation through the drip irrigation system with 10 equal applications of liquid ammonium sulphate injected every 2 wk from early spring to late summer each year. Yield increased with increasing N rate during the second and third years of fruit production. The yield response as well as flower number and plant size were greater with fertigation than with broadcast fertilizers. Fruit firmness also increased consistently with increased N rates, while fruit size either increased or decreased, depending on year. There were no effects of N on fruit oxygen radical absorbance capacity (ORAC), titratable acidity, or soluble solids. However, the composition of fruit anthocyanins changed, with concentrations of seven anthocyanins decreasing, and three others increasing, with N rate. In 2 of 3 yr, total anthocyanin concentration was greater in fertigated than in broadcast treatments. Soil ammonium and nitrate concentrations increased with N rate, but only soil nitrate differed between the two application methods. Soil nitrate was higher with fertigation than with granular fertilizers, particularly at the end of the season and when greater rates of N were applied. In summary, fertigation produced more shoot growth and greater yields with less N than broadcast applications of fertilizer.

Specialty fruit production in the Pacific Northwest: adaptation strategies for a changing climate

Specialty fruit crops represent a substantial portion of the value of agricultural production in the Pacific Northwest. Climate change may threaten water sources, lengthen the dry season, raise temperatures during both the winter chilling period and the growing season, and facilitate the spread of fungal diseases and insects. Such changes have the potential to substantially reduce net returns due to increased input costs and altered yields and product quality. Many management strategies that are already being used to prolong growing seasons in marginal production areas and to improve production and quality in established production regions may also be useful as adaptation strategies under a changing climate. These strategies mostly involve moderating temperatures and controlling or compensating for mismatches between phenology and seasonal weather conditions.

Potential of Deficit Irrigation, Irrigation Cutoffs, and Crop Thinning to Maintain Yield and Fruit Quality with Less Water in Northern Highbush Blueberry

Drought and mandatory water restrictions are limiting the availability of irrigation water in many important blueberry growing regions, such as Oregon, Washington, and California. New strategies are needed to maintain yield and fruit quality with less water. To address the issue, three potential options for reducing water use, including deficit irrigation, irrigation cutoffs, and crop thinning, were evaluated for 2 years in a mature planting of northern highbush blueberry (Vaccinium corymbosum L. ‘Elliott’). Treatments consisted of no thinning and 50% crop removal in combination with either full irrigation at 100% of estimated crop evapotranspiration (ETc), deficit irrigation at 50% ETc (applied for the entire growing season), or full irrigation with irrigation cutoff for 4–6 weeks during early (earlyto late-green fruit) or late (fruit coloring to harvest) stages of fruit development. Stem water potential was similar with full and deficit irrigation but, regardless of crop thinning, declined by 0.5–0.6 MPa when irrigation was cutoff early and by >2.0 MPa when irrigation was cutoff late. In one or both years, the fruiting season was advanced with either deficit irrigation or late cutoff, whereas cutting off irrigation early delayed the season. Yield was unaffected by deficit irrigation in plants with a full crop load but was reduced by an average of 35% when irrigation was cutoff late each year. Cutting off irrigation early likewise reduced yield, but only in the 2nd year when the plants were not thinned; however, early cutoff also reduced fruit soluble solids and berry weight by 7% to 24% compared with full irrigation. Cutting off irrigation late produced the smallest and firmest fruit with the highest soluble solids and total acidity among the treatments, as well as the slowest rate of fruit loss in cold storage. Deficit irrigation had the least effect on fruit quality and, based on these results, appears to be themost viable option for maintaining yield with less water in northern highbush blueberry. Relative to full irrigation, the practice reduced water use by 2.5 ML·ha per season. Most commercial blueberry (Vaccinium sp.) fields require a substantial amount of irrigation for profitable production. In the western United States, blueberry growers typically apply an average of 25–50 mm of water per week during the summer and up to 75 mm·week during periods of peak water use (Bryla, 2011). However, many growers are facing serious water limitations due to warmer and drier weather conditions, increased regulations, and greater demand by other sectors (Dalton et al., 2013). For example, in 2015, blueberry growers in Oregon and Washington lost an estimated 14 million pounds of fruit due to heat and inadequate water for cooling and irrigation as a consequence of reduced water allotments from irrigation districts (Schreiber, 2016). This was more than a

Chemigation with Micronized Sulfur Rapidly Reduces Soil pH in a New Planting of Northern Highbush Blueberry

20 million reduction in value to the industry. Growers in California are facing even more serious challenges due to an ongoing severe drought (Cooley et al., 2015). If water shortages continue to result in less water for irrigation, the total value of blueberry production and suitable farmland may be reduced substantially in the region. Although it is difficult to predict how small fruit producers will attempt to mitigate for water shortages, long-term solutions might include drought-resistant cultivars and switching to more efficient irrigation systems and management methods. Many blueberry growers have already switched from using sprinklers to drip to increase irrigation efficiency, and are scheduling irrigation based on soil and weather conditions (Bryla, 2011). Additional strategies may include deficit irrigation or cutting off (stopping) irrigation at key developmental stages. Deficit irrigation is used successfully in many fruit crops, including peach [Prunus persica (L.) Batsch] and wine grape (Vitis vinifera L.), but it has not been well tested in berry crops (Chalmers et al., 1981; Fereres and Soriano, 2007; Goldhamer, 2007). The technique consists of restricting irrigation water applications during either a particular growth period or the entire growing season, without causing significant reductions in yield. Irrigation cutoffs may likewise be effective at reducing water use, provided the cutoffs occur during periods when water demands by the crop are low or less critical to fruit production. Preharvest irrigation cutoffs had no effect on yield in almond [Prunus dulcis (Mill.) D.A. Webb] and virtually eliminated hull rot at harvest (Goldhamer and Viveros, 2000). Previous work indicated that there may be analogous benefits to reducing preharvest irrigation in northern highbush blueberry (Bryla et al., 2009; Ehret et al., 2012, 2015). In this case, underirrigation by drip had no effect on yield in blueberry but increased fruit firmness and the content of sugar and acid in the berries, primarily as a result of a slightly smaller berry size. Cropping thinning is also an effective strategy for dealing with soil water limitations in a number of fruit crops. For example, reducing crop loads during water deficits increased plant water status of peach (Lopez et al., 2006, 2010) and pear (Pyrus communis L.) (Marsal et al., 2008, 2010) and improved fruit quality of apple [Malus ·sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] (Mpelasoka et al., 2001; Neilsen et al., 2016). By thinning the crop when water is limited, competition for resources is reduced in the remaining fruit (Lopez et al., 2006; Proebsting and Middleton, 1980), resulting in larger fruit with better fruit quality and flavor (Crisosto et al., 1997; DeJong and Grossman, 1995; W€unsche and Ferguson, 2005). To avoid overthinning, thinning-intensity models have been developed according to the severity of water deficit for apple (Naschitz and Naor, 2005) and pear (Marsal et al., 2010). Similar models could easily be developed for blueberry, provided the strategy of reducing the crop load is cost-effective and actually Received for publication 27 Oct. 2016. Accepted for publication 26 Jan. 2017. We gratefully acknowledge Amber Shireman and Ruth Hamlyn for technical assistance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. Ph.D. student Corresponding author. E-mail: david.bryla@ars. usda.gov. HORTSCIENCE VOL. 52(4) APRIL 2017 625 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION

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

Northern highbush blueberry (Vaccinium corymbosum L.) is adapted to acidic soil conditions and often grows poorly when soil pH is greater than 5.5. When soil pH is high, growers will usually mix prilled elemental sulfur (S) into the soil before planting (converted to sulfuric acid by soil bacteria) and, if needed, inject acid into the irrigation water after planting. These practices are effective but often expensive, time consuming, and, in the case of acid, potentially hazardous. Here, we examined the potential of applying micronized S by chemigation through a drip system as an alternative to reduce soil pH in a new planting of ‘Duke’ blueberry. The planting was located in western Oregon and established on raised beds mulched with sawdust in Oct. 2010. The S product was mixed with water and injected weekly for a period of ’2 months before planting and again for period of ’2 months in late summer of the second year after planting (to assess its value for reducing soil pH once the field was established), at a total rate of 0, 50, 100, and 150 kg·ha S on both occasions. Each treatment was compared with the conventional practice of incorporating prilled S into the soil before planting (two applications of 750 kg·ha S each in July and Oct. 2010). Within a month of the first application of S, chemigation reduced soil pH (0–10 cm depth) from an average of 6.6 with no S to 6.1 with 50 kg·ha S and 5.8 with 100 or 150 kg·ha S. However, the reductions in pH were short term, and by May of the following year (2011), soil pH averaged 6.7, 6.5, 6.2, and 6.1 with each increasing rate of S chemigation, respectively. Soil pH in the conventional treatment, in comparison, averaged 6.6 a month after the first application and 6.3 by the followingMay. In July 2012, soil pH ranged from an average of 6.4 with no S to 6.2 with 150 kg·ha S and 5.5 with prilled S. Soil pH declined to as low as 5.9 following postplanting S chemigation and, at lower depths (10–30 cm), was similar between the treatment chemigated with 150 kg·ha S and the conventional treatment. None of the treatments had any effect on winter pruning weight in year 1 or on yield, berry weight, or total dryweight of the plants in year 2. Concentration of P, K, Ca,Mg, S, andMn in the leaves, on the other hand, was lower with S chemigation than with prilled S during the first year after planting, whereas concentration of N, P, and S in the leaves were lower with S chemigation during the second year. The findings indicate that S chemigation can be used to quickly reduce soil pH after planting and therefore may be a useful practice to correct high pH problems in established northern highbush blueberry fields; however, it was less effective and more time consuming than applying prilled S before planting. Northern highbush blueberry (Vaccinium corymbosum L.) is adapted to acidic soil conditions and is often most productive at soil pH between 4.5 and 5.5 (Retamales and Hancock, 2012). To grow blueberry at sites with a higher initial soil pH, elemental sulfur (S), which oxidizes to sulfuric acid by chemotrophic soil bacteria such as Thiobacillius species, is often mixed into soil before planting (Chapman, 1990; Germida and Janzen, 1993). In many soils, large amounts of S (>1500 kg·ha) are needed for the process, and in some cases, such as in soils with high amounts of calcium carbonate, S acidification is unfeasible (Horneck et al., 2006; Modaihsh et al., 1989; Neilsen et al., 1993). Soil incorporation of S is also limited after planting because blueberry has a fine, shallow root system (<0.3-m deep) that is easily damaged by cultivation (Bryla and Strik, 2007). Therefore, when soil pH is too high after planting, growers must either apply S on the soil surface or inject acid (e.g., sulfuric acid) into the irrigation water (chemigation). However, surface application of S is ineffective in dry environments and difficult to do in fields mulched with geotextile fabric (‘‘weed mat’’), while acid chemigation is hazardous and requires expensive, non-corrosive irrigation equipment (Burt et al., 1998). Soil acidification with S usually takes several months or more to change soil pH from 6.0 or higher to a desired level for northern highbush blueberry (Gough, 1994; Hart et al., 2006). The rate of the process is largely dependent on soil temperature, moisture, and aeration (Germida and Janzen, 1993), soil organic matter content (Cifuentes and Lindermann, 1993; Wainwright et al., 1986), and the size of the S particles (Germida and Janzen, 1993; Lee et al., 1988; Sholeh and Blair, 1997; Zhao et al., 2016). Generally, smaller S particles are oxidized faster than larger particles because of the greater surface area to volume ratio (Lawrence and Germida, 1988; Li and Caldwell, 1966). Currently, there are several micronized S products on the market labeled for chemigation that can be easily injected and applied through an irrigation system. These products contain micropropogules of S (<10mm) that readily dissolve in water. Although micronized S is primarily intended for use as a foliar fungicide, it could also be used to reduce soil pH. The objective of the present study was to determine the potential of applying micronized S by chemigation through a drip irrigation system to quickly reduce soil pH in a new planting of northern highbush blueberry. Micronized S was applied before planting, to evaluate its use as a preplant amendment, and after planting, to assess its value for reducing soil pH, once the field was established. We expected that applying S through a drip system would reduce soil pH faster and require less product than conventional applications of prilled S because the former would concentrate S directly beneath the drip emitters in a zone where many roots are located (Bryla et al., 2017), and where soil conditions remain moist (Bryla et al., 2011) and are favorable for rapid bacterial transformation of S to sulfuric acid (Konopka et al., 1986). Material and Methods Study site. The study was conducted at the Oregon State University Lewis-Brown Horticultural Research Farm in Corvallis, OR in HORTSCIENCE VOL. 52(10) OCTOBER 2017 1413 a field of ‘Duke’ northern highbush blueberry planted on 21 Oct. 2010. Soil at the site is a Malabon silty clay loam (fine, mixed, superactive, mesic Pachic Ultic Argixerolls) (Parsons and Herriman, 1970). The soil had an initial pH of 6.6 before any treatment and contained 2.4% organic matter. Plants were obtained from a commercial nursery as 2-year-old container stock and spaced 0.76 · 3.05 m apart on raised beds (0.4-m high and 0.9-m wide). The beds were shaped 3 months before planting to initiate the S treatments. A 5-cm-deep layer of douglas fir (Pseudotsuga menziesii Franco) sawdust was rototilled 20-cm deep into each row just before making the beds, and a bed shaper (Kennco Manufacturing, Inc., Ruskin, FL) was used to raise the beds. A 5-cm-deep layer of sawdust mulch was also applied on top of the beds after planting and reapplied in May 2012. Sulfur treatments.Micronized wettable S (0N–0P–0K–80S; Nufarm Americas Inc., Burr Ridge, IL) was mixed with irrigation water [pH 6.9, electrical conductivity (EC) of 0.13 dS·m, and 42 mg·L CaCO3 (alkalinity)] and applied once or twice weekly by chemigation at a total rate of 0, 50, 100, and 150 kg·ha S before planting (28 July to 27 Sept. 2010) and again during the second year after planting (3 Aug. to 12 Oct. 2012) (Fig. 1). Each treatment was compared with the conventional practice of incorporating a prilled S (0–0–0–90S; Tiger-Sul Products LLC, Atmore, AL) into the soil before planting. Prilled S was applied in two applications of 750 kg·ha each on 28 July and 10 Oct. 2010. The first application of prilled S was rototilled into the plots along with the sawdust before shaping the beds, and the second application was incorporated using a hand rake. The treatments were arranged in a randomized complete block design with five plots of four plants each per treatment. Drip tubing (Netafim USA, Fresno, CA) was installed on each side of the row at a distance of 20 cm from the base of the plants. The tubing had 2-L·h in-line emitters every 0.3 m and was installed immediately after the beds were shaped. The wettable S solution was injected through the drip system using water-powered proportional chemical injectors (Model D25F1; Dosatron, Clearwater, FL). Five injectors were installed in a manifold located at the head of each treatment. Irrigation was initiated 10 min before each injection to fully pressurize the system, and run for at least 10 min after injection to flush the lines. Water only was applied to treatments with prilled or no S. There was no evidence of emitter plugging during the study. Crop management. Weeds were controlled by cultivating between rows, as needed and were removed by hand at least once a month from the planting beds. Irrigation was scheduled up to 7 d/week, as needed, to meet crop water demands over each growing season (Bryla et al., 2011). Granular urea (46N–0P–0K) was applied by hand around the base of the plants at rate of 10 kg·ha N each on 27 April and 11 May 2011, and liquid urea (20N–0P–0K) was injected weekly through the drip system at a rate of 8 kg·ha N per application from 25 May to 27 July 2011 and 15 June to 27 July 2012. Overall, the plants received a total of 100 kg·ha N in 2011 and 56 kg·ha N in 2012. No other nutrients were applied to the plants, which is common in the region for northern highbush blueberry. There was no evidence of insect or disease problems in the plants, and therefore, no chemicals were used for pest control. Measurements. Plant growth occurred primarily from May to October each year. Plants were pruned immediately after planting in Oct. 2010 and before the second Fig. 1. Temperature (air and soil; lines) and precipitation (bars) in a new planting of ‘Duke’ blueberry that was either chemigated or treated conventionally with elemental sulfur (S). Hashed b

Influence of Irrigation Method and Scheduling on Patterns of Soil and Tree Water Status and Its Relation to Yield and Fruit Quality in Peach

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