Final Report for SW13-034
Project Information
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.
- 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).
- 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.
- 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).
- 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).
- 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).
Onion is a high value, intensively managed crop that utilizes short field rotations, high fertilizer rates, and aggressive pesticide applications to suppress thrips and weeds. Onion thrips vector Iris yellow spot virus (IYSV), a serious threat to onion production worldwide (Gent et al. 2006, Pappu et al. 2009). Excessive rates of fertilizers, increasing pesticide resistance, and crop loss from thrips, IYSV, and weeds threaten the sustainability of onion production. Altering cultural management practices and incorporating a whole systems approach can suppress pests while maintaining yields (Zitter and Simons 1980). Crop rotation can interrupt pest requisites in time and space, such as oviposition sites and food sources, and can limit their success (Altieri 1999, Zitter and Simons 1980). However, polyphagous pests such as onion thrips can sustain populations between hosts when crop intervals are short (Milne and Walter 1998).
Utah onion producer, Morgan Reeder, initiated interest in reduced nitrogen (N) inputs and novel crop rotation schemes. Morgan has reduced thrips sprays on his farm by over 70% since he changed management practices, and is achieving profitable yields. Several more Utah growers have adopted a similar approach. The Utah onion research and extension team has followed the growers’ lead and documented positive benefits of certain crop rotations and reduced N rates in a previous WSARE project (SW08-076, Buckland 2010).
Onion nutrient management is challenging due to onion’s shallow root system. Annual N application rates of 250-450 kg/ha is common. Malik et al. (2009) found that onions had 74% higher thrips populations at N rates of 250 kg/ha as compared to 100 kg/ha. In Utah, we showed a two-fold increase in onion thrips at the grower standard N rate of 400 kg/ha as compared to 130 kg/ha (Buckland 2010). Tissue N was greater in onions fertilized at the standard rate which could translate to greater reproductive efficiency of thrips (Butler et al. 2012); however, egg and hatch numbers did not bear this out. Nitrogen is also known to directly affect the expression of phenolic defense compounds in a wide range of plants, with high N fertility associated with reduced plant phenolic content (Koricheva et al. 1998, Stamp 2003). This apparent tradeoff between growth and defense suggests an alternative mechanism for reduced thrips on onions fertilized at a lower rate.
Eight weeds have been identified as hosts of IYSV (Nischwitz et al. 2007, Sampangi et al. 2007, Evans et al. 2009a and b). Onion thrips acquire IYSV as larvae and transmit it as adults; therefore, weeds must be common hosts for both thrips and IYSV to serve as a “green bridge”. In Utah, common mallow, prickly lettuce, and field bindweed have tested positive for IYSV and support thrips reproduction. We will focus on these three weeds in further study of their role in thrips and IYSV management.
Findings and educational products from our study will be relevant to producers of conventional and organic onions who also grow other vegetable and field crops (e.g., grains, forages) in rotation with onions.
Literature Cited
Altieri, M. A. 1999. The ecological role of biodiversity in agroecosystems. Agr. Ecosyst. Environ. 74: 19-31.
Buckland, K. 2010. Evaluating fertilizer rate, crop rotation and trap crops for effects on onion growth and yield, soil health, thrips densities and Iris yellow spot virus incidence. MS Thesis, Utah State University, Logan, UT.
Butler, J., Garratt, M.P.D., and Leather, S. R. 2012. Fertilizers and insect herbivores: a meta-analysis. Ann. Appl. Biol. 161: 223-233.
DeFrancesco, J. 2012. Pest management strategic plan for dry bulb storage onions in the United States. Report to the USDA NIFA, Washington, D. C.
Evans, C.K., Bag, S., Frank, E., Reeve, J.R., Ransom, C, Drost, D. and Pappu, H.R. 2009a. Natural infection of Iris yellow spot virus in two scale saltbush (Atriplex micrantha) growing in Utah. Pl. Dis. 93: 430.
Evans, C.K., Bag, S., Frank, E., Reeve, J.R., Ransom, C, Drost, D. and Pappu, H.R. 2009b. Green foxtail (Setaria viridis), a naturally infected grass host of Iris yellow spot virus in Utah. Pl. Dis. 93: 670.
Gent, D.H., du Toit, L. J., Fichtner, S.F., Mohan, S.K. , Pappu, H.R. , Schwartz, H.F. 2006. Iris yellow spot virus: an emerging threat to onion bulb and seed production. Pl. Dis. 90: 1468-1480.
Koricheva, J., Larsson, S., Haukioja, E. and Keinanen, M. 1998. Regulation of woody plant secondary metabolism by resource availability: hypothesis testing by means of meta-analysis. Oikos 83:212-226.
Malik, M.F., Nawaz, M., Ellington, J., Sanderson, R., El-Heneidy, A.H. 2009. Effect of different nitrogen regimes on onion thrips, Thrips tabaci Lindemann, on onions, Allium cepa L. Southwest. Entomol. 34: 219-225.
Milne, M., Walter, G.H. 1998. Significance of mite prey in the diet of the onion thrips Thrips tabaci Lindeman (Thysanoptera: Thripidae). Aust. J. Entomol. 37: 120-124.
Nischwitz, C., Gitaitis, R.D., Mullis, S., Csinos, A.S., Langston, Jr., D.B. and Sparks, A.N. 2007. First report of Iris yellow spot virus in spiny sow thistle (Sonchus asper) in the United States. Pl. Dis. 91: 1518.
Pappu, H.R., Jones, R.A.C., Jain, R.K. 2009. Global status of tospovirus epidemics in diverse cropping systems: successes achieved and challenges ahead. Virus Res. 141: 219-236.
Sampangi, R.K., Mohan, S.K. and Pappu, H.R. 2007. Identification of new alternative weed hosts for Iris yellow spot virus in the Pacific Northwest. Pl. Dis. 91: 1683.
Stamp, N. 2003. Out of the quagmire of plant defense hypotheses. Quart. Rev. Biol. 78:23-55.
Zitter, T.A., Simons, J.N. 1980. Management of viruses by alteration of vector efficiency and by cultural practices. Ann. Rev. Phytopathol. 18: 289-310.
Cooperators
Research
Objective 1: We surveyed 57 onion fields. Previous survey results identified key predictive parameters of yield and pests. Results for early and late sample dates were correlated; thus, we reduced sampling dates to June, July, and August. Across each field, we established a survey transect with 10 plots.
Thrips populations (counts of adults and larvae), IYSV incidence [visual symptoms and double antibody sandwich enzyme-linked immunosorbent assay (DAS ELISA) (Agdia Inc.) results], and plant growth (leaf number, fresh and dry weights) and tissue N were determined for three randomly selected onion plants in each transect plot. Soil inorganic and total N were determined from a soil sample collected from the crop root zone (0-12 inch) in each transect plot. Soil was dried and organic matter, pH, and microbial biomass and activity measured.
Cooperating growers agreed to share input information on fertility, herbicides, insecticides, fungicides, irrigation, crop rotation, bordering crops, and other farm practices that may impact pests and plant performance. At harvest, onion bulbs from four 3.1 m sections of bed were graded and weighed before storage. After 90 days, storage quality (weight loss, sprouting, rots, and thrips in bulb) were assessed. Production data was correlated with plant and soil N, plant growth, soil quality, and thrips and IYSV incidence to determine if growers using high or low N levels, and other management practices, alter feeding pressure by thrips and thus potential infection of IYSV.
Objective 2: Previous research revealed some onion management practices that were better for the environment were also better for the bottom line for farmers. This team created an onion enterprise budget for Utah. The onion field survey (Objective 1) gathered additional information that was used to develop an Excel-based interactive tool that will allow producers to examine how changing production practices will affect not only their costs and returns, but also their risk levels. The tool will allow producers to better understand how various production practices affect costs, risks, and profit. They will be able to customize the assumptions to reflect their own situation.
Objective 3: Forty onion plots (3.1 × 3.1 m) in a two-way factorial design (5 weed species × 2 mowing disturbance treatments × 4 replicates) were established at the USU research farm in Kaysville, UT. Weed treatments were monocultures of common mallow, prickly lettuce, or field bindweed; or the “natural” weed complex; or maintained weed-free through tillage and herbicides. Weeds were mowed in July before thrips populations peaked, or remained un-mowed. Weeds were established in a 0.6 m-wide border around each plot by selective removal, seeding, and transplanting. A vegetation-free buffer of 1.5 m was maintained between plots with tillage and herbicide. The proposed plot size was suitable to observe differences in thrips and IYSV responses based on previous research. Onions were seeded in spring of 2014 and 2015 on beds in double rows, and weeds were managed weekly through August.
Onion thrips and IYSV were sampled on three onion and weed plants in each plot biweekly from late May through August. In the lab, thrips were washed from plants for counting. From each plant, one leaf was stained with acid fuchsin to observe thrips eggs, and a second leaf was held at 25°C for 7 days to measure thrips egg hatch. An outer leaf from each onion and weed plant was tested for IYSV using DAS ELISA. Positive weed samples were confirmed with reverse transcription polymerase chain reaction (RT PCR).
Objective 4: We used onion plots with two N rates (high, 400 kg/ha; low, 130 kg/ha) in an ongoing N rate study located at the same research farm and analyzed as described in Objectives 1 and 3 for soil and plant nutrients, soil quality, onion growth and yield, and thrips and IYSV incidence. Onion tissue was analyzed for plant phenolic compounds and free amino acids in June of 2013 and 2014 by high gas chromatography-mass spectroscopy (GC-MS). Preliminary tests showed that amino acids were highest in the basal tissue of the 2nd and 3rd leaf which is where onion thrips prefer to feed. Tissue was soaked in HCl and submitted for EZfaast analysis (Phenomenex, Torence CA) sample preparation and derivatization using norvaline as an internal standard. Amino acid concentration was determined using calibration curves at 5, 10 and 20 nmol. Due to high concentrations of glutamine in onion samples, calibration curves of 50, 100 and 200 nmol were used for that amino acid. Due to high variability between blocks, all samples were analyzed twice to reduce processing error. Chloroformate derivatized samples were also analyzed by GC-MS. High levels of degradation products of aklylcysteine sulfoxides were detected including alliin, methiin, common C16 and C18 fatty acids and 14 unidentified compounds.
Objective 5: All outreach materials have been posted on the USU Extension website and disseminated to target audiences at meetings and field days. Extension Media created seven videos on onion crop and pest management. During the four years of the project, the team delivered over 45 presentations to the onion industry community at the annual conferences and field tours of the Utah Onion Association, and as invited speakers at industry and research meetings across the U.S. Slideshows, posters, and research and extension articles are posted on the USU Extension website and linked to from key national onion websites.
Progress and impacts of the project was assessed by: 1) grower roundtable discussions each year to gather input and feedback on farm surveys, research, crop risk tool, and outreach products; and 2) written surveys to evaluate changes in grower behaviors and adoption of crop and pest management practices from the beginning to the end of the project.
Objective 1. One goal of the commercial field survey was to determine which species of weeds are good overwintering and within-season hosts, or “green bridges”, for thrips and the virus. We have found that flixweed, common mallow, field bindweed, and to a lesser extent, prickly lettuce, are moderate to good reproductive hosts for onion thrips, an essential factor in thrips acquiring and transmitting Iris yellow spot virus. In addition, all four weed species tested positive for Iris yellow spot virus; however, virus incidence rates were low (1.1% in 200 weed samples in 2013, and 2.6% in 194 weed samples in 2014) indicating the virus is rare in the farm-scape in the spring.
Correlation analyses have revealed significant positive correlations among higher onion thrips densities and higher virus (IYSV) incidence. This confirms our hypothesis that more thrips feeding on onion plants in a field will translate to a higher infection of those plants with the virus. We found a negative correlation between thrips densities and bulb yield supporting the need to reduce thrips populations below economically damaging thresholds; however, we did not see a relationship between IYSV and yield. IYSV infection rates were low in most fields during the two years of the study. IYSV incidence did not exceed 22% in 2014, and in 2013, although one field had 64% infection, most fields were only 0 to 17%. These lower virus incidence rates likely did not contribute substantially to reduced yields (thus, our lack of finding a correlation between IYSV and yield).
Other important predictors of thrips and IYSV were low soil quality parameters, including microbial biomass, dehydrogenase enzyme activity, soil respiration, and low concentrations of zinc, iron and copper. High soil nitrate levels in June were positively associated with high onion thrips densities in August supporting previous findings that excessive nitrogen applications increase the risk for high onion thrips populations. Enhanced soil health is associated with more diverse soil microbial communities. Beneficial soil organisms are known to increase plant defense compounds through the phenomenon of induced systemic resistance. In addition, high levels of available soil nitrogen have been shown to inhibit soil microbial activity and may lead to accelerated breakdown of carbon rich soil organic matter.
Utah soils are high in calcium carbonate which in turns leads to high soil pH and reduced availability of soil phosphorus and trace elements. Increased soil carbon and microbial activity may lead to enhanced nutrient availability, which in turns improves plant growth. The nutrient imbalance hypothesis suggests that increased levels of pests are associated with plants that have bottlenecks in growth due to limitations of specific nutrients. Simple compounds such as amino acids build up in the cells as a result of these metabolic bottlenecks which in turn provide elevated levels of easily utilizable nutrients to pests. The negative association of certain soil nutrients with soil health indicators appears to support this hypothesis.
Objective 2. It is fairly common for interactive enterprise budgets to allow the user to input data and observe changes in costs and profits. Commonly a producer begins by entering their own prices and quantities to complete a customized budget. Enterprise budgets are for a single commodity or enterprise. They use the information from producer’s records on production, prices and inputs. Budgets use past information as assumptions to predict future outcomes. Budgets are an estimate of future performance and allow a producer to compare what they expected to happen with actual events. A static budget is a tool that allows comparison in changes of budget assumptions and can reveal estimated effects of varying management practices. These budgets are tools that allow producers to understand how various changes affect profit, understand the risk associated with each scenario, and also can be used mid-season to understand how changes in prices will affect overall profit. The goal of any risk tool is visualize how changes in production practices affects key costs, inform calculation of breakeven price and yield, and allow the producer to identify how changing a practice can impact the profitability or sustainability. Being informed of these factors is key to evaluating whether a change should be implemented. Growers need to be able to assess “how risky” is it to implement practice changes, such as to reduce nutrient inputs or insecticide sprays, improve border weed management, or assess how changes in seasonal pricing may influence future decisions, and thus profitability.
Few budgets however do everything. Our interactive model attempted to provide growers with an additional tool to better assess risk and to help them make better decisions. While we do not have any assessment yet of how the tool is being used, we intend to conduct follow-up evaluations with the Utah onion industry to gauge usefulness as part of our routine evaluations which are conducted at the yearly Onion Association winter meetings and summer field tour.
Objective 3. In our studies, weed populations were reduced by hand-weeding and mowing. However, in 2014, mowing resident weeds during mid-season caused a large migration of thrips into nearby onions. In this case, mowing increased the risk for transmission of IYSV by thrips. We did not observe a consistent relationship between weeds with highest IYSV infection translating into higher IYSV incidence in nearby onions. Detection of the antibody for IYSV in weed tissue does not confirm that the virus is replicating in the weed, but only that the virus has been placed in the plant by infected thrips. Therefore, a lack of a strong relationship between weeds with high incidence of IYSV and infection of nearby onions with IYSV is not completely surprising.
In 2015, all weeds tested for IYSV in June had very low infection rates, 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. The higher incidence of IYSV in witchgrass and black medic may indicate that these two weed species can pose higher risk for transmission of the virus into nearby onions; however, virus levels were low in this study, and so this result was not statistically significant.
Objective 4. In two years of the study we observed increased thrips densities on onion plants with higher rates of nitrogen. These results reflect our findings of higher soil available nitrogen and lower microbial biomass and activity under standard, or high, fertilization. Although we did not observe this same effect on increasing thrips in 2015; we hypothesize the difference was nitrogen fixation by alfalfa that was grown in the plots prior to onion. In 2015, while soil nitrate and ammonium were slightly higher in June with higher N treatments, this difference disappeared by August. Soil nitrate levels in August were double that in June suggesting N fixation and mineralization of alfalfa residue swamped the effects of the differential nitrogen application rates. Likewise, there were no differences in microbial biomass or soil health indicators in plots previously planted to alfalfa.
These results along with those in Objective 1 (presented in 2014 annual report) that plant size did not vary with location in the field although thrips densities were consistently higher on the field edge, support our hypothesis that there is a change in plant chemical or visual appearance that is influencing attraction and/or feeding and reproduction of thrips on onions with higher nitrogen levels. To test this hypothesis, we measured levels of sulfoxide defense compounds as well as free amino acids present in the 2rd and 3rd onion leaves. Preliminary studies suggested that differences were most pronounced in these actively growing leaves. In both years, 18 amino acids plus GABA and tyramine were identified in onion samples. In 2013 free amino acids were 76% greater in standard fertilized onions. The majority of the difference was accounted for by a 220% increase in glutamine, while most amino acids were just 40 – 80% higher in standard fertilized onions. Differences were less pronounced in 2014, however, with standard fertilized onions containing just 21% greater total free amino acids. In 2014 the onions were crushed prior to shipment for analysis so that actively growing leaf tissue could not be selected, the likely reason for the smaller differences detected that year. High levels of degradation products of aklylcysteine sulfoxides, (compounds implicated as having pesticidal activity) were also detected including alliin, methiin, common C16 and C18 fatty acids and 14 unidentified compounds half of which were significantly (p < 0.05) lower in reduced N fertilized onions. Several of the unknowns were also highly correlated with total amino acids. This suggests that thrips preference or performance was associated with high amino acid levels in highly fertilized onions, not increased levels of phenolic defense compounds in reduced N fertilized onions.
Objective 5. 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). 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 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.
We made significant progress in understanding critical interrelationships among onion crop nutrition, field cultural practices, pest management, yield, and profitability. Our results and outreach products will be especially relevant to onion production areas across the western U.S., as well as to other states and countries where onion thrips, IYSV, and weeds are economically important pests.
In the late-project assessment (2015) 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.
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, Entomological Society of America 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.
The seven videos produced on onion farm-scape management strategies for pests, optimal crop nutrition and irrigation, and demonstration of sampling protocols for crop and pest management (search Utah State University onion crop and pest management; https://www.youtube.com/watch?v=fkGcJDLfFr0) have been viewed a total of 20,808 times as of December 2016:
Onion Farm-scape Management for Weeds, Thrips, and Virus (https://www.youtube.com/watch?v=fkGcJDLfFr0): 1,395 views
Onion drip irrigation (https://www.youtube.com/watch?v=it8EJw7cGnk): 15,525 views
Onion furrow irrigation (https://www.youtube.com/watch?v=SkIyYrl-R9c): 2,832 views
Onion nutrient management (https://www.youtube.com/watch?v=mUESUs_C2Fo): 446 views
How to do Onion Plant Sampling (https://www.youtube.com/watch?v=jg6bSrdZqiQ): 243 views
How to Soil Sample in an Onion Field ( https://www.youtube.com/watch?v=BFW-5wIIHcs): 192 views
How to sample for onion thrips (https://www.youtube.com/watch?v=KINMAvjHzv8): 175 views
Research Outcomes
Education and Outreach
Participation Summary:
Refereed Research Articles
Schwartz, H. F. D. Alston, J. Alwang, M. Bartolo, T. Blunt, C. O. Boateng, B. Bunn, C. S. Cramer, W. Cranshaw, J. Davidson, M. Derie, J. Doran, K. Douce, D. Drost, L. J. du Toit, J. Gao, T. Gourd, B. Gugino, B. Hammon, J. Hardin, M. Hausbeck, G. Jibilian, J. Lafferty, J. LaForest, M. S. McMillan, S. K. Mohan, J. Morrice, B. A. Nault, C. Nischwitz, G. Norton, K. Otto, H. R. Pappu, M. Petersen, R. Sampangi, B. Schroeder, W. Secor, S. Szostek, N. Tisserat, M. E. Uchanski, J. VanKirk, T. Waters, P. Wiriyajitsomboon, and C. Wohleb. 2014. Onion ipmPIPE: A coordinated effort to improve the management of onion thrips and Iris yellow spot virus for the U.S. onion industry. Plant Health Review 15 (4): 172-183.
Buckland, K., J. Reeve, D. Alston, C. Nischwitz, and D. Drost. 2013. Effects of nitrogen fertility and crop rotation on onion growth and yield, thrips densities, Iris yellow spot virus and soil properties. Agriculture, Ecosystems and Environment 177: 63-74.
Refereed Abstract
Swain, A., C. Ransom, D. G. Alston, and C Nischwitz. 2015. Influence of weed species on thrips and iris yellow spot virus in onion. Proceedings of the Western Society of Weed Science 68.
Extension Publications
Drost, D., C. Cannon, D. Alston, M. Murray, C. Nischwitz, M. Pace, B. Bunn, B. Hunter, and T. Beddes. 2016. Utah Vegetable Production and Pest Management Guide (206 pp). Utah State University Extension, Logan, UT
http://utahpests.usu.edu/IPM/files/uploads/Publications/UT-veg-guide-2016.pdf
Maughan, T., D. Drost, N. Allen. 2015. Vegetable Irrigation: Onion. Utah State University Extension, Logan, UT. http://digitalcommons.usu.edu/extension_curall/723/ (298 downloads)
Trade Magazine Articles
Drost, D. 2016. Experts Share Forecast, Advise at Utah Onion Meetings. Onion World. May/June Pg. 22-23. http://reader.mediawiremobile.com/ColumbiaMediaGroup/issues/101732
Drost. 2015. Utah Onion Association Covers Hot Topics at Winter Meetings. Onion World. May/June Pg. 8. http://reader.mediawiremobile.com/ColumbiaMediaGroup/issues/104416
Drost, D. 2014. Creating a Balance in the Plant. Onion World. May/June Pg. 10-12. http://reader.mediawiremobile.com/ColumbiaMediaGroup/issues/104424
Newsletter Articles
Alston, D. 2015. Growing cover crops for pest management. Utah Pests News Vol 9 (Fall): 4-6. Utah State University Extension, Logan, UT
http://utahpests.usu.edu/files/uploads/UtahPests-Newsletter-fall15.pdf
Alston, D. 2014. Systems-based onion pest management in Utah. Utah Pests News 8 (Winter): 1-2. Utah State University Extension, Logan, UT
http://utahpests.usu.edu/files/uploads/UtahPests-Newsletter-winter14.pdf
Posters
Alston, D. G., C. Nischwitz, D. Drost, J. Reeve, C. Ransom, and R. Ward. 2016. System drivers of IPM for onion thrips and iris yellow spot virus in onion. Pacific Branch of the Entomological Society of America, April 3-6, Honolulu, HI.
Alston, D. G., C. Nischwitz, D. Drost, J. Reeve, C. Ransom, and R. Ward. 2015. System drivers of IPM for onion thrips and iris yellow spot virus in onion. International Integrated Pest Management Symposium, March 23-26, Salt Lake City, UT.
Drost, D. A. Alston, C. Nischwitz. 2016. Do Crop Rotations and Nitrogen Levels Impact Thrips, IYSV and Onion Productivity? National Allium Research Conference. Dec. 1-3, Savannah GA.
Education and Outreach Outcomes
Areas needing additional study
To further elucidate novel project findings:
- Collect additional onion tissue element data to confirm the nutrient imbalance hypothesis put forth based on project results.
- Confirm ISYV infection in two novel weeds that may serve as reservoirs for the virus- black medic and witchgrass.
- Repeat onion-weed-thrips-IYSV interactive studies in larger plots to reduce the strong border effects of the smaller plots used in this study.
- Conduct further data analyses on the large dataset of onion production and pest management factors generated in this project, to further elucidate important cropping systems interrelationships.
- Expand economic analyses and gather longer-term impact data on the utility of the onion crop risk tool developed in this project. Modify the onion risk tool as determined based on feedback.
- Produce more how-to videos on complex cropping systems. The growers expressed enthusiasm for the onion crop and pest management video series.