Malheur Experiment Station Information for Sustainable Agriculture

Management of onion Cultural Practices to control The Expression of Iris Yellow Spot Virus

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders

Malheur Experiment Station, Oregon State University, Ontario, OR

Lynn B. Jensen

Malheur County Extension Service, Oregon State University, Ontario, OR

S. Krishna Mohan and Ram S. Sampangi

University of Idaho, Parma, ID

Hanu Pappu

Washington State University, Pullman, WA

Introduction

Onion plants infected with iris yellow spot virus (IYSV) can progressively lose leaf area, resulting in reduced yield and reduced bulb size. The virus is transmitted by onion thrips (Thrips tabaci). The incidence of IYSV can be increased by inadequate control of onion thrips. Thrips control has become more difficult through increased thrips resistance to insecticides. A certain degree of varietal tolerance to thrips and IYSV has been determined (Shock et al. 2007, 2008). However, management factors such as irrigation and fertilization that reduce plant stress might reduce the intensity of thrips and IYSV infestations. Onion stress that results in greater expression of negative IYSV effects could be caused by high temperature or by excessively dry soil. This trial tested the response of four onion varieties to water stress level, irrigation system, and nitrogen fertilizer rate.

Materials and Methods

The onions were grown on a Greenleaf silt loam previously planted to wheat. In the fall of 2007 prior to planting, the wheat stubble was shredded and the field was irrigated and disked. Soil analysis indicated the need for 100 lb phosphate (P₂O₅)/acre, 100 lb sulfur (S)/acre, 7 lb maganese (Mn)/acre, and 5 lb zinc (Zn)/acre, which were broadcast in the fall of 2007 after disking. The field was then moldboard-plowed, groundhogged, roller-harrowed, fumigated with Telone^® C-17 at 20 gal/acre, and bedded at 22 inches.

Onion seed of four varieties ('Evolution', D. Palmer Seed Co., Yuma AZ; 'Vaquero' and 'Joaquin', Nunhems, Parma ID; 'Charismatic', Seminis Seed Co., Saint Louis, MO) was planted on March 21 at 260,000 seeds/acre in double rows spaced 3 inches apart.

Each double row was planted on a 22-inch bed with a customized planter using John Deere Flexi Planter units equipped with disc openers. Drip tape (T-tape, T-systems International, San Diego, CA) was laid at 4-inch depth between 2 onion beds at the same time as planting. The distance between the tape and the center of each double row was 11 inches. The drip tape had emitters spaced 12 inches apart and emitter flow rate of 0.22 gal/min/100 ft.

On March 24 the onion beds received a narrow band of 3.7 oz of Lorsban 15G^® per 1,000 ft of bed (0.82 lb ai/acre) and the soil surface was rolled. Onion emergence started on April 15. On May 19, alleys 7 ft wide were cut between plots, leaving plots 23 ft long. On May 20 and May 21, the seedlings were hand thinned to a plant population of 2 plants/ft of single row (6-inch spacing between individual onion plants equivalent to 95,000 plants/acre).

The experimental design was a randomized complete block design with split-split plots and four replicates. The main plots were the irrigation treatments (Table 1). Each irrigation plot was split into two nitrogen (N) rates (100 or 200 lb N/acre). Each N-rate split plot was split into the four varieties. Each variety in each split-split plot was 88 inches wide (4 double onion rows) and 23 ft long.

Table 1. Irrigation treatment specifications, Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.

^asoil water tension at 8-inch depth.

^b15 cb until July 31, then 25 cb thereafter.

^c0.24 inch/irrigation until July 31, then 0.48 inch/irrigation thereafter.

^dtotal water applied based on limited furrow inflow measurements.

The sprinkler- and furrow-irrigated plots (treatments 6,8, and 9) had the unneeded drip tape removed in early May. The sprinkler-irrigated onions (treatments 6,7, and 9) had four sprinklers at the top and another four sprinklers at the bottom of each plot. The risers were 3 ft tall and spaced 14.7 ft apart. The first riser was located at the plot corner. Each riser had a guard preventing the nozzle from applying water to the adjacent plots. The sprinklers were R10 turbos (Nelson Irrigation, Walla Walla, WA) with 0.75-gal/min flow-control nozzles. The sprinkler water application rate was designed to apply 0.16 inch/hour at a pressure of 35 psi.

Each furrow-irrigated plot (treatments 8 and 9) had a gated pipe at the upper and a tail ditch at the lower end. At each irrigation, the onions were irrigated for 8 to 12 hours. The last furrow irrigation occurred on August 28.

Onions in each drip- and sprinkler-irrigated plot (treatments 1-7) were irrigated automatically and independently according to the irrigation criterion and intensity predetermined for each treatment (Table 1). The irrigation duration for each treatment was adjusted so that when irrigated the maximum number of times, all irrigation systems had the capacity to deliver up to a maximum of 0.48 inch of water per day. The furrow-irrigated onions were irrigated manually when the soil water tension (SWT) at 8-inch depth reached 25 cb. The drip- and furrow-irrigated onions with optional sprinkler irrigation (treatments 7 and 9) received sprinkler irrigation of 0.48 inch of water once a week during the hottest part of the growing season starting on June 30 and ending on August 28.

Soil water tension was measured in each plot 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. 1998a). The GMS were connected to the datalogger via four multiplexers (AM 410 multiplexer, Campbell Scientific, Logan, UT). The datalogger read the sensors and recorded the SWT every hour.

The datalogger made irrigation decisions for each drip- and sprinkler-irrigated plot every 6 hours. The irrigation decisions for each plot were based on the average SWT. The irrigations were controlled by the datalogger using a controller (SDM CD16AC controller, Campbell Scientific, Logan, UT) connected to solenoid valves in each plot. The water for the drip and sprinkler plots was supplied by a well that maintained a continuous and constant water pressure of 35 psi. The pressure in the drip lines was maintained at 10 psi by pressure regulators in each plot. The amount of water applied to each plot was recorded daily at 8:00 a.m. from a water meter installed between the solenoid valve and the drip tape. The automated irrigation system was started on June 2. Automated irrigations were terminated on September 5.

The amount of water applied to and the amount of water infiltrated in the furrow-irrigated plots was estimated from measurements of furrow inflow and outflow. The difference between inflow and outflow is the amount of water infiltrated. The inflow and outflow measurements were taken every 2 hours on 4 furrows in each of 2 plots during 3 irrigations. Inflow was determined by measuring the volume of water coming out of the gate in 10 seconds. To measure outflow, trapezoidal flumes were installed at the furrow ends. Outflow was determined by measuring the level of the water in the flume. Flume water level measurements were converted to flow rate using an equation developed by measurements of flume water height and outflow.

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 on April 8 until the onions were lifted.

Starting on June 6, the high-N split plots in each irrigation main plot received 40 lb N/acre and the adequate-N split plots received 20 lb N/acre weekly until the last application on July 2. The N fertilizer for the drip plots was injected through the drip tape as uran. The N fertilizer for the sprinkler plots was applied as urea broadcast on the surface of the onion beds. The N fertilizer for the furrow plots was applied as urea to the top half of each furrow. The total amount of N applied during the growing season was 200 lb N/acre for the high N treatment and 100 lb N/acre for the adequate N treatment. A composite sample of the soil from the first and second foot depths was taken from each replicate on June 2. The soil was analyzed for nitrate and ammonium and for potentially mineralizable N.

Soil temperature was measured with four temperature probes installed at 2-inch depth in the bed center in each irrigation plot in replicates one and two. Soil temperature measurements were recorded hourly using a datalogger (CR10, Campbell Scientific).

The onions were managed to minimize yield reductions from weeds, pests, diseases, and other nutrient deficiencies. Weeds were controlled with an application of Roundup^® at 1 lb ai/acre on April 1 prior to onion emergence. On April 28, Prowl H₂O^® at 0.95 lb ai/acre and Select^® at 0.125 lb ai/acre were applied for weed control. On May 13, Goal^® at 0.16 lb ai/acre, Buctril^® at 0.19 lb ai/acre, and Volunteer^® at 0.25 lb ai/acre were applied for weed control. Goal at 0.16 lb ai/acre and Buctril at 0.19 lb ai/acre were again applied on May 16. Starting on June 1 and ending August 7, the field was sprayed every 10 days with Aza-Direct^® at 0.0062 lb ai/acre and Success^® at 0.25 lb ai/acre with a backpack sprayer for thrips control.

Thrips populations were measured every 10 days in each split-split-split plot starting June 17 and ending August 21. Thrips populations were measured by counting the total number of thrips on each of 15 plants per split-split plot.

Onions in each plot were evaluated subjectively for severity of symptoms of IYSV on August 18 and on August 27. Twenty consecutive plants in one of the two middle rows in each plot were rated. Each plant was given a rating on a scale of 0 to 5 of increasing severity of symptoms, where the rating was 0 if there were no symptoms, 1 if 1 to 25 percent of foliage was diseased, 2 if 26 to 50 percent of foliage was diseased, 3 if 51 to 75 percent of foliage was diseased, 4 if 76 to 99 percent of foliage was diseased, and 5 if 100 percent of foliage was diseased. Onions in each plot were evaluated subjectively for severity of symptoms of powdery mildew (Leveillula taurica) on September 15 using the same rating scale as for IYSV.

Presence of IYSV in leaf tissue was determined by the double antibody sandwich-enzyme-linked immunosorbent assay (DAS-ELISA, Agdia, Inc., Elkhart, IN). Three leaf tissue samples for ELISA were collected from each split-split-split plot on August 27. Each sample consisted of two fully developed leaves from one plant.

Onion plant maturity was evaluated subjectively in each plot by visually rating the percentage of onions with the tops down and the percent dryness of the foliage. The percent maturity was calculated as the average percentage of onions with tops down and the percent dryness. All plots were evaluated for maturity on August 26 and September 5. The number of bolted onion plants in each plot was counted.

Bulbs in each plot were rated for single centers on September 24, 25, and 26. In each split-split-split plot, 25 onions ranging in diameter from 3.5 to 4.25 inches were rated. The onions were cut equatorially through the bulb middle and, if multiple centered, the long axis of the inside diameter of the first single ring was measured. These multiple-centered onions were ranked according to the inner diameter of the first single ring: "small" had diameters less than 1.5 inches, "medium" had diameters of 1.5-2.25 inches, and "large" had diameters greater than 2.25 inches. Onions were considered "functionally single centered" for processing if they were single centered or had a small multiple center.

The onions were lifted on September 17 to cure in the field. Onions from the middle two rows in each plot were topped by hand and bagged on September 30. The bags were put in storage on October 30. Precipitation on September 20, 21, and 22 totaled 1.25 inches. To reduce the risk of disease in storage, a propane heater was used to raise the air temperature in the storage shed to 85°F. When the air temperature reached 85°F, the heat was turned off. The storage shed was subsequently ventilated to maintain air temperature as close to 34°F as possible. Onions were graded out of storage on December 17 and 18, 2008.

During grading, bulbs were separated according to quality: bulbs without blemishes (No. 1s), split bulbs (No. 2s), neck rot (bulbs infected with the fungus Botrytis allii in the neck or side), plate rot (bulbs infected with the fungus Fusarium oxysporum), and black mold (bulbs infected with the fungus Aspergillus niger). The No. 1 bulbs were graded according to diameter: small (<2.25 inches), medium (2.25-3 inches), jumbo (3-4 inches), colossal (4-4.25 inches), and supercolossal (>4.25 inches). 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 in yield, grade, maturity, single centeredness, thrips counts, and disease severity ratings were compared using three-way analysis of variance (ANOVA) where factor A was irrigation treatment, factor B was replicate, factor C was N rate, and factor D was variety. Treatment differences in water applied and water use efficiency were compared using one-way ANOVA, where factor A was irrigation treatment and factor B was replicate. Means separation was determined with Fisher's least significant difference test at the 5 percent probability level, LSD (0.05). Within the group of drip-irrigated treatments (averaged over N rate and variety), regressions were run between the average season SWT, IYSV severity on August 18, and marketable yield.

Results and Discussion

Drip irrigation at 30 cb was among the treatments with the highest average maximum daily soil temperature, with the other drip-irrigated treatments being among the treatments with the lowest average maximum daily soil temperature (Table 2).

The average season SWT increased with the increasing irrigation criteria for the drip-irrigated treatments (Table 3). The average SWT for the furrow-irrigated treatment was not different than drip-irrigation at 20 cb. As expected, the furrow-irrigated treatments had a greater oscillation in soil water than the wetter drip-irrigated treatments, getting wetter during irrigations and drier between irrigations (Figs. 1a and 1b).

The furrow-irrigated treatment with optional sprinkler irrigation had the highest total amount of water applied, followed by the furrow-irrigated treatment (Table 3). The treatment drip irrigated at 30 cb had the lowest total water applied. The total ET_c for the season was 38.6 inches. The two furrow-irrigated treatments, drip irrigation at 10 and 15 cb, and drip irrigation at 20 cb with occasional sprinkler irrigation applied more water than ET_c. Drip irrigation at 20 cb and at 15/25 cb were among the treatments with the highest water-use efficiency. The furrow-irrigated treatments had the lowest water-use efficiency.

The thrips population reached a maximum in early to mid-July and then decreased (Fig. 2). The thrips population was low for all plots in June, but exceeded the recommended threshold for the initiation of control measures in July, based on 15-25 thrips/plant (Jensen 2005). There were significant differences in thrips counts only between treatments on July 11, July 21, and for the weekly June 17 through August 21 season average (Table 4). On July 11, the drip-irrigated treatment at 20 cb with occasional sprinkler irrigation had among the highest thrips counts, followed by the two furrow-irrigated treatments and by drip irrigation at 20 cb. Drip irrigation at 10, 15, 15/25, and 30 cb, and sprinkler irrigation were among the treatments with the lowest thrips counts on July 11.

On the two IYSV symptom severity rating dates (August 18 and 27), onions that were drip irrigated at 30 cb had the highest rating (Table 4). The two furrow-irrigated treatments and sprinkler irrigation were among the treatments with the lowest severity ratings. Drip irrigation at 30 cb was among the treatments with the lowest thrips counts and furrow irrigation was among the treatments with the highest thrips counts, indicating a lack of association between thrips population level and IYSV severity.

Powdery mildew symptoms were very few, with the sprinkler-irrigated treatment and the treatments that were occasionally sprinkler irrigated showing no symptoms. Drip irrigation at 30 cb also did not exhibit any symptoms of powdery mildew. The variety Evolution had the highest powdery mildew severity rating, followed by Joaquin. Charismatic and Vaquero had no powdery mildew (Table 4).

Averaged over varieties and N rates, drip irrigation at 30 cb resulted in significantly lower marketable, colossal, and colossal plus supercolossal bulb yield than the other treatments (Table 5). Averaged over varieties and N rates, sprinkler irrigation resulted in lower marketable and colossal yield than drip irrigation at 10 and 15 cb. Drip or furrow irrigation with optional sprinkler irrigation did not significantly increase onion yield. Increasing the SWT irrigation criterion from 15 to 25 cb after August 1 did not affect onion yield in this trial.

For the drip-irrigated treatments (averaged over N rate and variety), increases in average SWT (drier soil) was related to increases in IYSV symptom severity on August 18 (Fig. 3) and to decreases in marketable yield (Fig. 4). Increases in IYSV symptom severity on August 18 was also related to decreases in marketable yield (Fig. 5).

Averaged over irrigation systems and N rates, Charismatic had the highest and Joaquin had the lowest marketable yield, and Charismatic had the highest and Vaquero had the lowest supercolossal yield.

There was no significant interaction of N rate with irrigation and variety. Averaged over irrigation treatments and varieties, there was no significant difference in yield or grade between N rates. The analysis of the soil sample taken on June 2 showed that in the top 2 ft of soil there was 145 lb of available N. The N mineralization test showed that in the top 2 ft of soil, there was 158 lb of mineralizable N that could be available to the crop during the season. The N requirements for a 900-cwt/acre onion crop are estimated to be 342 lb N/acre (Sullivan et al. 2001). Nonfertilizer sources of N in this trial could have fulfilled a large part of the N requirements of the crop.

Averaged over varieties and N rates, drip irrigation at 10 cb resulted in a significantly higher percentage of single-centered bulbs than any of the other treatments except drip irrigation at 15 cb (Table 5). Drip irrigation at 20 and 30 cb, and furrow irrigation were among the treatments with the lowest percentage of single-centered bulbs. Averaged over irrigation systems and N rates, the varieties differed significantly in percentages of single-centered bulbs; from highest to lowest were Joaquin, Evolution, Vaquero, and Charismatic.

Conclusions

The results of this trial show that higher soil moisture achieved with the wetter drip irrigation treatments (10, 15, and 20 cb) and with furrow irrigation at 25 cb resulted in higher onion yield and grade and low expression of IYSV. Soil water tensions of 15 to 20 cb for drip irrigation and of 25 cb for furrow irrigation are recommended for onion on silt loam soil in the Treasure Valley (Shock et al. 1998b, 2000). Previous research at the Malheur Experiment Station has also shown that drip irrigations at a SWT wetter than 20 cb or furrow irrigations at a SWT wetter than 25 cb can result in high yield and grade, but in years with rainfall, very wet conditions also result in higher losses to storage decomposition (Shock et al. 1998b, 2000). Heavy rainfall occurred after the onions were lifted in 2008, but the bulbs were cured when put in storage.

For the drip-irrigated treatments, as the soil was maintained slightly drier, the severity of IYSV symptoms increased and the marketable yield decreased. Drip irrigation at 30 cb resulted in the lowest marketable yield and yield of colossal plus super-colossal bulbs, and in the highest IYSV severity rating. These results are important when considering the irrigation management. The datalogger checked the SWT and irrigated the plots if necessary every 6 hours. This resulted in the SWT of the drip treatments not exceeding the criterion (Figs. 1a and 1b). In commercial production the irrigation frequency could be less, resulting in the SWT often exceeding the criterion. These results reinforce the importance of careful irrigation management to avoid yield and grade losses to IYSV.

The lack of difference in onion yield and grade between N rates in this trial is in agreement with previous results of onion yields often showing little response to N fertilizer in furrow or drip irrigations (Miller et al. 1993; Shock et al. 2001, 2004). As shown in this trial and in previous research, high amounts of N can be provided to an onion crop by natural sources, e.g., residual available N, N mineralization, and N in irrigation water (Shock et al. 2001, 2004). The only reason that the 200 lb N/acre was used in this trial was the possibility that higher N might help IYSV-infected plants to sustain leaf area and productivity, an advantage that did not occur. The furrow-irrigation management in this trial might not be realistic on a commercial scale. The plots in this trial were only 23 ft long, allowing extremely uniform furrow irrigation that is difficult to achieve on a commercial scale without risk of over-irrigation, N leaching, and bulb decomposition in a considerable part of the field. In commercial fields, well-managed drip irrigation will probably result in smaller losses of N from the onion root zone than furrow irrigation.

Acknowledgements

This project was partially funded by the Western Sustainable Agricultural Research and Education Program, the Idaho- Eastern Oregon Onion Committee, the Irrometer Co., cooperating onion seed companies, and Oregon State University.

References

Jensen, L. 2005. Controlling thrips in onions. Onion World December 2005 issue.

Miller, J.M., C. Shock, and M. Saunders. 1993. Efficiency of nitrogen fertilization on onions 1992 trials. Oregon State University Agricultural Experiment Station Special Report 924:79-90.

Shock, C.C., J.M. Barnum, and M. Seddigh. 1998a. Calibration of Watermark Soil Moisture Sensors for irrigation management. Pages 139-146 in Proceedings of the International Irrigation Show, Irrigation Association, San Diego, CA.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 1998b. Onion yield and quality affected by soil water potential as irrigation threshold. HortScience 33:1188-1191.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2001. Plant population and nitrogen fertilization for subsurface drip-irrigated onions. Oregon State University Agricultural Experiment Station Special Report 1029:52-60.

Shock, C.C., E.B.G Feibert, and L.D. Saunders. 2004. Plant population and nitrogen fertilization for subsurface drip-irrigated onion. HortScience 39:1722-1727.

Shock, C.C., E.B.G. Feibert, L.D. Saunders, L. Jensen, and K. Mohan. 2007. Onion variety trials. Oregon State University Agricultural Experiment Station Special Report 1075:33-42.

Shock, C.C., E. Feibert, L. Jensen, S.K. Mohan, and L.D. Saunders. 2008. Onion variety response to Iris Yellow Spot Virus. HortTechnology 18:539-544.

Sullivan, D.M., B.D. Brown, C.C. Shock, D.A. Horneck, R.G. Stevens, G.Q. Pelter, and E.B.G. Feibert. 2001. Nutrient management for onions in the Pacific Northwest. Pacific Northwest Extension Publication No. 546.

Wright, J.L. 1982. New evapotranspiration crop coefficients. Journal of Irrigation and Drainage Division, American Society of Civil Engineers 108:57-74.

Table 2. Average daily maximum soil temperature from July 1 to September 5 at 2-inch depth for onions submitted to 9 irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.

Table 3. Average hourly soil water tension, total water applied, marketable yield, and water use efficiency (cwt marketable yield per inch of water applied) for onions submitted to nine irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.

^aTotal water applied plus precipitation (1.56 inches) from emergence to the last irrigation: includes 6.8 inches of drip irrigation applied to all plots from emergence to the start of the irrigation treatments on June 2. For the furrow-irrigated plots, the total water applied was estimated from limited furrow inflow measurements. Based on inflow and outflow measurements, the total amount of water infiltrated was estimated to be 25.3 inches for the furrow-irrigated treatment and 37.2 inches for the furrow-irrigated with optional sprinkler irrigation treatment.

Table 4. Thrips counts, iris yellow spot virus (IYSV), and powdery mildew (PM) leaf damage severity ratings on onions in response to irrigation system and variety, Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.

^a Disease severity rating: 0 = no symptoms, 1 = 1-25% of foliage diseased, 2 = 26-50% of foliage diseased, 3 = 51-75% of foliage diseased, 4 = 76-99% of foliage diseased, and 5 = 100% of foliage diseased.

Table 5. Onion yield and grade in response to irrigation system, N rate, and variety, Malheur Experiment Station, Oregon State University, Ontario, OR, 2008.