Research Advances in Adopting Drip Irrigation for California Organic Spinach: Preliminary Findings

agriculture

Article Research Advances in Adopting Drip Irrigation for California Organic Spinach: Preliminary Findings

Ali Montazar 1,*, Michael Cahn 2 and Alexander Putman 3

1 Division of Agriculture and Natural Resources, University of California, UCCE Imperial County, 1050 East Holton Road, Holtville, CA 92250, USA 2 Division of Agriculture and Natural Resources, University of California, UCCE Monterey County, 1432 Abbott Street, Salinas, CA 93901, USA 3 Department of Microbiology and Plant Pathology, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA * Correspondence: [email protected]

Received: 13 June 2019; Accepted: 6 August 2019; Published: 9 August 2019

Abstract: The main objective of this study was to explore the viability of drip irrigation for organic spinach production and the management of spinach downy mildew disease in California. The experiment was conducted over two crop seasons at the University of California Desert Research and Extension Center located in the low desert of California. Various combinations of dripline spacings and installation depths were assessed and compared with sprinkler irrigation as control treatment. Comprehensive data collection was carried out to fully understand the differences between the irrigation treatments. Statistical analysis indicated very strong evidence for an overall effect of the irrigation system on spinach fresh yields, while the number of driplines in bed had a significant impact on the shoot biomass yield. The developed canopy crop curves revealed that the leaf density of drip irrigation treatments was slightly behind (1–4 days, depending on the irrigation treatment and crop season) that of the sprinkler irrigation treatment in time. The results also demonstrated an overall effect of irrigation treatment on downy mildew, in which downy mildew incidence was lower in plots irrigated by drips following emergence when compared to the sprinkler. The study concluded that drip irrigation has the potential to be used to produce organic spinach, conserve water, enhance the efficiency of water use, and manage downy mildew, but further work is required to optimize system design, irrigation, and nitrogen management practices, as well as strategies to maintain productivity and economic viability of utilizing drip irrigation for spinach.

Keywords: downy mildew; drip irrigation; irrigation management; organic production; spinach

1. Introduction Spinach (Spinacia oleracea L.) is a fast-maturing, cool-season vegetable crop. In California, spinach is mainly produced in four areas of the southern desert valleys, the southern coast, the central coast, and the central San Joaquin Valley. The farm gate value of Californian non-processing spinach from a total planted area of 13,557 ha was nearly 242 million dollars in 2017 [1]. The area in California under spinach production increased more than 30% over the last 10 years; the planted area increased from 1478 ha in 2008 to 3893 ha in 2017 in the Imperial Valley [2,3]. Water and nitrogen are generally essential drivers for plant growth and survival, and for spinach, are two key factors that considerably aﬀect yield and quality [4]. Spinach is highly responsive to 1 nitrogen fertilization [5] and accumulates as much as 134 kg ha− of nitrogen in 30 days [6]. Spinach yield was shown to increase as a result of high nitrogen application rates but decreased when nitrogen application rate was excessive [6]. Researchers reported that with the increase of nitrogen fertilizer

Agriculture 2019, 9, 177; doi:10.3390/agriculture9080177 www.mdpi.com/journal/agriculture Agriculture 2019, 9, 177 2 of 14 application, nitrogen use efficiency of spinach was significantly decreased; however, water use efficiency (as the ratio of yield to the seasonal crop water use) of spinach was increased in most cases [5,7]. Reducing nitrogen fertilizer use in spinach will be a challenge. The crop has a shallow root system, a high N demand, which occurs over a short period, and strict quality standards for a deep green color that tends to encourage N applications beyond the agronomic requirements to maximize yield [6]. Addressing these challenges requires that nitrogen fertilizer is applied at the optimal time and rate based on crop uptake and the soil–nitrogen test, and that irrigation water is efficiently applied to minimize leaching. The unpublished data from a survey conducted by the University of California Cooperative Extension Monterey county indicates that spinach-growers generally apply more water than that needed by crop water (100% to 150% more). Downy mildew on spinach is a widespread and very destructive disease in California. It is a disease that is the most significant issue facing the spinach industry, and crop losses can be significant in all areas where spinach is produced [8]. Peronospora effusa is an obligate oomycete that causes downy mildew of spinach. Spinach growers rely on host resistance and fungicides to manage downy mildew. This dependence on resistance was relatively effective until recently, when malignant forms of P. effusa began to emerge in rapid succession [9]. In recent years, several new downy mildew races have appeared in the state of California, raising concerns about the ability to manage this threat and causing the industry to consider research strategies to address the problem. Organic spinach producers are especially vulnerable to these virulent strains because synthetic fungicide use is prohibited, and choice in regard to variety is determined at planting. This has led to significant yield losses in organic spinach production. Like all downy mildew pathogens, P. effusa requires cool and wet conditions for infection and disease development [8,10]. The California industry is known for using very high planting densities and a large number of seed lines per bed [10]. For the baby leaf or clipped markets, the planting density is usually 8.6–9.8 million seeds per hectare [11]. The dense canopy of spinach retains much moisture, and creates ideal conditions for infection and disease development. Spores (called sporangia) are dispersed at short distances via wind or splashing water, or at medium distances via wind. In addition, most conventional and organic spinach fields are irrigated by sprinkler irrigation in California. Overhead irrigation deposits free moisture on leaf surfaces and increases relative humidity in the canopy, which contributes to the speed and severity of downy mildew epidemics within a field when other conditions, such as temperature, are favorable. Production practices that reduce the favorability of the spinach canopy for downy mildew development are needed to reduce losses to downy mildew in both organic and conventional production. Because most conventional and organic spinach fields are irrigated by sprinkler irrigation in California, overhead irrigation could contribute to the speed and severity of downy mildew epidemics within a field when other conditions, such as temperature, are favorable. While preventing downy mildew on spinach is of high interest of organic vegetable production in California, integrated approaches are greatly required to reduce yield losses from this major disease. New irrigation management techniques and practices in spinach production may have a significant economic impact to the leafy greens industry through the control of downy mildew. In addition to losses from plant pathogens, new irrigation practices could reduce risks to food safety caused by overhead application of irrigation water. Drip irrigation has revolutionized crop production systems in western states of the US by increasing yields and water-use efficiency in many crops [12]. Sub-surface drip irrigation was successfully implemented on vegetable crops, such as processing tomato, where yields have increased by 20–50% over recent years since this practice was adapted [13]. While sub-surface drip irrigation has been utilized primarily for high-value specialty crops, several studies have reported the benefits of its application for agronomic low-value crops [12–16]. Drip irrigation was evaluated versus micro-sprinkler irrigation in spinach [17]. The drip was found to be better than the micro-sprinkler because of greater yields and the lower installation cost. The effectiveness of drip fertigation for Agriculture 2019, 9, 177 3 of 14 reducing nitrate in spinach was reported by Takebe et al. [18]. Drip fertigation was considered to reduce nitrate more stably. In addition to higher yield, sub-surface and surface drip irrigation can reduce risks of plant diseases in vegetables by minimizing leaf wetness and waterlogged soil conditions. Adapting drip irrigation for high-density spinach plantings may be a possible solution to reduce losses from downy mildew, improve crop productivity and quality, and conserve water and fertilizer. Currently, drip irrigation is not used for producing spinach in California, and there is a lack of information on the viability of this technology and optimal practices for irrigating spinach with drips. In fact, to our knowledge, few studies have been conducted to assess the potential benefit of drip irrigation for this high-density planted spinach. As an initial test, the main objective of this project was to evaluate the viability of adapting drip irrigation for organic spinach production. The project was particularly aimed at understanding the system design to successfully produce spinach, and to conduct a preliminary assessment on the impact of drip irrigation on the management of spinach downy mildew.

2. Materials and Methods The field experiments were carried out over two crop seasons at the organic field of the University of California Desert Research and Extension Center (UC DREC) (32◦48035” N; 115◦ 26039” W; 22 m below mean sea level) located in the Imperial Valley, California. The field had a silty clay soil (the top 30 cm soil surface contains 14% sand, 42% silt, and 44% clay). Soil characteristics referring to three genetic horizons selected from the soil survey are presented in Table1. Soil pH ranged between 7.94 1 and 8.04, and its electric conductivity was from 1.3 to 2.9 dS m− . The area has a desert climate with a mean annual rainfall of 76 mm and air temperature of 22 ◦C. Spinach can typically grow from October to March in this region. The average pH and electrical conductivity of water supply (surface water from the Colorado River) was 8.0 and 1.18 ds/m during the experiment, respectively.

Table 1. Soil characteristics of the study site.

Soil Parameters Soil Depth Generic horizon (cm) 0–30 30–60 60–90 Texture silty clay clay loam clay Sand (%) 14 21 8 Silt (%) 42 41 34 Clay (%) 44 38 58 pH 7.94 8.04 8.02 1 EC (dS m− ) 1.3 1.4 2.9 Cation exchange capacity (meq/1000 g) 48.6 50.7 54.0 3 Bulk density (g cm− ) 1.54 1.48 1.45 Exchange sodium percentage (%) 3.5 4.1 6

2.1. Experimental Design and Treatments Experiment 1 (fall experiment): Land preparation was conducted in late September 2018, and 1 untreated Viroflay spinach seeds were planted at a rate of 37 kg ha− on 31st October. For the research trial (Figure1a), five irrigation system treatments consisted of two drip depths (driplines on the soil surface and driplines at 3.8 cm depth), two dripline spacings (three driplines on a 203 cm bed and four driplines on an 203 cm bed), and sprinkler irrigation (203 cm bed). The experiment was arranged in a randomized complete block with four replications. Each drip replication had three beds, and each sprinkler replication had six beds. The beds were 61 m long. Figure2 presents the individual experimental beds with four driplines at 3.8 cm and on the soil. All treatments were germinated by sprinklers (two sets of five-hour irrigations). A flow control drip tape from the Toro company was used with a hose diameter of 15.88 mm, wall thickness of 0.154 mm, emitter spacing of 20.3 cm, and operating emitter flowrate of 0.49 L/h using the Nelson sprinklers R200WF Rotator (280 L/h @ 3.45 Bar) were used with spacings of 12.2 m 9.1 m. Water × Agriculture 2019, 9, 177 4 of 14

distribution uniformity was measured for the sprinkler irrigation using the ASABE (American Society Agricultureof Agricultural 2019, 9, x and FOR Biological PEER REVIEW Engineers) standard method [19]. The average distribution uniformity4 of 14 for the sprinkler irrigation was 80%, and an average of 92% was assumed as water distribution AgricultureTrue 2019 6-6-2, 9, x FOR(a homogeneous PEER REVIEW pelleted fertilizer from True Organic Products) was applied at4 aof rate 14 uniformity of the drip irrigation. of 89 kg of N per hectare as pre-plant fertilizer, and True 4-1-3 (a liquid fertilizer from True Organic TrueTrue 6-6-2 6-6-2 (a (a homogeneous homogeneous pelleted pelleted fertilizer fertilizer from from True True Organic Organic Products) Products) was was applied applied at at a arate rate Products) was applied as complementary fertilizer through injection into the irrigation system. For ofof 89 89 kg kg of of N N per per hectare hectare as as pre-plant pre-plant fertilizer, fertilizer, and and True True 4-1-3 4-1-3 (a (a liquid liquid fertilizer fertilizer from from True True Organic Organic the drip treatments, True 4-1-3 was applied three times after germination (by crop harvest) at a rate Products)Products) was was applied applied as as complementary complementary fertilizer throughthrough injectioninjection into into the the irrigation irrigation system. system. For For the of 45, 33, and 44 kg of N per hectare. This liquid fertilizer was applied at a rate of 56, 42, and 50 kg of thedrip drip treatments, treatments, True True 4-1-3 4-1-3 was wa applieds applied three three times times after after germination germination (by (by crop crop harvest) harvest) at a at rate a rate of 45, N per hectare for the sprinkler irrigation system. Soil tests conducted before planting were used to of33, 45, and 33, 44and kg 44 of kg N of per N hectare.per hectare. This This liquid liquid fertilizer fertilizer was appliedwas applied at a rateat a ofrate 56, of 42, 56, and 42, 50and kg 50 of kg N of per determine the status of available nitrogen/nutrients to develop fertilizer recommendations to achieve Nhectare per hectare for the for sprinkler the sprinkler irrigation irrigation system. system. Soil tests Soil conducted tests conducted before planting before planting were used were to determine used to optimum crop production and prevent nitrogen deficiency. Initially, the crop was irrigated with more determinethe status the of availablestatus of available nitrogen /nitrogen/nutrientsnutrients to develop to fertilizerdevelop fertilizer recommendations recommendations to achieve to optimumachieve water than required, as determined by following crop evapotranspiration (ET, as the sum of soil optimumcrop production crop production and prevent and prevent nitrogen nitrogen deﬁciency. deficie Initially,ncy. Initially, the crop the was crop irrigated was irrigated with with more more water evaporation and plant transpiration), and using soil moisture data, we tried to irrigate spinach trials waterthan required,than required, as determined as determined by following by following crop evapotranspiration crop evapotranspiration (ET, as the (ET, sum as of the soil sum evaporation of soil more than crop water requirements to make sure there was no water stress for the entire crop season. evaporationand plant transpiration),and plant transpiration), and using and soil moistureusing soil data,moisture we tried data, to we irrigate tried to spinach irrigate trials spinach more trials than However, according to our data, this led to over-irrigation at some points in the early and mid-crop morecrop than water crop requirements water requirements to make sureto make there sure was th noere water was no stress water for stress the entire for the crop entire season. crop However,season. season. Irrigation events were typically scheduled once a week for the sprinkler treatments and twice However,according according to our data, to our this data, led this to over-irrigation led to over-irri atgation some at points some inpoints the earlyin the and early mid-crop and mid-crop season. a week for the drip treatments at the required running times of each system/treatment. season.Irrigation Irrigation events events were typically were typically scheduled scheduled once a once week a forweek the for sprinkler the sprinkler treatments treatments and twice and atwice week a forweek the for drip the treatments drip treatments at the requiredat the requir runninged running times oftimes each of system each system/treatment./treatment.

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(a) (b)

(b)

Figure 1. The fall (a) and winter (b) experiment layouts (not to scale). Sprinkler: treatment irrigated byFigure sprinklers; 1. The fall3.8D-3B: (a) and treatment winter (bwi)th experiment three driplines layouts in (noteach to bed scale). installed Sprinkler: at 3.8 cm treatment depth; irrigated3.8D-4B: Figure 1. The fall (a) and winter (b) experiment layouts (not to scale). Sprinkler: treatment irrigated treatmentby sprinklers; with 3.8D-3B: four driplines treatment in witheach threebed installed driplines at in each3.8 cm bed depth; installed 0D-4B: at 3.8 treatment cm depth; with 3.8D-4B: four by sprinklers; 3.8D-3B: treatment with three driplines in each bed installed at 3.8 cm depth; 3.8D-4B: driplinestreatment in with each four bed driplines on the soil in each surface; bed installed0D-3B: tr ateatment 3.8 cm with depth; three 0D-4B: driplines treatment in each with bed four on driplines the soil treatment with four driplines in each bed installed at 3.8 cm depth; 0D-4B: treatment with four surface.in each bed on the soil surface; 0D-3B: treatment with three driplines in each bed on the soil surface. driplines in each bed on the soil surface; 0D-3B: treatment with three driplines in each bed on the soil surface.

(a) (b)

Figure 2. Beds with four driplines in the fall crop season.season. The pictures demonstrate individual beds with four driplines at 3.8 cm depth 30 days after planting ( a) and on the soil surface 12 days after Figure 2. Beds with four driplines in the fall crop season. The pictures demonstrate individual beds planting ( b). with four driplines at 3.8 cm depth 30 days after planting (a) and on the soil surface 12 days after plantingExperiment (b). 2 2 (winter (winter experiment) experiment): :UntreatedUntreated Viroflay Viroflay spinach spinach seeds seeds were were planted planted at a at rate a rate of 38 of 1 kg38 kgha− ha1 on− 28thon 28thJanuary. January. For the For research the research trial (Figure trial (Figure1b), three1b), irrigation three irrigation system treatments system treatments consisted Experiment 2 (winter experiment): Untreated Viroflay spinach seeds were planted at a rate of 38 of two dripline spacings (three driplines on a 203 cm bed and four driplines on a 203 cm bed), and kg ha−1 on 28th January. For the research trial (Figure 1b), three irrigation system treatments consisted sprinkler irrigation (203 cm bed). All driplines were installed at 3.8 cm depth. The experiment was of two dripline spacings (three driplines on a 203 cm bed and four driplines on a 203 cm bed), and arranged in a randomized complete block with four replications. Each drip and sprinkler treatment sprinkler irrigation (203 cm bed). All driplines were installed at 3.8 cm depth. The experiment was replication consisted of three beds × 61 m length. All treatments were germinated by sprinklers (two arranged in a randomized complete block with four replications. Each drip and sprinkler treatment replication consisted of three beds × 61 m length. All treatments were germinated by sprinklers (two Agriculture 2019, 9, 177 5 of 14 consisted of two dripline spacings (three driplines on a 203 cm bed and four driplines on a 203 cm bed), and sprinkler irrigation (203 cm bed). All driplines were installed at 3.8 cm depth. The experiment was arranged in a randomized complete block with four replications. Each drip and sprinkler treatment replication consisted of three beds 61 m length. All treatments were germinated by sprinklers (two × sets of five hours). The buffer beds located between sprinkler and drip treatments were not planted. The drip treatments were more frequently irrigated (three times a week with shorter irrigation events) than the drip treatments in the fall trial. True 6-6-2 organic fertilizer was applied before planting at a rate of 78 kg of N per hectare, and the crop was supplemented with True 4-1-3 liquid fertilizer injection into irrigation system. For the drip system, True 4-1-3 was applied four times after germination (by crop harvest) at a rate of 34, 45, 33, and 44 kg of N per hectare. In the sprinkler system, the same fertilizer was applied at a rate of 45, 45, 50, and 45 kg of N per hectare.

2.2. Field Measurements and Analysis The actual crop ET (evapotranspiration) was measured using Tule Technology sensor (www. tuletechnologis.com) which uses the residual of energy balance method as surface renewal equipment. Using the actual crop water-use data measured and spatial California Irrigation Management Information System (CIMIS) data (http://wwwcimis.water.ca.gov/SpatialData.aspx), the actual crop coefficient curve was developed for each crop season. The reference ET (ETo) was retrieved from spatial CIMIS as well. Images were taken on weekly basis utilizing an infrared camera (NDVI digital camera, NDVI stands for the Normalized Difference Vegetation Index) to quantify the development of the crop canopy of each treatment over the crop seasons. The images were analyzed using PixelWrench2 software. Decagon 5TE sensors were installed at three depths (20, 30, and 45 cm) to monitor soil water content on a continuous basis. The numerous horizontal roots of spinach typically remain in the top 30 cm of soil with a spread of approximately 40 cm, while soil water storage at the top 45 cm (spinach crop root zone) was calculated using the soil water content data. The applied water for the irrigation treatments was measured throughout the crop seasons using magnetic flowmeters. The NDVI values and plant leaf wetness values were measured using Spectral Reflectance sensors (combination of SRS-Pi Hemispherical Sensor and SRS-Pr Field Stop Sensor) and dielectric leaf wetness sensors (PHYTOS 31), respectively (METER Group, Inc. USA) on a continuous basis. Leaf chlorophyll was measured using an atLEAF CHL STD sensor (FT Green LLC: Wilmington, DE, USA) on a weekly basis. Shoot biomass (sum of the weight of leaves and stem represents shoot biomass) measurements at the final harvests were carried out in three sample areas of 0.56 m2 (0.92 m 0.61 m) per replicate and × treatment. The bed located in the center of each replication in each of the treatments was selected as the sample bed (four sample beds per each treatment, a total of 20 sample beds for five irrigation treatments in the fall trial and 14 sample beds for three irrigation treatments in the winter trial). Fresh weight was measured in order to determine shoot biomass accumulation. Leaf chlorophyll content was measured on 20 individual leaves in each of the sample beds. Weekly plant samples (shoots) were analyzed for total nitrogen at the UC Davis Analytical Lab. The statistical significances were performed using generalized linear mixed model using the GLIMMIX (generalized linear mixed models) procedure in SAS 9.4. (SAS Institute Inc., Cary, NC, USA)

2.3. Downy Mildew Both replicate runs of the experiment were visually scouted by walking down arbitrarily selected rows of all treatments. When disease symptoms were observed, the number of plants exhibiting downy mildew symptoms was counted for the entire length of two of the three beds in each plot. To calculate disease incidence, or the percentage of plants affected by downy mildew, the number of plants in each bed was divided by the estimated plant population. The estimated plant population was determined from the seeding rate and the germination rate averaged over each treatment at emergence. Downy mildew incidence was analyzed in a generalized linear mixed model using the GLIMMIX Agriculture 2019, 9, 177 6 of 14 procedure in SAS 9.4. A block was treated as a random effect, and options in the model statement included use of the beta distribution and the logit link function. Due to evidence for a significant effect of treatment, means were separated using the lsmeans statement with the Tukey-Kramer adjustment forAgriculture multiple 2019 comparisons., 9, x FOR PEER REVIEW 6 of 14

3. Results and Discussion

3.1. Weather ConditionsConditions The daily air temperature, relative humidity, and wind speed variations were different different over the fall and thethe winterwinter experimentsexperiments (Figure (Figure3 ).3). At At the the fall fall trial, trial, we we observed observed a meana mean daily daily air air temperature temperature of 18.8of 18.8◦C °C at theat the early- early- and and mid-seasons mid-seasons when when the temperaturethe temperature stayed stayed above above 6.5 ◦C 6.5 at nighttime.°C at nighttime. The mean The dailymean temperaturedaily temperature decreased decreased to 12.1 ◦Cto during12.1 °C the du latering season, the late and season, relative and humidity relative dramatically humidity 1 increaseddramatically over increased the last 10over days the before last 10 the days final before harvest. the Anfinal average harvest. daily An average wind speed daily of wind 1.5 m speed s− was of observed1.5 m s−1 was during observed the fall during experiment. the fall experiment. While a moremore variablevariable airair temperaturetemperature was observedobserved at thethe winterwinter experiment,experiment, the meanmean dailydaily temperature was 12.6 ◦°CC at the early- and mid-seasons, and the temperature fell below 1.0 °C◦C for a few nights. Higher Higher daytime daytime relative relative humidity humidity was was measur measureded during during the the early- early- and and mid-seasons mid-seasons compared compared to tothe the fall. fall. Although Although there there was was not not a asignificant significant diffe diffrenceerence between between the the average average wind wind speed speed of of the crop seasons, several windy windy days days occurred occurred during during the the winter winter season season (hours (hours with with a awind wind speed speed of of more more than than 8 1 8m m s−1 s).− The). The cumulative cumulative ETo ETforo thefor fall the and fall winter and winter cropscrops was 122 was and 122 177 and mm, 177 respectively. mm, respectively.

(a)

(b)

Figure 3. a a b Figure 3. Daily air temperature ( a),), relative relative humidity humidity ( (a),), and and wind speed ( b) over the fall seasonseason (October through December, left side of the figures) and the winter season (January through March, (October through December, left side of the figures) and the winter season (January through March, right side of the figures). right side of the figures). 3.2. Crop Water Use and Applied Water 3.2. Crop Water Use and Applied Water Total crop water use (seasonal ET) of 92.3 and 154.9 mm was measured in the fall and the winter trials,Total respectively. crop water Variable use (seasonal daily crop ET) ET of and92.3 crop and coe154.9fficient mm was values measured were measured in the fall during and the the winter entire trials, respectively. Variable daily crop ET and crop coefficient values were measured during 1the crop season (Figure4). The maximum and minimum crop ET observed was 3.8 and 0.7 mm d − in entire crop season (Figure 4). The maximum1 and minimum crop ET observed was 3.8 and 0.7 mm d−1 the fall experiment and 5.3 and 0.3 mm d− in the winter experiment, respectively. A similar range −1 ofin the crop fall coe experimentfficient value and (0.2–1.2) 5.3 and was0.3 mm obtained d in forthe bothwinter crop experiment, seasons. Piccinni respectively. et al. A reported similar arange crop waterof crop use coefficient of 157.5 mmvalue and (0.2 crop–1.2) coe wasfficient obtained range for of 0.2–1.5both crop for winterseasons. spinach Piccinni in Texaset al. reported using lysimeter a crop measurementswater use of 157.5 [20]. mm and crop coefficient range of 0.2–1.5 for winter spinach in Texas using lysimeterThe totalmeasurements applied water [20]. for the sprinkler treatment was 144.8 and 225.2 mm in the fall and the winterThe total trials, applied respectively. water for The the totalsprinkler applied treatment water forwas the 144.8 drip and treatments 225.2 mm was in the 130.5 fall mm and over the thewinter fall trials, season respectively. and 202.4 mm The overtotal theapplied winter water season. for the Overall, drip treatments an average was of 130.5 11% moremm over water the was fall appliedseason and through 202.4 themm sprinkler over the systemwinter comparedseason. Over withall, the an dripaverage system of 11% to compensate more water forwas the applied lower through the sprinkler system compared with the drip system to compensate for the lower water application uniformity of sprinkler irrigation system. A well-designed and properly managed drip irrigation usually has high distribution uniformity, and unlike conventional sprinkler irrigation systems, has the potential to conserve water because of a lower potential for tailwater runoff and deep percolation losses. Agriculture 2019, 9, 177 7 of 14 water application uniformity of sprinkler irrigation system. A well-designed and properly managed drip irrigation usually has high distribution uniformity, and unlike conventional sprinkler irrigation systems, has the potential to conserve water because of a lower potential for tailwater runoff and deep percolationAgriculture 2019 losses., 9, x FOR PEER REVIEW 7 of 14

(a2) (a1) (a2)

(b1) (b2)

FigureFigure 4.4. DailyDaily spinach spinach crop crop evapotranspiration evapotranspiration or or ET ET (a1 (a1 and,a2) a2 and) and crop crop coe coefficientfficient (b1 (,b1b2 )and values b2) invalues the experimentalin the experimental seasons. seasons. The subfigures The subfigures a1 and a1 a2 and show a2 cropshow ET crop in theET in fall the and fall spring and spring trials, respectively,trials, respectively, and the and subfigures the subfigures b1 and b2 b1 show and crop b2 sh coeowffi cientcrop valuescoefficient in the values fall and in spring the fall experimental and spring seasons,experimental respectively. seasons, respectively.

SoilSoil waterwater storagestorage inin thethe profileprofile waswas calculated,calculated, assumingassuming thatthat thethe waterwater contentcontent sensedsensed atat eacheach depthdepth waswas representative representative of of the the soil soil from from that that depth depth to the to midpointthe midpoint between between the next the uppernext upper and lower and depths.lower depths. A comparison A comparison of the soilof the water soil storagewater storage between between sprinkler sprinkler and drip and (3.8–4B) drip (3.8 treatments–4B) treatments versus soilversus water soil storage water storage in the average in the average field capacity field capacity (FC) of the(FC) top of 45the cm top of 45 the cm soil of is the shown soil is in shown Figure 5in. EvenFigure though 5. Even there though were there differences were differences between the between daily availablethe daily soilavailable water soil in both water treatments in both treatments and crop seasons,and crop the seasons, soil water the soil storage water amounts storage were amounts uniformly were uniformly maintained maintained at around theat around average the soil average water storagesoil water of fieldstorage capacity of field (19.35 capacity cm at (19.35 the top cm 45 at cm the of top the soil).45 cm More of the uniform soil). More soil water uniform availability soil water in theavailability drip treatment in the drip over treatment the winter over experiment the winter could ex beperiment as a result could of morebe as frequenta result of irrigation more frequent events withirrigation shorter events durations. with shorter durations.

(a) (b)

FigureFigure 5.5. SoilSoil waterwater storagestorage at thethe surfacesurface toto aa 4545 cm-deepcm-deep proﬁleprofile atat thethe dripdrip (3.8–4B)(3.8–4B) andand sprinklersprinkler treatmentstreatments inin thethe fallfall ((aa)) andand winterwinter ((bb)) cropcrop seasons.seasons. TheThe blueblue lineline showsshows thethe soilsoil waterwater storagestorage 33 −3 calculatedcalculated usingusing anan averageaverage ﬁeldfield capacitycapacity ofof thethe toptop 4545 cmcm ofof thethe soilsoil (0.43(0.43 mm3 mm−−3).).

3.3. Crop Canopy over the Season Crop canopy cover is defined as the percentage of plant material which covers the soil surface, and can be a very useful index for estimating the crop coefficient. Here, the canopy cover percentage was developed for each of the irrigation treatments (Figure 6). Canopy cover percentages show that the leaf density of drip irrigation treatments was slightly behind (1–4 days depending upon the irrigation treatment and crop season) than that of the sprinkler irrigation treatments. Agriculture 2019, 9, 177 8 of 14

3.3. Crop Canopy over the Season Crop canopy cover is deﬁned as the percentage of plant material which covers the soil surface, and can be a very useful index for estimating the crop coeﬃcient. Here, the canopy cover percentage was developed for each of the irrigation treatments (Figure6). Canopy cover percentages show that the leaf density of drip irrigation treatments was slightly behind (1–4 days depending upon the irrigation treatmentAgriculture 2019 and, 9 crop, x FOR season) PEER REVIEW than that of the sprinkler irrigation treatments. 8 of 14

(a)

(b)

Figure 6. Canopy crop curve for the different irrigation treatments in the (a) winter crop season and (b) the fall crop season. Figure 6. Canopy crop curve for the different irrigation treatments in the (a) winter crop season and The(b) the individual fall crop season. canopy cover curves for each season demonstrate that spinach crop water requirements and irrigation scheduling could be different in fall and winter seasons. For instance, in this study,The individual an average canopy of 52% andcover 70% curves of canopy for cropeach coverageseason demonstrate was observed that 30 daysspinach after crop planting water in therequirements winter and and the irrigation fall crop season, scheduling respectively. could be We different may expect in fall a longerand winter season seasons. for spinach For instance, planted inin thethis winter study, than an average the fall of in the52% Imperial and 70% Valley, of canopy though crop it maycoverage change was depending observed on 30 the days specific after planting weather conditions.in the winter Figure and 6the shows fall crop the canopy season, cover respecti forvely. individual We may experimental expect a longer beds withseason four for driplines spinach plantedplanted atin twothe diwinterfferent than dates. the fall in the Imperial Valley, though it may change depending on the specific weather conditions. Figure 6 shows the canopy cover for individual experimental beds with 3.4.four Crop driplines Growth planted and Greenness at two different dates. Few differences between drip and sprinkler treatments were visible for the winter planted trial, while3.4. Crop more Growth differences and Greenness were observed in the fall experiment in November. Leaves began to yellow in betweenFew the differences drip laterals between in plots drip withand sprinkler the three-dripline treatments treatments. were visible A for possible the winter reason planted for this trial, may while be thatmore the differences fertigation were did observed not move in thethe Nfall in experime betweennt the in November. driplines. The Leaves total be plantgan to tissue yellow N in content between of thethe dripdrip treatmentslaterals in plots with with three the driplines three-dripline in bed tr waseatments. less than A possible the sprinkler reason treatment for this may on thebe that 30th the of Novemberfertigation did (1.8% not N move for the the drip N vs.in between 3.2% N forthe thedriplines. sprinkler), The whiletotal plant the di tissuefference N incontent N content of the of drip the tissuetreatments was lesswith between three driplines the sprinkler in bed treatment was less than and thethe dripsprinkler treatment treatment with on four the driplines 30th of November in the bed (Figure(1.8% N7 ).for Overall, the drip avs. higher 3.2% plant-tissueN for the sprinkler), nitrogen while content the difference was observed in N forcontent the sprinklerof the tissue treatment was less comparedbetween the to sprinkler drip treatments. treatment and the drip treatment with four driplines in the bed (Figure 7). Overall, a higher plant-tissue nitrogen content was observed for the sprinkler treatment compared to drip treatments. Agriculture 2019, 9, x FOR PEER REVIEW 9 of 14 Agriculture 20192019,, 99,, 177x FOR PEER REVIEW 9 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 9 of 14

Figure 7. The trends of total plant nitrogen content over the fall crop season. The results are presented Figure 7. The trends of total plant nitrogen content over the fall crop season. The results are presented for the individual irrigation treatments. forFigure thethe individualindividual7. The trends irrigationirrigation of total treatments.treatments.plant nitrogen content over the fall crop season. The results are presented for the individual irrigation treatments. TheThe leafleaf chlorophyllchlorophyll contentcontent (Figure(Figure8 8))8) was waswas also alsoalso higher higherhigher in inin the thethe sprinkler sprinklersprinkler treatments treatmentstreatments compared comparedcompared to toto thethe dripdripdripThe treatments,treatments, treatments,leaf chlorophyll thoughthough content thethe dripdrip (Figure treatmenttreatmenttreatment 8) was withwith also fourfour fourhigher driplinesdriplinesdriplines in the sprinkler atat 3.83.83.8 cmcmcm treatments depthdepthdepth waswaswas numericallycompared numericallynumerically to morethemore drip similarsimilar treatments, toto sprinklersprinkler though treatmentstreatments the drip and andtreatment sometimesometime with hadhad four aa greatergreaterdriplines leafleaf at chlorophyll chlorophyll3.8 cm depth contentcontent was numerically duringduring thethe cropmorecrop seasons.seasons. similar to AA greatersprinklergreater levellevel treatments ofof leafleaf chlorophyllchlorophyll and sometime contentcontent had inain greater thethe latelate leaf seasonseason chlorophyll waswas observedobserved content atat theduringthe winterwinter the experimentcropexperiment seasons. than than A greaterthe the fall fall levelexperiment experiment experiment of leaf forchlorophyll for for both both both sprink sprink sprinkler contentlerler and and andin drip thedrip drip late treatments. treatments. treatments. season was For For For observedinstance, instance, instance, theat the the total thetotal winter total leaf leaf leafchlorophyllexperimentchlorophyll chlorophyll content thancontent content the of offall the the ofexperiment sprinkler thesprinkler sprinkler treatment treatmentfor treatmentboth sprink two two twodays daysler daysand before before drip before the the treatments. final final the ﬁnal harvest harvest harvestFor atinstance,at the the at winter winter the the winter trial totaltrial trialwas wasleaf −2 4 µg cm−2 more2 than leaf chlorophyll content of sprinkler treatment at the fall trial. Variable leaf was4chlorophyll µg 4cmµgcm more content− more than of thanleaf the sprinklerchlorophyll leaf chlorophyll treatment content content two of sprinklerdays of sprinkler before treatment the treatment final atharvest the at fall the at thetrial. fall winter trial. Variable Variabletrial leafwas leafchlorophyll4chlorophyll µg chlorophyll cm−2 more contentcontent contentthan waswas leaf was observed observedchlorophyll observed overover overcontent thethe the plantplant of plant sprinkler developmentdevelopment development treatment inin in eacheach eachat the ofof of thefall the treatments, trial.treatments, Variable whichwhichwhich leaf correspondschlorophyllcorresponds totocontentto thethethe plantplantplant was N N Nobserved accumulation accumulationaccumulation over over overtheover plantthe thethe growing growinggrowing development seasons. seasons.seasons. in each of the treatments, which corresponds to the plant N accumulation over the growing seasons.

(a)(a) (b)(b) (a) (b)

FigureFigure 8.8.8. The The trendstrends ofof averageaverageaverage leafleaf chlorophyllchlorophyll contentcontent ofof spinachspinach atat didifferentdifferentﬀerent irrigation irrigationirrigation treatmentstreatmentstreatments overFigureover ((aa)) ) 8. the thethe The fall fallfall trends crop cropcrop seasonseason seasonof average and andand ( (leafb(bb))) winter winter winterchlorophyll cropcrop crop season. season. season.content of spinach at different irrigation treatments over (a) the fall crop season and (b) winter crop season. Figure9 shows the average day NDVI values for the drip treatment with four driplines (the FigureFigure 9 9 shows shows thethe average average day day NDVI NDVI valuesvalues for for thethe dripdrip treatmenttreatment with with four four driplinesdriplines (the (the 3.8D-4B 3.8D-4B 3.8D-4B treatment) and the sprinkler treatment for the fall season experiment. The results indicated treatment)treatment)Figure and and 9 shows the the sprinkler sprinkler the average treatment treatment day NDVI for for the the values fall fall se se forasonason the experiment. experiment. drip treatment The The withresults results four indicated indicated driplines that that (the the the 3.8D-4B NDVI NDVI that the NDVI values varied from 0.2 (two weeks after planting) to 0.84 (the day before ﬁnal harvest). valuestreatment)values varied varied and from from the 0.2sprinkler 0.2 (two (two weeks weeks treatment after after for planting) planting) the fall toseto 0.84ason 0.84 (the (theexperiment. day day before before The final final results harvest). harvest). indicated By By late late that November, November, the NDVI By late November, the NDVI values were very similar in both irrigation treatments, where this index thevaluesthe NDVINDVI varied valuesvalues from werewere 0.2 (two veryvery weeks similarsimilar after inin bothboth planting) irrigationirrigation to 0.84 treatments,treatments, (the day before wherewhere final thisthis harvest). inindexdex hadhad By higherhigher late November, valuesvalues inin had higher values in the drip treatment even over a short period. The results demonstrated lower thethe dripNDVIdrip treatmenttreatment values were eveneven very overover similar aa shortshort in period. period.both irrigation TheThe resultsresults treatments, demonstrateddemonstrated where lowerlowerthis in NDVIdexNDVI had valuesvalues higher inin values thethe dripdrip in NDVI values in the drip treatment than sprinkler treatment over the last two weeks of the crop season. treatmentthetreatment drip treatment thanthan sprinklersprinkler even treatmentovertreatment a short overover period. thethe lastlast The twotwo results weeksweeks demonstrated ofof thethe cropcrop season.season. lower NDVI values in the drip treatment than sprinkler treatment over the last two weeks of the crop season.

FigureFigure 9.9. NDVI NDVI values values ofof sprinklersprinkler andand dripdrip (3.8D-4B)(3.8D-4B) treatmentstrtreatmentseatments overover thethe fall fall cropcrop season. season.season. TheThe dailydaily valuesFigurevalues vary vary9. NDVI betweenbetween values a aa maximum maximummaximumof sprinkler during duringduring and drip daytime daytimedaytime (3.8D-4B) and andand zero zerotrzeroeatments duringduring during over nighttime. nighttime.nighttime. the fall crop season. The daily values vary between a maximum during daytime and zero during nighttime. Agriculture 2019, 9, 177 10 of 14 Agriculture 2019, 9, x FOR PEER REVIEW 10 of 14

TheThe valuesvalues ofof total total plant plant nitrogen nitrogen content, content, leaf chlorophyllleaf chlorophyll content content of spinach, of spinach, and NDVI and confirmed NDVI confirmedthat N uptake that at N the uptake drip at treatments the drip treatments was not entirely was no ast eentirelyffective as as effective the sprinkler as the treatment, sprinkler particularly treatment, particularlyin the fall experiment. in the fall Theexperiment. nutrient managementThe nutrient issue management in spinach issue drip irrigationin spinach in drip combination irrigation with in combinationwater management with water is likely management a critical is issue likely that a critic we needal issue to learn that we more need about, to learn since more it may about, affect since the itadoption may affect and the viability adoption of theand drip viability for spinach of the drip production. for spinach Spinach production. is a very Spinach fast short-season is a very fast crop, short- and seasonhence, crop, water and and hence, nitrogen water management and nitrogen may manage significantlyment may influence significantly the leaf influence as the major the leaf organ as the for majorimportant organ physiological for important processes physiological and essential processe indicators and essential used to measureindicator the used growth to measure and yield the of growththe crop. and While yield the of leafthe areacrop. and While shoot the biomass leaf area of and spinach shoot are biomass greatly of aff spinachected by are water greatly and nitrogenaffected bylevels water [4], and this nitrogen dependency levels is [4], likely this more dependency critical in is organic likely more spinach. critical in organic spinach.

3.5.3.5. Leaf Leaf Wetness FigureFigure 1010 showsshows the the probe probe output output of of the the leaf leaf we wetnesstness sensors sensors placed placed in in the the sprinkler sprinkler treatment treatment andand drip drip treatment treatment (3.8D-4B) (3.8D-4B) for for a a period period of of 12 12 days days during during the the fall fall season season experiment. experiment. At At this this period, period, therethere were were two two irrigation irrigation events events in in each each of of the treatments, and two rainy days. The The sensor sensor output output fromfrom dew dew was was typically typically lower lower (less (less than than 700 700 counts) counts) th thanan that that from from the the rain rain or or the the irrigation irrigation event. event. TheThe results results revealed revealed that that sprinkler-irrigated sprinkler-irrigated crop canopies remained wet 24.3% (= (= (70(70 h/288 h/288 h) h) × 100)100) × timestimes longer longer during during this this period period than than the the cr cropop canopy canopy irrigated with drip treatment.

irrigation + dew irrigation rain

dew dew

FigureFigure 10. 10. TheThe row row counts counts of of leaf leaf wetness sensors at the sprinkler andand dripdrip (3.8D-4B)(3.8D-4B)treatments treatments over over a a12-day 12-day period period in in the the fall fall crop crop season. season.

SpinachSpinach downy downy mildew mildew requires requires a a cool cool environment environment with with long long peri periodsods of of leaf leaf wetness wetness or or high high humidity.humidity. Wet Wet foliage is especially favorable. Cons Consideringidering the the above above analysis, analysis, and and in in the the case case where where thethe weather weather and and farming farming conditions conditions are are similar, similar, there there is is higher higher risk risk for for infection infection and and downy downy mildew mildew diseasedisease development inin spinachspinach irrigated irrigated by by sprinklers sprinklers in comparisonin comparison with with spinach spinach irrigated irrigated by drips. by drips.The air The temperature air temperature and relative and relative humidity humidity pattern patte (Figurern (Figure2) indicate 2) indicate that there that was there a desirable was a desirable weather weathercondition condition and more and possibility more possibility for downy for mildew downy disease mildew in disease mid-December, in mid-December, but the fall but experiment the fall experimentwas just terminated was just at terminated the time. The at nextthe time. desirable The periodnext desirable was mid-February period was when mid-February temperatures when had temperaturescooled to the rangehad cooled believed to tothe be range optimal believed for downy to be mildew, optimal days for became downy windier, mildew, and days there became was a windier,period of and leaf there surface was wetness a period caused of leaf by surface sprinkler wetness irrigation, caused rainfall, by sprinkler or high relative irrigation, humidity. rainfall, or high relative humidity. 3.6. Shoot Biomass 3.6. ShootThe eBiomassffects of various irrigation treatments on spinach shoot biomass yield over the two experimental seasons are summarized in Table2 and Figure 11. In the fall trial, the mean biomass yield The effects of various irrigation treatments on spinach shoot biomass yield over the two in the sprinkler treatment was 13,905 kg ha 1, approximately 9% more than the 3.8D-4B treatment. experimental seasons are summarized in Table− 2 and Figure 11. In the fall trial, the mean biomass The lowest mean yield (11,136 kg ha 1) was observed in the 0D-3B treatment. Statistical analysis yield in the sprinkler treatment was− 13,905 kg ha−1, approximately 9% more than the 3.8D-4B indicated very strong evidence (p = 0.001) for an overall effect of the irrigation system on spinach yield. treatment. The lowest mean yield (11,136 kg ha−1) was observed in the 0D-3B treatment. Statistical A significant difference between the individual treatments was investigated using the Tukey-HSD analysis indicated very strong evidence (p = 0.001) for an overall effect of the irrigation system on analysis. The results demonstrated a significant yield difference between the sprinkler irrigation and spinach yield. A significant difference between the individual treatments was investigated using the each of the drip irrigation treatments (p values of 0.0001 to 0.0009). Even though no significant yield Tukey-HSD analysis. The results demonstrated a significant yield difference between the sprinkler difference was obtained between the surface drip and sub-surface drip (driplines at 3.8 cm depth) irrigation and each of the drip irrigation treatments (p values of 0.0001 to 0.0009). Even though no with the same dripline number in bed (p value of 0.8276 for the three-dripline and 0.1995 for the significant yield difference was obtained between the surface drip and sub-surface drip (driplines at 3.8 cm depth) with the same dripline number in bed (p value of 0.8276 for the three-dripline and 0.1995 for the four-dripline), the number of driplines in bed had a very significant impact on spinach biomass yield (p values of 0.0001 to 0.0009). Agriculture 2019, 9, 177 11 of 14 four-dripline),Agriculture 2019, the9, x numberFOR PEER of REVIEW driplines in bed had a very significant impact on spinach biomass yield11 of 14 (p values of 0.0001 to 0.0009). In the winter trial, the mean biomass yield in the sprinkler treatment was 14,886 kg ha1−1, In the winter trial, the mean biomass yield in the sprinkler treatment was 14,886 kg ha− , approximatelyapproximately 7% 7% more more than than the the 3.8D-4B 3.8D-4B treatment.treatment. Statistical analysis analysis indicated indicated strong strong evidence evidence (p (p== 0.0424)0.0424) forfor an an overall overall effect effect of irrigation system onon spinachspinach freshfresh yield. yield.While While we we could could not not find find a a significantsignificant di differencefference between between the the impact impact of of the the sprinkler sprinkler and and the the 3.8D-4B 3.8D-4B irrigation irrigation treatments treatments on on spinachspinach yield yield (p (p= =0.1161), 0.1161), there there was was a a statistically statistically significant significant yield yield di differencefference between between the the sprinkler sprinkler andand the the 3.8D-3B 3.8D-3B irrigation irrigation treatments treatments (p (=p =0.04147). 0.04147).

TableTable 2. 2.Mean Mean spinach spinach fresh fresh yield yield values values of of each each irrigation irrigation treatment treatment in in each each of of the the fall fall and and winter winter experiments.experiments. Yields Yields with with di differfferentent letters letters significantly significantly di ffdifferer (p (

FallFall 2018 2018 Winter Winter 20192019 −1 −1 Irrigation Treatment Fresh Yield (kg 1ha ) Irrigation Treatment Fresh Yield (kg ha 1 ) Irrigation Treatment Fresh Yield (kg ha− ) Irrigation Treatment Fresh Yield (kg ha− ) Sprinkler 13,905 a Sprinkler 14,886 a Sprinkler 13,905 a Sprinkler 14,886 a 3.8D-4B 12,753 b 3.8D-4B 13,914 ab 3.8D-4B 12,753 b 3.8D-4B 13,914 ab b b 0D-4B0D-4B 12,27312,273b 3.8D-3B3.8D-3B 13,58013,580 b 0D-3B0D-3B 11,13611,136c c --- - 3.8D-3B3.8D-3B 11,35111,351c c --- -

(a) (b)

Figure 11. Mean yield data of the sample beds in each of the irrigation treatments at the fall experiment andFigure (a) and 11. theMean winter yield experiment data of the (sampleb). Horizontal beds in each red linesof the indicate irrigation the treatments average for at eachthe fall treatment. experiment and (a) and the winter experiment (b). Horizontal red lines indicate the average for each treatment. The yield reduction in drip irrigation treatments compared to the sprinkler irrigation ranged betweenThe 7% yield (the reduction 3.8D-4B treatment in drip irrigation against the treatmen sprinklerts treatmentcompared in to the the winter sprinkler trial) irrigation and 24.8% ranged (the 0D-3Bbetween treatment 7% (the against 3.8D-4B the treatment sprinkler against treatment the insprinkler the fall trial).treatment The yieldin the diwinterfference trial) may and have 24.8% likely (the been0D-3B caused treatment by suboptimal against the irrigation sprinkler andtreatment nutrient in the management fall trial). The conditions yield difference of the drip may treatments. have likely Sincebeen drip caused irrigation by suboptimal was tested irrigation for the firstand timenutrient for spinach management in this conditions study, subsequent of the drip trials treatments. need to planSince for drip irrigation irrigation and was nutrient tested improvements for the first time and fo ber conductedspinach in inthis di ffstudy,erent aspects.subsequent These trials practices need to hadplan to for be adjustedirrigation in and real nutrient time as improvements the study progressed. and be conducted The biomass in different yield reported aspects. here These are practices related tohad a final to be harvest, adjusted which in real was time on theas the same study day prog for allressed. the treatments. The biomass Since yield the reported findings here showed are related that theto leafa final density harvest, of drip which irrigation was on treatments the same day was fo morer all behindthe treatments. than sprinkler Since the treatment, findings scheduling showed that a dithefferent leafharvest density time of drip for theirrigation drip treatments treatments may was compensate more behind for somethan sprinkle of the yieldr treatment, reductions scheduling against thea different sprinkler harvest treatment. time for the drip treatments may compensate for some of the yield reductions againstSeveral the factorssprinkler influence treatment. appropriate drip irrigation management, including system design, soil characteristics,Several factors and environmental influence appropriate conditions. drip The irriga influencestion management, of these factors including can besystem integrated design, into soil acharacteristics, practical and e andfficient environmental system which conditions. determine The the in quantityfluences of and these timing factors of drip can be irrigation. integrated Drip into irrigationa practical off ersand the efficient potential system for precisewhich waterdetermine management, the quantity and and also timing provides of thedrip ideal irrigation. vehicle Drip to irrigation offers the potential for precise water management, and also provides the ideal vehicle to deliver nutrients in a timely and efficient manner. However, achieving high water and nutrient use efficiency while maximizing crop productivity requires intensive and proper management. However, the 7% through 13% shoot biomass difference between the four dripline treatments and the sprinkler treatment demonstrates the potential of sub-surface drip irrigation for profitable Agriculture 2019, 9, 177 12 of 14 deliver nutrients in a timely and efficient manner. However, achieving high water and nutrient use efficiency while maximizing crop productivity requires intensive and proper management. However, the 7% through 13% shoot biomass difference between the four dripline treatments and the sprinkler treatment demonstrates the potential of sub-surface drip irrigation for profitable spinach production. This yield difference could be reduced through optimal system design and better irrigation and nutrient management practices for the drip system. Optimizing water and nitrogen management had not been an objective for this study, therefore, evaluating water use efficiency of the treatments may not be an interest to be discussed. With the 7% lower biomass yield and 11% less applied water in the 3.8D-4B than the sprinkler treatment, we may Agriculture 2019, 9, x FOR PEER REVIEW 12 of 14 simply conclude that drip irrigation may have the potential to enhance the efficiency of water use in spinachspinach production production. or at This least yield at difference this point, could there be reduced is no through major optimal difference system between design and the better two systems regardingirrigation this indicator. and nutrient High management water use practices efficiency for the fordrip sub-surface system. drip irrigation was reported by Optimizing water and nitrogen management had not been an objective for this study, therefore, researchersevaluating for multiple water use crops efficiency [12,13 of,15 the,21 treatments]. may not be an interest to be discussed. With the 7% lower biomass yield and 11% less applied water in the 3.8D-4B than the sprinkler treatment, we may 3.7. Downysimply Mildew conclude Incidence that drip irrigation may have the potential to enhance the efficiency of water use in spinach production or at least at this point, there is no major difference between the two systems Downy mildew was not observed in the fall experiment. In the winter experiment, downy mildew regarding this indicator. High water use efficiency for sub-surface drip irrigation was reported by activity wasresearchers first confirmed for multiple in crops the [12,13,15,21]. study area on 5 March. Disease incidence was rated on 11 March. Downy mildew incidence was low on this date, with only two beds (0.12% and 0.20%) exhibiting incidence3.7. values Downy above Mildew 0.1% Incidence (Figure 12). Mean downy mildew incidence in plots irrigated with sprinklers followingDowny mildew emergence was not was observed 0.08%, in approximatelythe fall experiment. 4 toIn 5the higherwinter experiment, than treatments downy irrigated × with drip followingmildew activity emergence. was first confirmed Statistical in the analysis study area indicated on 5 March. evidence Disease (incidencep = 0.0461) was forrated an on overall 11 effect March. Downy mildew incidence was low on this date, with only two beds (0.12% and 0.20%) of irrigationexhibiting treatment incidence on downy values above mildew. 0.1% (Figure 12). Mean downy mildew incidence in plots irrigated Analysiswith ofsprinklers means following suggested emergence that all was three 0.08%, treatments approximately were 4 to statistically 5× higher than similar. treatments However, a pairwise comparisonirrigated with revealed drip following some emergence. evidence Statistical that downy analysis mildew indicated incidence evidence was (p = lower 0.0461) in for plots an irrigated with 3.8D-4Boverall (p = effect0.0671) of irrigation or 3.8D-3B treatment (p = on0.1139) downy followingmildew. emergence when compared to the sprinkler. Analysis of means suggested that all three treatments were statistically similar. However, a pairwise The likelycomparison mechanism revealed some causing evidence this that e downyffect was mildew a reduction incidence was under lower in drip plots irrigation irrigated with of 3.8D- leaf wetness, which is critical4B (p = 0.0671) for infection or 3.8D-3B and (p = sporulation0.1139) following by emergence the downy when mildew compared pathogen. to the sprinkler. Additional repetitions of this experimentThe likely in higher mechanism disease causing pressure this effect situations was a reduction are needed under drip for furtherirrigation evaluation of leaf wetness, of the ability of drip irrigationwhich is critical to reduce for infection downy and sporulation mildew. by Another the downy mechanism mildew pathogen. that Additional could partially repetitions account for of this experiment in higher disease pressure situations are needed for further evaluation of the ability the observedof drip di irrigationfferences to amongreduce downy the treatments mildew. Anot isher that mechanism the leaf that density could partially in drip-irrigated account for the plots was slightly behindobserved that differences of the sprinkler among the irrigatedtreatments is plots that th ine time.leaf density A less in drip-irrigated dense canopy plots could was slightly reduce the leaf wetness potential,behind that and of the in sprinkler turn,disease irrigated incidenceplots in time.potential. A less dense However,canopy could it reduce is unclear the leafif wetness the magnitude of differencespotential, in density and in couldturn, disease account incidence for the potential. magnitude However, in di itff erencesis unclear in if downythe magnitude mildew of incidence differences in density could account for the magnitude in differences in downy mildew incidence between sprinkler-between sprinkler- and drip-irrigated and drip-irrigated treatments. treatments.

Figure 12.FigureRaw data12. Raw (all data plots (all andplots beds and beds within within plots) plots) of of downy downy mildew incidence. incidence. Horizontal Horizontal bars bars and labels indicateand labels the meanindicate for the each mean treatment. for each treatment. Points Points are jittered are jittered to to reduce reduce overlap. overlap. The The top top line line of of x-axis x-axis labels indicate what the plot was germinated with, and the bottom row indicates the irrigation labels indicate what the plot was germinated with, and the bottom row indicates the irrigation used used following emergence. following emergence. Perhaps the most important benefit of drip for spinach production could be less yield loss as a result of downy mildew management. Spinach is a high-value crop—for instance, the value of Agriculture 2019, 9, 177 13 of 14

Perhaps the most important beneﬁt of drip for spinach production could be less yield loss as a result of downy mildew management. Spinach is a high-value crop—for instance, the value of spinach per kg was about $2 in 2016 [22]. While more data are needed to conduct an economic feasibility analysis of drip irrigation for spinach production, the results of this study demonstrated a positive impression. The initial cost associated with a drip system for spinach ﬁelds is estimated at about $3000 to $4000 per hectare in the region.

4. Conclusions This study demonstrated the potential for drip irrigation to be used to produce organic spinach, conserve water, enhance the eﬃciency of water use, and reduce incidence of downy mildew. Further work is needed to comprehensively evaluate the viability of utilizing drips—speciﬁcally, the optimal system design, the impacts of irrigation and nitrogen management practices in various soil types and climates, and strategies to maintain productivity and economic viability at spinach. Assessing drip irrigation for the entire crop season, including germination, could be another research interest since spinach is a short-season crop and combining the sprinkler for crop germination and drip for such a short period might cause some practical issues.

Author Contributions: Conceptualization, A.M. and M.C.; Data curation, A.M.; Formal analysis, A.M.; Investigation, A.M. and A.P.; Methodology, A.M., M.C. and A.P.; Project administration, A.M.; Supervision, A.M.; Writing—original draft, A.M.; Writing—review & editing, A.M., M.C. and A.P. Funding: This research was funded by the California Leafy Greens Research Board. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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