Onion Systems Management Strategies for Crop Nutrition, Weeds, Thrips, and Iris Yellow Spot Virus

View the project final report

• 2013 annual report
• 2014 annual report
• 2015 annual report

Commodities

• Agronomic: corn, wheat
• Vegetables: onions

Practices

• Crop Production: crop rotation, application rate management, tissue analysis
• Education and Training: decision support system, demonstration, extension, farmer to farmer, focus group, networking, on-farm/ranch research, participatory research, workshop
• Farm Business Management: whole farm planning, budgets/cost and returns, agricultural finance, risk management
• Pest Management: biological control, chemical control, cultural control, disease vectors, field monitoring/scouting, integrated pest management, prevention, sanitation, weed ecology
• Production Systems: agroecosystems, holistic management
• Soil Management: soil analysis, nutrient mineralization, soil microbiology, organic matter, soil quality/health
• Sustainable Communities: sustainability measures

Research and outreach activities were completed in 2016, Year 4 of the project. 

Objective 1: Results for the grower-collaborative survey of commercial onion fields to refine predictors of pests (onion thrips and Iris yellow spot virus), yield and soil health for 57 fields in four counties found significant correlations among higher onion thrips densities and higher virus (IYSV) incidence (R = 0.47, p = 0.0003), and lower onion yields (R = -0.49, p = 0.025).  Onion thrips counts in August were better predictors of IYSV incidence and bulb yields than thrips counts in July. Other important predictors of thrips and IYSV were low soil quality parameters, including microbial biomass (R = -0.36, p = 0.006), dehydrogenase enzyme activity (R = -0.50, p < 0.0001), soil respiration (R = -0.30, p = 0.025), and lower concentrations of zinc, iron and copper (R = -0.30 to 0.34, p = 0.025 to 0.01).  High soil nitrate levels in June were positively associated with high onion thrips densities in August (R = 0.41, p = 0.002), supporting findings from Objective 4 that excessive nitrogen applications increase the risk for high onion thrips populations. 

Soil phosphorus and potassium concentrations were positively correlated with each other (R = 0.58, p < 0.0001); phosphorus levels were good predictors of onion bulb diameter in August (R = 0.45, p = 0.001).  Phosphorus and potassium levels were also associated with high soil quality metrics and readily mineralizable carbon levels (R = 0.57, p < 0.0001).  Bulb size and yield were negatively associated with pH (high pH=low yield; R = -0.50 to 0.57, p < 0.0001), low soil ammonium (R = -0.45, p = 0.002) and low readily mineralizable carbon levels (R = -0.31, p = 0.03), but were positively correlated to high soil quality parameters.

We identified four weed species that served as overwintering and spring reproductive hosts for onion thrips and as reservoirs of Iris yellow spot virus (IYSV): flixweed, common mallow, field bindweed, and prickly lettuce.  Weeds must support thrips reproduction in order for thrips to acquire and transmit the virus (propagative, persistent mode); thus, these weeds may serve as a “green bridge” for thrips and IYSV across growing seasons. 

Objective 2: The project economist and vegetable specialist developed an onion risk factor assessment tool to evaluate how different onion crop and pest management practices influence crop performance.

Ten years of monthly pricing data were collected for a range of bulb sizes of hybrid yellow Sweet Spanish onions.  Pricing data were based on the Atlanta (GA) and Dallas (TX) terminal markets, two common markets for Utah-produced onions.  Pricing data was used to estimate both seasonal prices and risk and variability in pricing.  Onion bulb data (weight/acre by bulb size) collected from Utah State University research trials and commercial producers’ fields (Obj. 1) was used to assess yield variability in 2013-2015.  Input price data (fertilizers, chemicals, etc.) was gathered from local farm supply businesses.  Estimates for labor, equipment and other operating costs came from existing Extension budgets.  Research results from this project (correlation of risk factors described above) were incorporated into the model.  This information was used to develop an interactive Excel tool that allows the user to vary assumptions to generate expected results, including the range of values.  Larger ranges indicate greater variation in expected results, and hence greater risk.  Producers are able to view the risk from pricing alone, yield alone, and the combination of both.  Because the tool is interactive, growers can input their own yield values and operating costs. Given that: 1) onion prices fluctuate from year-to-year and also from month-to-month, 2) yields vary each year, and 3) various costs of production change, growers now have a tool to assess long term historic trends and get a more immediate evaluation of present season risk.  

Objective 3: A Master’s of Science student was employed to manage the weed-thrips-IYSV interaction experiment.  Replicated treatments consisted of weedy borders (0.6 m wide) surrounding onion plots (9.2 m²):  monocultures of common mallow or prickly lettuce, resident weeds (mowed mid-season, or not), or hand-weeded.  The mowed resident weed treatment served to assess thrips dispersal into onions following a major disturbance when thrips densities peak in July.  We observed that mid-season mowing of resident weed borders pushed thrips into adjacent onions in one of the two years of study.  Weeds with the highest rates of IYSV infection did not necessarily translate into transmission of IYSV by thrips at higher incidence rates in adjacent onions. Weed removal (hand-weeding) and mowing were the most effective in reducing the incidence of IYSV in onions; however, the induced dispersal of thrips by mowing may increase the risk for higher IYSV.  IYSV incidence rates did not exceed 20%, which is fairly low; thus, we did not observe an outbreak of IYSV in the onions with mowed weed borders.  These results may have differed at higher IYSV infection levels.

In June 2015, IYSV infection rates in weeds were very low, less than 5% across all weed border treatments.  In July, IYSV infection rates for witchgrass and black medic increased to 18% and 13%, respectively; however, onion infection rates remained below 5%.  In August, IYSV infection rates in onions spiked to 20% in treatments with weedy borders, but remained low in plots with hand-weeded borders.

Objective 4: To determine linkages among nitrogen concentration in onion leaves and soil, phenolic defense compounds, and thrips populations, onion plots treated with low and high rates of fertilizer were sampled in June through August in 2013-2015.  In 2013 and 2014, fewer onion thrips were found on onion plants fertilized with reduced (120 lb N per acre) as compared to standard (350 lb N per acre) nitrogen rates.  In 2015, onion plots followed three years of alfalfa in the scheduled rotation scheme.  We observed slightly increased soil nitrate and ammonium levels in June, but not in August, and did not observe an effect of nitrogen rate on thrips likely because nitrogen fixation from the alfalfa overrode differences in N application rates.  Results for higher fertilizer rate applications increasing thrips reflect higher soil available nitrogen, and lower microbial biomass and activity under standard fertilization.  Our research results have shown that onion plants containing higher levels of N are more attractive to onion thrips and may boost their reproductive potential.  Additionally, gas chromatograph-mass spectrocopy (GC-MS) analyses in this project found that onion tissue in the standard N treatment had significantly higher levels of free amino acids, particularly glutamine. Degradation products of sulfoxide defense compounds were largely positively correlated with amino acids; however, suggesting onion thrips preference or performance was associated with high amino acid levels on highly fertilized onions as opposed to being deterred by greater defense compounds in onions fertilized with reduced levels of N. There was no effect of previous crop on either amino acids or sulfoxide compounds.

Objective 5: We have developed numerous outreach products and completed early-, mid- and late-project impact assessments.  In the late-project assessment of producers and industry professionals, 100% of respondents confirmed that results shared from this onion systems project had increased and provided them with new knowledge and skills, and 92% reported that the project had modified their attitudes about onion crop and pest management.  One hundred percent of producers responded that they would adopt one or more practices discussed, increase their networking with other producers, and felt that the knowledge gained would improve their farm profitability. Sixty-seven percent said that knowledge gained would help them increase their operation’s diversification, and 80% thought the information would help them reduce purchase of off-farm inputs. 

Seven videos were produced on onion farm-scape management strategies for pests, optimal crop nutrition and irrigation, and demonstration of sampling protocols for crop and pest management.  The USU extension media specialist produced the videos using a drone-mounted camera to capture images of crop health, soil issues, weedy field borders, and the interface with adjacent crops that can serve as sources of thrips and IYSV.  The videos were premiered at the Utah Onion Association (UOA) conference in February 2015, and were well received.  The videos are available on YouTube (search Utah State University onion crop and pest management; https://www.youtube.com/watch?v=fkGcJDLfFr0) and have been viewed a total of 20,808 times as of December 2016. 

Across the four-year project, face-to-face presentations and contacts were made with over 2,260 growers, researchers, and industry and extension professionals at Utah Onion Association meetings and field days, the Pacific Northwest Vegetable Association Annual Meeting, Agricultural Experiment Station Multi-State Coordinating Committee Annual Meeting W-2008, the National Allium Research Conference, National Onion Association meeting, and the International Integrated Pest Management Symposium.  Thirteen publications disseminating project results have been produced: two refereed research articles, one refereed abstract, two extension publications, three trade magazine articles, two newsletter articles, and three posters.

1. Collaborate with Utah onion producers in a survey of ~60 onion fields to expand and refine production system predictors of pests and yield. A total of 57 commercial onion fields were surveyed in 2013 and 2014.  Correlation analyses have revealed positive and negative predictors of thrips and virus levels, bulb diameter and yield, and soil health parameters.  Measures of key soil characteristics, such a pH, macro- and micro-nutrient concentrations and electrical conductivity were associated with risks for pest densities and crop yield (see Summary Objective 1 for more details).
2. Develop a crop risk model based on onion production and pest management parameters associated with profitable yield and quality. The creation of an interactive Excel tool enables onion producers to alter yield values, input and operating costs.  The tool uses historic/actual onion prices to determine the risks associated with changes in farm management practices (e.g., reduced fertilizer/chemical inputs, lower yields, etc.). Tools of this nature provide additional insights into how alterations in practice impact sustainable production and reduce some risks associated with growing onions.  The tool will also help with assessing total risk by incorporating the potential variability in multiple factors to show the variability in expected profit.
3. Determine how management of key weed hosts influences incidence of onion thrips and IYSV in onion. Field plot experiments were conducted by the MS student in 2014 and 2015. Weed removal (hand-weeding) and mowing were the most effective in reducing the incidence of IYSV in onions; however, the induced dispersal of thrips by mowing may increase the risk for higher IYSV.  Witchgrass and black medic had the highest infection incidence of IYSV, and could increase the risk for IYSV in onions when thrips densities are high.  Weed removal on plot borders resulted in the lowest IYSV incidence in adjacent onions (see Summary Objective 3 for more details).
4. Determine linkages among reduced nitrogen (N) fertilization, onion tissue N, phenolic defense compounds, and thrips populations. Field plot experiments were conducted in 2013-2015.  The reduced N rate of 120 lb N per acre reduced onion thrips densities in two of the years. Results for higher fertilizer rate applications increasing thrips reflect higher soil available nitrogen, and lower microbial biomass and activity under standard fertilization.  Onion plants containing higher levels of N are more attractive to onion thrips, and may boost their reproductive potential. GC-MS analysis of onion tissue showed significantly higher levels of free amino acids, particularly glutamine in the standard fertilized onions. Degradation products of sulfoxide defense compounds were largely positively correlated with amino acids; however, suggesting onion thrips preference or performance was associated with high amino acid levels in highly fertilized onions as opposed to deterred by phenolic compounds in onions fertilized with reduced levels of N. There was no effect of previous crop on total free amino acids or sulfoxide compounds (see Summary Objective 4 for more details). 
5. Develop and deliver outreach products to the onion industry and research, extension, and regulatory communities of relevance, and assess project impacts. Results of a late-project stakeholder assessment showed strongly positive impacts of this onion systems project on the onion industries in Utah, Idaho and Oregon.  Educational videos and publications have been disseminated to onion industry stakeholders (see Summary Objective 5 for more details).