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

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

Summary

Research and outreach activities continued on schedule in 2015. Preliminary results for the grower-collaborative survey of commercial onion fields (Objective 1) were analyzed (57 fields were sampled). We found that higher thrips densities were correlated to higher virus (IYSV) incidence, and to lower onion yields. We identified four weed species that serve 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. 

Still to be completed, we will summarize all grower field practice data (nitrogen applications, insecticide use, etc.), and combine it with our field sample measurements (soil nutrients, thrips abundance, IYSV incidence, and yield) to create a comprehensive database to inform the onion production risk model under development (Objective 2). The project economist has designed a format for the risk model and companion interactive spreadsheet to evaluate how different management practices influence crop performance. The database and model will be completed in 2016.

A Master’s of Science student was employed to manage the weed-thrips-IYSV interaction experiment (Objective 3). 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.

To determine linkages among nitrogen concentration in onion leaves and soil, phenolic defense compounds, and thrips populations (Objective 4), onion plots treated with low and high rates of fertilizer were sampled in June through August in 2013-2015. In two years of the study, 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, we did not observe an effect of nitrogen rate on thrips because all of the onion plots in the rotation scheme followed three years of alfalfa where nitrogen fixation overrode differences in N application rates. Results for higher fertilizer rate applications increasing thrips reflect higher soil available nitrogen, phosphorus and trace elements, 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. Processing of samples to measure concentrations of secondary compounds and free amino acids in onion tissue from standard and reduced onion plots remain to be completed. We are assessing concentrations of these compounds because we hypothesize that they will be related to thrips attraction to onion plants and the ability of plants to deter thrips feeding and virus infection.

We have developed numerous outreach products and completed pre- and mid-project impact assessments (Objective 5). 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. He used a drone-mounted video 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 they were well received. The videos are available on YouTube (https://www.youtube.com/playlist?list=PLMnDQoXFVBEYZRVFYWJyijJDDon0AnqcA). In 2015, twelve presentations on project results were delivered to over 565 growers, researchers, and industry and extension professionals at the UOA Winter Meeting and Field Tour, Pacific Northwest Vegetable Association Annual Meeting, Agricultural Experiment Station Multi-state Coordinating Committee Annual Meeting W-2008, and the International Integrated Pest Management Symposium. Three publications disseminating project results were disseminated: one research article, one extension fact sheet, and one newsletter article.

Objectives/Performance Targets

All project team members met on March 18 and November 10, 2015 to plan and coordinate research and outreach activities, and discuss results and budgets. 

Objective 1: Collaborate with Utah onion producers in a survey of commercial onion fields to expand and refine production system predictors of pests and yield. Field data collection was completed in 2014; however, we are still gathering grower practice data for each field included in the 2013 and 2014 surveys. We have made significant progress, but it has been challenging to obtain crop and pest management data from some growers. We sampled a total of 57 fields in thee onion producing counties of northern Utah for soil nutrient content (N, P, and K), thrips abundance, IYSV incidence, onion bulb size, and yield. Initial correlation analyses reveal the following positive relationships among variables (increases in one variable are associated with increases in another variable): thrips numbers in July and thrips numbers in August, thrips numbers in August and IYSV incidence in onion plants, soil nitrate and soil available potassium concentrations, soil available phosphorous and soil potassium concentrations, and soil phosphorous and onion bulb diameter. Negative correlations were found for thrips abundance in August with yield, lending support to higher thrips densities causing decreased bulb yield. The dataset is still incomplete; final analyses will be conducted on the complete dataset. Final grower practice data will be obtained during winter and early spring. We will compile field survey data from 2013 and 2014 with data from a previous onion field survey conducted in 2008 and 2009 (and also funded by WSARE). This enlarged database will inform the crop risk model that is under development in Objective 2. We will use Random Forest or other appropriate statistical analyses to determine key drivers of risk for thrips and virus in onion fields, and reduced yields.

Objective 2: Develop a crop risk model based on onion production and pest management parameters associated with profitable yield and quality. The database developed in Objective 1 combined with data from a previous WSARE grant will be used to develop the crop risk model. The project economist has begun developing the format for the model and companion interactive spreadsheet. Users of the model spreadsheet will be able to select pest management practices and inputs and follow how changes influence the risks for obtaining desired yield, and bulb size and bulb quality. This interactive model will allow growers and other users to select crop and pest management options that lower risks for yield and profit losses.

Objective 3: Determine how management of key weed hosts influences incidence of onion thrips and IYSV in onion. This experiment was managed by M.S. student Andrew Swain in 2014 and 2015. The experiment was conducted at the USU Horticultural Crops Research Farm in Kaysville, UT. All 2014 samples have been completed and the data analyzed; some 2015 thrips samples remain to be completed. Results from 2014 showed that mid-season mowing of resident weed borders resulted in a large increase in thrips abundance in adjacent onions. Thrips counts on onions in other treatments did not significantly differ (Fig. 1). Of the three weed monoculture border treatments, thrips preferred field bindweed followed by common mallow. Prickly lettuce was the least-preferred host of the three monoculture species tested (Figs. 2 and 3). Border biomass analysis revealed the dominance of common lambsquarters in all resident weed borders. In the 2014 trial we learned that un-mowed resident weeds grew too tall and may have blocked access of thrips to onion plots. In 2015, we kept the resident weeds mowed to a height not exceeding 1.6 ft (50 cm) with a hedge-trimmer. A second lesson we learned in 2014 was the need for a larger sample size of weeds and onions to adequately measure virus incidence, and this was incorporated into the 2015 study methods, resulting in more consistent detection of IYSV in weed and onion plants. 

In 2015, purslane and wild oats, which were present in 2014, were essentially absent from the resident weed border plots. Thrips population trends within a season differed between 2014 and 2015, and late season counts were much lower in 2015 (Fig, 4). Preliminary results indicate that thrips counts between treatments in 2015 do not differ significantly (Fig. 5). Positive IYSV samples were obtained from all weed species tested; however, IYSV levels in onions were low (Tables 1 and 2). The highest virus infection rates in weeds occurred in July, while highest infection rates in onions occurred in August. Weed species with the highest IYSV infection were witchgrass, black medic, lambsquarters, prickly lettuce, and nightshade (Table 1). Infection rates in common mallow and field bindweed did not exceed 3% on any sample date. Weed border treatments that contributed to the highest IYSV infection rates in adjacent onion plants were common mallow and prickly lettuce (Table 2). Resident weed borders mowed to the ground once during July and hand-weeding resulted in the lowest virus infection rates in the adjacent onions. In summary based on preliminary results to-date, mid-season mowing of resident weed borders pushed thrips into adjacent onions in one of the two years of study. By maintaining resident weed height at no more than 50 cm in 2015, thrips populations may not have built to as high of levels as in 2014 when high thrips migration followed the July mowing event. Weeds with the highest rates of IYSV infection did not necessarily translate into transmission of IYSV at higher incidence rates in adjacent onions. Overall, weed removal (hand-weeding) and mowing were the most effective in reducing the incidence of IYSV in onions.

Objective 4: Determine linkages among reduced N fertilization rate, onion tissue N, phenolic defense compounds, and thrips populations.  This is the third year of this experiment.  In 2013 and 2014, fewer thrips were found on onion plants fertilized with reduced (120 lb N per acre) as compared to standard (350 lb N per acre) N rates.  The thrips results support our initial hypothesis that elevated N application rates will increase the risk for thrips and IYSV.  In 2015, all onion plots followed three years of rotation to alfalfa and one year to onion, and there was no difference in thrips populations among reduced and standard N fertilizer treatments.  We hypothesize that this was due to a higher availability of soil N following nitrogen fixation by the previous alfalfa crop; abundant N produced by the alfalfa washed out the effects of the differential N application rates to the onion crop in 2015.

Samples were collected for soil properties in May and July of each 2013, 2014, and 2015. Available soil nitrogen and microbial biomass and activity were measured. Onion secondary compounds, free amino acids and total N was measured on freeze dried samples from the 2013 and 2014 growing seasons. The 2015 onion tissue samples are still being analyzed for total nutrients.  In 2013 and 2014, soil nitrate was 33 and 37% lower in the reduced than standard fertilizer treatment (Fig. 6) and soil ammonium was 80% lower in 2013. This represented a significant reduction in N leaching potential as available soil N was adequate to high in all treatments. Soil pH was lower and electrical conductivity (EC), Olsen P, and trace elements higher in the standard fertilizer treatments in both years. Interestingly, readily mineralizable C was also higher in the standard fertilizer treatments (Fig. 7), indicating that the high level of available N may have made soil C more available to the soil microbial biomass. Soil respiration and microbial biomass and activity were higher in the reduced fertilizer treatments (Fig. 8), confirming our findings generated previously. Soil nitrate was 32% higher after alfalfa than after 2 years of wheat and 27% higher than after 2 years of corn. Soil nitrate was 12% higher after corn than after wheat in both years, with wheat corn and corn-wheat rotations intermediate.

Objective 5: Develop and deliver outreach products to the onion industry and research, extension, and regulatory communities of interest; and assess project impacts.  The team made great strides in reaching performance targets in outreach: seven educational videos, gathered data on early-project perceptions and impacts, and continued outreach presentations to industry and research audiences on a state and national scale.  We will continue to gather more project impact data in our final year, and finish the risk model database and predictive tool by the end of the project period.

• Project Results Figures and Tables

Accomplishments/Milestones

Objective 1. One goal of the commercial field survey is 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. Preliminary analyses have revealed positive correlations between onion thrips abundance and 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 also 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. Our field survey dataset is still incomplete, and so more and stronger relationships may appear when analyses are completed. Additionally, 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, and, 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).

Objective 2. All of the data for the risk model has been collected. We need to finish gathering grower practice data for 2014. Statistical analyses seeking primary factors that are driving reduced yields and higher risk will be completed in the coming year, and a user-friendly risk model will be completed and disseminated to the growers and industry.

Objective 3. We demonstrated that reduction in weed populations reduced incidence of IYSV in nearby onions. 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. In our analyses completed so far, we have not observed 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. Final samples remain to be completed and analyzed, and final conclusions will be consolidated during the coming year.

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, phosphorus and trace elements, 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. The higher concentration of nitrogen following alfalfa swamped the effects of the differential nitrogen application rates. 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.

Objective 5.  Pre- and early-project data on industry perceptions and impacts have been collected and summarized. Grower roundtable discussions were held at the Utah Onion Association winter meetings in both 2014 and 2015. At the 2016 onion meeting, we will survey growers and industry representatives for their feedback on project results and accomplishments, and document their changes in onion crop and pest management practices following exposure to educational activities and resources produced by this project.

Impacts and Contributions/Outcomes

In an early-project survey of growers and industry professionals (conducted February 2014), 100% of respondents indicated that project results improved their awareness of onion management issues and provided new knowledge and skills. One hundred percent of producers said that during the next year they would adopt one or more of the practices discussed, increase networking with other producers, and use the information to improve their farm profitability. One hundred percent of professionals who provide support to the onion industry reported that they will use some aspect of the project information to improve their advice, 78% will use it as a resource to make available to producers, and 44% will use the information in an educational program. Seven videos demonstrating project results and showcasing sustainable crop and pest management practices were premiered at the Utah Onion Association meeting in February 2015. The videos are posted on YouTube (https://www.youtube.com/playlist?list=PLMnDQoXFVBEYZRVFYWJyijJDDon0AnqcA) and were well received by the conference attendees. The risk model tool is under development. The dataset to support the model will be completed in early 2016, and the user interface tool, a fillable spreadsheet, will be released for beta testing.

Collaborators: