ONION RESPONSE TO LATE-SEASON WATER STRESS AND HIGH NITROGEN UNDER TWO PLANT POPULATIONS

ONION RESPONSE TO LATE-SEASON WATER STRESS AND HIGH NITROGEN UNDER TWO PLANT POPULATIONS Jose M. Pinto, Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders, Malheur Experiment Station, Oregon State University, Ontario, OR, 2014 Deron Beck, Monsanto Co., Payette, ID Introduction A split onion bulb happens when the bottom of the bulb splits open, and split bulbs can reduce onion crop marketable yields. Anecdotal evidence suggests that late-season irrigation interruptions and/or excessive nitrogen (N) fertilizer can cause bulb splitting. Past research at the Oregon State University Malheur Experiment Station demonstrated the sensitivity of onion yield and grade to soil water tension (Shock et al. 2000). In many other countries onions are grown at higher plant populations than in the Treasure Valley. This trial tested the effect of late-season irrigation interruption and excessive N fertilizer on bulb splitting at two plant populations with two varieties. Materials and Methods Onions were grown in 2014 on an Owyhee silt loam. The field had been planted to wheat in 2013. In the fall of 2013, the wheat stubble was shredded and the field was irrigated. The field was then disked, moldboard plowed, and groundhogged. A soil analysis taken in the fall of 2013 showed a ph of 7.9, 1.27% organic matter, 181% base saturation, 21 ppm of phosphorus, and 418 ppm of potassium, and less than 1.5% lime. Based on the soil analysis, 500 lb of sulfur/acre, 3 lb of manganese/acre, and 1 lb of boron/acre were broadcast before plowing. After plowing, the field was fumigated with Vapam at 15 gal/acre and bedded at 22 inches. Seed was planted on March 18 in double rows spaced 3 inches apart at 9 seeds/ft of single row. Each double row was planted on beds spaced 22 inches apart. Planting was done with customized John Deere Flexi Planter units equipped with disc openers. Immediately after planting, the onions received a narrow band of Lorsban 15G at 3.7 oz/1,000 ft of row (0.82 lb ai/acre), and the soil surface was rolled. Onion emergence started on April 7. The field had drip tape laid at 4-inch depth between 2 onion beds during planting. The drip tape had emitters spaced 12 inches apart and emitter flow rate of 0.22 gal/min/100 ft (Toro Aqua- Traxx, Toro Co., El Cajon, CA). The distance between the tape and the center of each double row of onions was 11 inches. The experimental design was a split-split plot randomized complete block with six replicates. The main treatment plots were 4 double rows wide by 54 ft long. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 60

The four treatments consisted of two nonstressed check treatments (an adequate N fertilizer check and a high N fertilizer check) and two water-stressed treatments (an adequate N fertilizer stress treatment and a high N fertilizer stress treatment). The stress treatments used a soil water tension of 20 cb until July 14, when temporary water stress was imposed. Two onion varieties ( Vaquero, Nunhems, Parma, ID and Swale, Seminis, Payette, ID) were planted as split plots within each main plot. Each variety split plot was divided into two plant population split-split plots (120,000 and 450,000 plants/acre). Variety split plots were 27 ft long and plant population split-split plots were 13.5 ft long. On March 21, a mixture of humic acid (CHB Premium 6, Bio-Gro, Mabton, WA, 5% humic acids, 6 gal/acre), phosphoric acid (NUE 0-30-0, Bio-Gro, 26 lb P/acre), and Avail (Simplot, Caldwell, ID, 0.5% of the final volume) was sidedressed between the seed row and the drip tape at 3-inch depth. On May 16, the population split-split plots were thinned by hand. The plots thinned to 120,000 plants/acre had onions thinned to 4.75 inches between plants in each single row. The plots thinned to 450,000 plants/acre had onions thinned to 1.4 inches between plants in each single row. Onions were irrigated to maintain the soil water tension (SWT) in the onion root zone below 20 cb (Fig. 1). Soil water tension was measured in each 450,000-plant/acre split-split plot in the Vaquero split plot in each main plot. Soil water tension in each split-split plot was measured with four granular matrix sensors (GMS, Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, CA) installed at 8-inch depth in the center of the double row. Sensors had been calibrated to SWT (Shock et al. 1998). The sensors were read manually at 8 a.m. from Monday through Friday. The irrigation decisions were based on the average SWT of the four GMS in each plot. The irrigation durations were 24 hours (1.3 inches of water). The temporary water stress was started on July 14, and was imposed by withholding irrigations until the SWT in each plot reached 60 cb; at that point, irrigations were restarted to maintain the SWT at 20 cb. The water stress period was ended on July 21 for 10 plots (one plot was re-irrigated on July 18 and one on July 22). The irrigations were controlled using a controller (SDM CD16AC controller, Campbell Scientific, Logan, UT) connected to a solenoid valve in each main plot. The water for the drip system was supplied by a well that maintained a continuous and constant water pressure of 30 psi. The pressure in the drip lines was maintained at 10 psi by pressure regulators in each plot. Irrigations for the whole trial were terminated on September 2. Onion evapotranspiration (ET c ) was calculated with a modified Penman equation (Wright 1982) using data collected at the Malheur Experiment Station by an AgriMet weather station. Onion ET c was estimated and recorded from crop emergence until the onions were lifted. Starting on June 4 and every week thereafter, plants with their bulbs and roots were sampled from the nonharvest rows of each plot of treatment 1 (nonstressed, adequately fertilized) and treatment 3 (nonstressed, high N fertilizer) and the bulbs and roots washed in deionized water. One composite sample from each treatment consisting of the roots from all plots in each treatment was sent to Western Laboratories, Inc. (Parma, ID) for nutrient analysis. All nutrients except N were applied based on root tissue analysis. For the adequate N treatments (1 and 2), urea ammonium nitrate solution was applied from early June to early July to total 100 lb of N/acre (Table 1). For the high N treatments (3 and 4), urea ammonium nitrate solution was Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 61

applied from early June to early July to total 400 lb of N/acre. Nutrients were injected into the drip irrigation system of each treatment using an Ozawa Precision Metering Pump (Ozawa R and D, Ontario, OR). The onions were managed to minimize yield reductions from weeds, pests, diseases, water stress, and nutrient deficiencies (Table 2). For weed control, the following herbicides were applied. On April 21, Prowl H 2 O at 0.83 lb ai/acre (2 pt/acre) was broadcast. On May 8, Goal Tender at 0.09 lb ai/acre (4 oz/acre), Buctril at 0.25 lb ai/acre (16 oz/acre), and Poast at 0.38 lb ai/acre (24 oz/acre) were broadcast. On June 5, Poast at 0.38 lb ai/acre (24 oz/acre) was broadcast. For thrips control, the following insecticides were applied: Movento at 5 oz/acre on May 27 and June 5 (ground applications), Radiant at 10 oz/acre on June 12 (ground application), Agri-Mek at 16 oz/acre on June 19 (ground application), and June 30 (aerial application), Lannate at 0.9 lb ai/acre on July 6 and 13 (aerial applications), and Radiant at 10 oz/acre on July 22 and 27 (aerial applications). For disease control, Quadris Opti was broadcast aerially on August 18. All split-split plots were evaluated for maturity and for severity of symptoms of iris yellow spot virus (IYSV) on August 20. Onions were evaluated subjectively for maturity by visually rating the percentage of onions with the tops down and the percent dryness of the foliage. The number of bolted onion plants was counted in each plot. Each plot was given a subjective rating on a scale of 0 to 5 of increasing severity of IYSV symptoms. The rating was 0 if there were no symptoms, 1 if 1-25% of foliage was diseased, 2 if 26-50% of foliage was diseased, 3 if 51-75% of foliage was diseased, 4 if 76-99% of foliage was diseased, and 5 if 100% of foliage was diseased. The onions were lifted on September 10 to field cure. Onions from the 9 ft of the middle 2 rows in each split-split plot were topped by hand and bagged on September 17. The bags were put in storage on September 23. The storage shed was ventilated and the temperature was slowly decreased to maintain air temperature as close to 34 F as possible. Onions were graded out of storage on November 10. During grading, all bulbs from each split-split plot were counted. Split bulbs were counted and weighed. Bulbs were then separated according to quality: bulbs without blemishes (No. 1s), double bulbs (No. 2s), bulbs infected with neck rot (Botrytis allii) in the neck or side, plate rot (Fusarium oxysporum), or black mold (Aspergillus niger). The No. 1 bulbs were graded according to diameter: small (<2¼ inches), medium (2¼-3 inches), jumbo (3-4 inches), colossal (4-4¼ inches), and supercolossal (>4¼ inches). For the purposes of this report, all No. 1 bulbs were considered marketable. In markets where colossal and supercolossal bulbs are desired and plant populations are low, very few of the bulbs are small. In markets where medium and small bulbs are desired and plant populations are high, there are very few colossal and supercolossal bulbs. Bulb counts per 50 lb of supercolossal onions were determined for each plot of every variety by weighing and counting all supercolossal bulbs during grading. Treatment differences were compared using analysis of variance (ANOVA) and regression analysis. Means separation was determined using a protected Fisher s least significant difference test at the 5% probability level, LSD (0.05). Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 62

Results Soil water tension over time oscillated around the target of 20 cb (Fig. 1). The amount of water applied with irrigation at 20 cb paralleled crop evapotranspiration (ET c ), which totaled 34.8 inches for the season (Fig. 2, Table 1). There was no statistically significant difference in total water applied between treatments, but the high N fertilizer treatments had slightly lower total applied water. Plant population effects The plots thinned to 120,000 plants/acre actually contained close to 117,000 plants/acre and the plots thinned to 450,000 plants/acre actually contained close to 328,000 plants/acre. Averaged over varieties and treatments, colossal and jumbo yield were higher with 117,000 plants/acre (Table 5). Total yield, marketable yield, medium yield, small yield, total rot, and bolting were higher at 328,000 plants/acre. The percentages of tops down and bolting on August 20 were higher at 328,000 plants/acre. Water stress effects For Vaquero, water stress reduced marketable yield for the adequately fertilized onions at 120,000 plants/acre and for the high N fertilizer treatment at 450,000 plants/acre. For Swale, water stress reduced marketable yield for the adequately fertilized onions at 450,000 plants/ acre. Split bulbs were not found in this trial. N fertilizer level effects Root nitrate concentrations during the season for the high N fertilizer treatments were higher than for the adequate N treatments (Table 3). For Vaquero, high N fertilizer (400 lb N/acre) reduced total yield and marketable yield for nonstressed onions under both plant populations and for stressed onions under the 328,000- plants/acre plant population. For Swale, high N fertilizer (400 lb N/acre) reduced total yield and marketable yield for nonstressed and stressed onions under both plant populations. Averaged over water stress treatments, high N fertilizer increased storage rot for both varieties at 328,000 plants/acre. Averaged over varieties, plant populations, and water stress treatments, the percent of tops down, percent of leaf dryness, and IYSV severity were higher with high N fertilizer on August 20. Bulb single centers Averaged over treatments and varieties, the 330,000-plants/acre population resulted in higher single-centered and functionally single-centered bulbs than the 120,000-plants/acre population (Table 6). Bulb single centeredness was not affected by either July water stress or the N fertilization rate. Averaged over treatments and varieties, the 330,000-plants/acre population resulted in a higher percentage of tops down on August 20. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 63

Discussion The results of this study agree with a study at the Malheur Experiment Station (Shock et al. 2007) that tested the effect of single episodes of a 60-cb water stress at different timings of onion development. Shock et al. (2007) found that water stress at the 4- to 6-leaf stages (May through June) caused decreases in single centeredness, but not at the 8-leaf stage (late June to early July). In the present study water stress in the second half of July did not decrease single centeredness. In a similar study (Pinto et al. 2013), water stress to 60 cb decreased total and marketable yield, but the stress was initiated on July 31 and lasted 12 days compared to 2014 when the stress was initiated on July 14 and lasted 8 days. The negative effect of the high N would be expected considering that the soil in the adequately fertilized plots supplied substantial amounts of N during the season (Table 4). The adequate supply of soil available N, based on the soil samples, disagrees with the root tissue N contents, which were below the critical level for both N treatments until the middle of July. Acknowledgements This project was funded by the Idaho-Eastern Oregon Onion Committee, cooperating onion seed companies, Oregon State University, the Malheur County Education Service District and supported by Formula Grant no. 2014-31100-06041 and Formula Grant no. 2014-31200-06041 from the USDA National Institute of Food and Agriculture. References Shock, C.C., J. Barnum, and M. Seddigh. 1998. Calibration of Watermark soil moisture sensors for irrigation management. Irrigation Association. Proceedings of the International Irrigation Show. Pages 139-146. San Diego, CA. Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2000. Irrigation criteria for drip-irrigated onions. HortScience 35:63-66. Shock, C.C., E. Feibert, and L.D. Saunders. 2007. Short-duration water stress decreases onion single centers without causing translucent scale. HortScience 42(6):1450-1455. Pinto, J.M., C.C. Shock, E.B.G. Feibert, and L.D. Saunders. 2013. Onion response to late-season water stress at two plant populations. Oregon State University Agricultural Experiment Station Ext/CrS 149:57-65. Wright, J.L. 1982. New evapotranspiration crop coefficients. Journal of Irrigation and Drainage Division, American Society of Civil Engineers 108:57-74. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 64

Table 1. Total water applied (includes 1.7 inches of precipitation) from onion emergence to the last irrigation and average soil water tension. Evapotranspiration from emergence to lifting totaled 34.8 inches. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Irrigation criterion N fertilizer level Total water applied inches Average soil water tension cb 20 cb adequate 32.2 18.4 20 cb high 26.1 17.1 20 cb/stress adequate 31.2 21.4 20 cb/stress high 28.0 21.2 LSD (0.05) NS 3.2 Table 2. Nutrients applied (lb/acre) through the drip tape. All nutrients, except nitrogen, were applied based on root tissue analysis. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. No stress, adequate N Stress, adequate N No stress, high N Stress, high N Date N P K S Zn Mn B N P K S Zn Mn B N P K S Zn Mn B N P K S Zn Mn B 5-Jun 40 40 120 120 12-Jun 40 40 120 120 23-Jun 20 20 100 100 3-Jul 10 0.3 0.2 10 0.3 0.2 60 10 0.3 0.2 60 10 0.3 0.2 10-Jul 10 10 10 0.3 0.3 0.2 10 10 10 0.3 0.3 0.2 20 10 10 10 0.3 0.3 0.2 20 10 10 10 0.3 0.3 0.2 Total 100 10 20 10 0.6 0.3 0.4 100 10 20 10 0.6 0.3 0.4 420 10 20 10 0.6 0.3 0.4 420 10 20 10 0.6 0.3 0.4 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 65

Table 3. Root nitrate content for onions submitted to two nitrogen fertilizer regimes. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Date N fertilizer level Sufficiency range adequate high ------------- NO 3 - N, ppm ----------- 17-Jun 7108 1372 3789 24-Jun 5020 2603 4549 1-Jul 4411 3468 4654 8-Jul 3802 1653 3390 15-Jul 3193 3107 7226 22-Jul 2584 2719 9253 5-Aug 1366 3250 3003 14-Aug 583 2585 3529 Table 4. Soil and soil solution plant available nitrogen (N) contents from weekly top foot soil samples for adequately fertilized onions (total 100 lb N/acre). The critical level of soil solution N for onions is 2 lb N/day. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Date Soil solution Soil lb/day lb/acre 17-Jun 9.9 69 24-Jun 13.3 93 1-Jul 10.3 72 8-Jul 5.1 36 15-Jul 26.6 186 22-Jul 12.4 87 29-Jul 24 168 5-Aug 17.1 120 14-Aug 27.9 195 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 66

Figure 1. Soil water tension at 8-inch depth for onions irrigated at 20 cb without water stress and at 20 cb with temporary stress. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 67

Figure 2. Water applied plus precipitation and evapotranspiration (ET c ) for onions irrigated at 20 cb without water stress and at 20 cb with temporary stress. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 68

Table 5. Onion yield and grade for two varieties under two plant populations in response to late-season water stress and nitrogen fertilizer level. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Continued on next page. Plant population Marketable yield by grade Split bulbs Variety Water stress N level Target Actual Total yield Total >4¼ in 4-4¼ in 3-4 in 2¼-3 in Small Total rot % by No. Yield -- plants/acre -- --------------------------- cwt/acre --------------------------- % by yield % cwt/acre Vaquero no adequate 120,000 111,088 731.4 677.9 0.0 52.9 573.9 45.5 5.6 7.5 0.0 0.0 yes adequate 120,000 119,008 695.9 628.4 0.0 17.6 533.3 63.5 14.0 9.0 0.0 0.0 average 115,048 713.7 653.1 0.0 35.3 553.6 54.5 9.8 8.2 0.0 0.0 no high 120,000 118,568 664.3 607.6 0.0 11.3 444.2 126.5 25.6 8.5 0.0 0.0 yes high 120,000 116,412 709.4 648.8 0.0 25.9 534.8 80.1 8.1 8.2 0.0 0.0 average 117,588 684.8 628.2 0.0 17.9 485.4 105.4 17.6 8.4 0.0 0.0 no average 120,000 114,828 697.9 642.8 0.0 32.1 509.0 86.0 15.6 8.0 0.0 0.0 yes average 120,000 117,828 702.1 638.6 0.0 21.4 534.0 71.0 11.3 8.6 0.0 0.0 average 116,318 699.2 640.7 0.0 26.6 519.5 80.0 13.7 8.3 0.0 0.0 no adequate 450,000 318,307 934.5 823.6 0.0 0.0 247.7 398.6 177.2 11.9 0.0 0.0 yes adequate 450,000 357,243 951.7 806.7 0.0 0.0 191.2 410.9 204.6 15.3 0.0 0.0 average 337,775 943.1 815.1 0.0 0.0 219.4 404.8 190.9 13.6 0.0 0.0 no high 450,000 314,347 791.8 604.1 0.0 1.2 141.9 297.8 163.3 24.7 0.0 0.0 yes high 450,000 287,510 797.0 553.2 0.0 0.0 126.3 266.7 160.2 31.1 0.0 0.0 average 300,929 794.4 578.6 0.0 0.6 134.1 282.2 161.7 27.9 0.0 0.0 no average 450,000 316,327 863.1 713.8 0.0 0.6 194.8 348.2 170.2 18.3 0.0 0.0 yes average 450,000 322,376 874.3 679.9 0.0 0.0 158.7 338.8 182.4 23.2 0.0 0.0 average 319,352 868.7 696.9 0.0 0.3 176.8 343.5 176.3 20.7 0.0 0.0 average 217,835 784.0 669.2 0.0 13.4 348.1 211.7 95.0 14.5 0.0 0.0 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 69

Table 5. Continued. Onion yield and grade for two varieties under two plant populations in response to late-season water stress and nitrogen fertilizer level. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Continued on next page. Plant population Marketable yield by grade Split bulbs Variety Water stress N level Target Actual Total yield Total >4¼ in 4-4¼ in 3-4 in 2¼-3 in Small Total rot % by No. Yield -- plants/acre -- -------------------------- cwt/acre -------------------------- % by yield % cwt/acre Swale no adequate 120,000 115,488 764.6 714.9 0.0 34.6 615.0 56.5 8.7 6.6 0.0 0.0 yes adequate 120,000 125,387 769.0 721.5 0.0 30.1 605.8 76.0 9.5 6.1 0.0 0.0 average 120,437 766.8 718.2 0.0 32.4 610.4 66.3 9.1 6.3 0.0 0.0 no high 120,000 111,924 665.0 635.7 0.0 23.3 526.3 71.9 14.1 4.3 0.0 0.0 yes high 120,000 119,888 662.8 613.1 0.0 2.2 511.0 85.8 14.1 7.3 0.0 0.0 average 116,268 663.8 624.4 0.0 11.8 518.0 79.5 14.1 5.9 0.0 0.0 no average 120,000 113,868 719.3 675.3 0.0 29.5 574.7 63.5 11.2 5.6 0.0 0.0 yes average 120,000 122,637 715.9 667.3 0.0 16.2 558.4 80.9 11.8 6.7 0.0 0.0 average 118,353 715.3 671.3 0.0 22.1 564.2 72.9 11.6 6.1 0.0 0.0 no adequate 450,000 347,784 1060.3 995.1 0.0 0.0 270.3 520.4 204.5 6.1 0.0 0.0 yes adequate 450,000 349,104 969.3 904.9 0.0 0.0 166.4 494.6 244.0 6.7 0.0 0.0 average 348,444 1014.8 950.0 0.0 0.0 218.3 507.5 224.2 6.4 0.0 0.0 no high 450,000 316,239 788.6 694.2 0.0 0.0 130.6 363.4 200.2 12.3 0.0 0.0 yes high 450,000 331,945 774.5 682.2 0.0 0.0 89.9 378.4 213.9 11.8 0.0 0.0 average 324,806 780.9 688.2 0.0 0.0 108.4 371.6 207.7 12.0 0.0 0.0 no average 450,000 333,445 936.8 844.7 0.0 0.0 206.8 449.0 202.6 8.9 0.0 0.0 yes average 450,000 340,525 871.9 793.6 0.0 0.0 128.1 436.5 229.0 9.3 0.0 0.0 average 336,625 897.9 819.1 0.0 0.0 163.3 439.5 216.0 9.2 0.0 0.0 average 227,489 806.6 748.7 0.0 11.1 363.8 256.2 113.8 7.7 0.0 0.0 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 70

Table 5. Continued. Onion yield and grade for two varieties under two plant populations in response to late-season water stress and nitrogen fertilizer level. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Plant population Marketable yield by grade Split bulbs Variety Water stress N level Target Actual Total yield Total >4¼ in 4-4¼ in 3-4 in 2¼-3 in Small Total rot % by No. Yield -- plants/acre -- ------------------------- cwt/acre -------------------------- % by yield % cwt/acre Average no adequate 120,000 113,288 748.0 696.4 0.0 43.8 594.4 51.0 7.1 7.0 0.0 0.0 yes adequate 120,000 122,197 732.5 674.9 0.0 23.9 569.6 69.8 11.8 7.5 0.0 0.0 average 117,743 740.3 685.7 0.0 33.8 582.0 60.4 9.5 7.3 0.0 0.0 no high 120,000 115,548 664.6 620.4 0.0 16.8 481.5 101.7 20.4 6.6 0.0 0.0 yes high 120,000 118,308 684.0 629.3 0.0 13.0 521.8 83.2 11.3 7.7 0.0 0.0 average 116,928 674.3 624.8 0.0 14.9 501.7 92.5 15.8 7.2 0.0 0.0 no average 120,000 114,369 708.1 658.4 0.0 30.9 540.4 75.2 13.5 6.8 0.0 0.0 yes average 120,000 120,337 709.3 652.1 0.0 18.7 546.7 76.2 11.6 7.6 0.0 0.0 average 117,353 708.7 655.2 0.0 24.8 543.6 75.7 12.5 7.2 0.0 0.0 no adequate 450,000 333,045 997.4 909.3 0.0 0.0 259.0 459.5 190.8 9.0 0.0 0.0 yes adequate 450,000 353,173 960.5 855.8 0.0 0.0 178.8 452.7 224.3 11.0 0.0 0.0 average 343,109 978.9 882.6 0.0 0.0 218.9 456.1 207.6 10.0 0.0 0.0 no high 450,000 315,207 790.3 645.1 0.0 0.6 136.7 327.6 180.1 19.1 0.0 0.0 yes high 450,000 309,728 785.8 617.7 0.0 0.0 108.1 322.5 187.1 21.5 0.0 0.0 average 312,348 788.0 631.4 0.0 0.3 121.8 325.0 183.7 20.3 0.0 0.0 no average 450,000 324,514 898.4 777.2 0.0 0.3 200.5 396.4 185.7 13.8 0.0 0.0 yes average 450,000 331,450 873.1 736.8 0.0 0.0 143.4 387.6 205.7 16.2 0.0 0.0 average 328,056 885.5 757.0 0.0 0.2 171.4 391.9 195.9 15.0 0.0 0.0 no adequate average 223,167 872.7 802.9 0.0 21.9 426.7 255.3 99.0 8.0 0.0 0.0 yes adequate average 237,685 846.5 765.4 0.0 11.9 374.2 261.2 118.0 9.3 0.0 0.0 average average 230,426 859.6 784.1 0.0 16.9 400.4 258.3 108.5 8.6 0.0 0.0 no high average 215,378 727.5 632.7 0.0 8.7 309.1 214.7 100.2 12.8 0.0 0.0 yes high average 218,179 737.1 623.2 0.0 6.2 305.9 208.1 103.0 14.9 0.0 0.0 average average 214,638 731.1 628.0 0.0 7.6 311.7 208.7 99.8 13.7 0.0 0.0 LSD (0.05) Water Stress NS NS NS NS NS NS NS NS NS NS N level 64.7 75.3 NS NS 64 NS NS NS NS NS Variety NS NS NS NS NS 8.2 NS NS NS NS Population 16.1 16.9 NS 6.5 22.9 16.8 9.5 1.6 1.6 NS Variety X Population NS NS NS NS NS NS NS 2.3 2.2 NS Water Stress X Pop. NS NS NS NS 32.4 NS NS NS NS NS N level X Pop. 22.8 24 NS NS NS 23.8 13.5 2.2 2.2 NS N level X Variety X Pop. NS NS NS NS NS NS NS 3.2 3.2 NS Water Stress X N level X Var. X Pop. 45.6 47.9 NS 18.4 NS NS NS NS NS NS Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 71

Table 6. Onion single-center ratings, maturity, and iris yellow spot virus (IYSV) severity for two varieties under two plant populations in response to late-season water stress. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Continued on next page. Plant population Maturity Aug 20 Multiple center Single center Variety Water stress N level target tops down leaf dryness bolting large medium small functional a single IYSV plants/acre --------------------------------------- % ----------------------------------------- 0-5 Vaquero no adequate 120,000 23.3 25.0 4.0 1.3 7.3 12.7 91.3 78.7 2.2 yes adequate 120,000 26.7 23.3 3.2 4.0 4.0 12.7 92.0 79.3 2.2 average 25.0 24.2 3.6 2.7 5.7 12.7 91.7 79.0 2.2 no high 120,000 26.7 26.7 1.3 0.7 9.3 10.0 90.0 80.0 2.5 yes high 120,000 23.3 25.0 1.8 0.8 8.0 12.8 91.2 78.4 2.2 average 25.0 25.8 1.5 0.7 8.7 11.4 90.6 79.2 2.3 no average 120,000 25.0 25.8 2.6 1.0 8.3 11.3 90.7 79.3 2.3 yes average 120,000 25.0 24.2 2.6 2.4 6.0 12.7 91.6 78.9 2.2 average 25.0 25.0 2.6 1.7 7.2 12.0 91.1 79.1 2.3 no adequate 450,000 58.3 26.7 9.1 0.0 1.3 4.0 98.7 94.7 1.8 yes adequate 450,000 71.7 25.0 8.3 0.7 0.7 9.3 98.7 89.3 2.0 average 65.0 25.8 8.7 0.3 1.0 6.7 98.7 92.0 1.9 no high 450,000 71.7 31.7 7.8 0.0 0.0 2.0 100.0 98.0 2.5 yes high 450,000 71.7 25.0 6.6 0.0 3.3 7.3 96.7 89.3 2.2 average 71.7 28.3 7.2 0.0 1.7 4.7 98.3 93.7 2.3 no average 450,000 65.0 29.2 8.5 0.0 0.7 3.0 99.3 96.3 2.2 yes average 450,000 71.7 25.0 7.5 0.3 2.0 8.3 97.7 89.3 2.1 average 68.3 27.1 8.0 0.2 1.3 5.7 98.5 92.8 2.1 average 46.7 26.0 5.3 0.9 4.2 8.9 94.8 86.0 2.2 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 72

Table 6. Continued. Onion single-center ratings, maturity, and IYSV severity for two varieties under two plant populations in response to late-season water stress. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Continued on next page. Plant population Maturity Aug 20 Multiple center Single center Variety Water stress N level target tops down leaf dryness bolting large medium small functional a single IYSV plants/acre ----------------------------------------- % ------------------------------------------ 0-5 Swale no adequate 120,000 23.3 20.0 12.0 2.7 1.3 15.3 96.0 80.7 1.2 yes adequate 120,000 25.0 23.3 2.7 0.0 6.0 12.0 94.0 82.0 1.8 average 24.2 21.7 7.4 1.3 3.7 13.7 95.0 81.3 1.5 no high 120,000 20.0 16.7 5.5 0.0 6.4 15.2 93.6 78.4 1.7 yes high 120,000 23.3 25.0 0.8 0.0 4.7 12.7 95.3 82.7 2.0 average 21.7 20.8 2.9 0.0 5.5 13.9 94.5 80.5 1.8 no average 120,000 21.7 18.3 9.0 1.3 3.9 15.3 94.8 79.5 1.4 yes average 120,000 24.2 24.2 1.8 0.0 5.3 12.3 94.7 82.3 1.9 average 22.9 21.3 5.1 0.7 4.6 13.8 94.7 80.9 1.7 no adequate 450,000 66.7 20.0 9.2 0.0 0.0 1.3 100.0 98.7 1.2 yes adequate 450,000 65.0 18.3 6.0 0.0 0.7 4.7 99.3 94.7 1.7 average 65.8 19.2 7.6 0.0 0.3 3.0 99.7 96.7 1.4 no high 450,000 76.7 26.7 2.9 0.0 2.4 4.8 97.6 92.8 2.0 yes high 450,000 73.3 26.7 3.5 0.0 0.7 1.3 99.3 98.0 2.0 average 75.0 26.7 3.2 0.0 1.5 3.1 98.5 95.4 2.0 no average 450,000 71.7 23.3 6.3 0.0 1.2 3.1 98.8 95.7 1.6 yes average 450,000 69.2 22.5 4.8 0.0 0.7 3.0 99.3 96.3 1.8 average 70.4 22.9 5.4 0.0 0.9 3.0 99.1 96.0 1.7 average 46.7 22.1 5.3 0.3 2.8 8.4 96.9 88.5 1.7 Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 73

Table 6. Continued. Onion single-center ratings, maturity, and IYSV severity for two varieties under two plant populations in response to late season water stress. Malheur Experiment Station, Oregon State University, Ontario, OR, 2014. Plant population Maturity Aug 20 Multiple center Single center Variety Water stress N level target tops down leaf dryness bolting large medium small functional a single IYSV plants/acre ----------------------------------------- % ----------------------------------------- 0-5 Average no adequate 120,000 23.3 22.5 8.0 2.0 4.3 14.0 93.7 79.7 1.7 yes adequate 120,000 25.8 23.3 3.0 2.0 5.0 12.3 93.0 80.7 2.0 average 24.6 22.9 5.5 2.0 4.7 13.2 93.3 80.2 1.8 no high 120,000 23.3 21.7 3.2 0.4 8.0 12.4 91.6 79.3 2.1 yes high 120,000 23.3 25.0 1.2 0.4 6.2 12.7 93.5 80.7 2.1 average 23.3 23.3 2.2 0.4 7.1 12.5 92.5 80.0 2.1 no average 120,000 23.3 22.1 5.7 1.2 6.2 13.2 92.7 79.5 1.9 yes average 120,000 24.6 24.2 2.2 1.2 5.6 12.5 93.2 80.7 2.0 average 24.0 23.1 3.9 1.2 5.9 12.9 92.9 80.1 2.0 no adequate 450,000 62.5 23.3 9.2 0.0 0.7 2.7 99.3 96.7 1.5 yes adequate 450,000 68.3 21.7 7.2 0.3 0.7 7.0 99.0 92.0 1.8 average 65.4 22.5 8.2 0.2 0.7 4.8 99.2 94.3 1.7 no high 450,000 74.2 29.2 5.6 0.0 1.1 3.3 98.9 95.6 2.3 yes high 450,000 72.5 25.8 5.1 0.0 2.0 4.3 98.0 93.7 2.1 average 73.3 27.5 5.3 0.0 1.5 3.8 98.5 94.7 2.2 no average 450,000 68.3 26.3 7.5 0.0 0.9 3.0 99.1 96.2 1.9 yes average 450,000 70.4 23.8 6.1 0.2 1.3 5.7 98.5 92.8 2.0 average 69.4 25.0 6.8 0.1 1.1 4.3 98.8 94.5 1.9 no adequate average 42.9 22.9 8.6 1.0 2.5 8.3 96.5 88.2 1.6 yes adequate average 47.1 22.5 5.1 1.2 2.8 9.7 96.0 86.3 1.9 average average 45.0 22.7 6.8 1.1 2.7 9.0 96.3 87.3 1.8 no high average 48.8 25.4 4.4 0.2 4.5 7.8 95.3 87.5 2.2 yes high average 47.9 25.4 3.2 0.2 4.0 8.3 95.8 87.5 2.1 average average 48.3 25.4 3.8 0.2 4.3 8.1 95.5 87.5 2.1 LSD (0.05) Water stress NS NS NS NS NS NS NS NS NS N level 3.0 2.6 NS NS 3.1 NS NS NS 0.2 Variety NS 2.8 NS NS NS NS NS NS 0.4 Population 2.6 NS 2.4 0.6 1.6 1.7 2.1 2.2 NS Variety X Population NS NS NS NS NS NS NS NS NS Water stress X Pop. NS NS NS NS NS NS NS NS NS Water stress X Variety X Pop. NS NS NS 1.2 NS NS NS NS NS N level X Pop. 3.7 1.7 NS NS NS NS NS NS NS N level X Variety X Pop. NS NS NS NS NS NS NS NS NS Water stress X N level X Var. X Pop. NS NS NS NS NS NS NS NS NS a Single center plus small multiple center. Onion Response to Late-season Water Stress and High Nitrogen Under Two Plant Populations 74