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 ramped up in Year 2 (2014) of the Utah integrated onion systems project. The grower-collaborative survey of commercial onion fields (Objective 1) was continued: 31 fields operated by 13 different farms were sampled (26 fields were sampled in 2013). 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. 

Numerous parameters were measured in each field in June and August (and July for thrips and IYSV): soil available nitrogen (N), macro- and micro-nutrient levels, microbial biomass, pH, and EC; thrips densities; IYSV incidence; and onion plant and bulb growth (weight and leaf number). Estimated field yield (bulb weight and size grade) was determined in September. We are preparing 2014 cooperator reports to provide feedback on field conditions to growers. This winter, cooperators will be interviewed to gather data on crop and pest management practices for each survey field. These data will be added to the onion production risk model database (Objective 2). In 2014, the project economist designed formats for the risk model and companion interactive spreadsheet to evaluate how different management practices influence crop performance. 

A MS student was hired 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, or hand-weeded. There were two resident weed treatments; one was mowed as onion thrips densities peaked in July, and the other was left un-mowed to determine if mid-season host removal will increase thrips and virus on nearby onion plants. IYSV incidence in weeds and onions was low in June and July and increased in August. Weeds that tested positive for IYSV included common lambsquarters, field bindweed, nightshade, and wheatgrass.  Completion of thrips samples and data analysis is in-progress. 

To determine linkages among N concentration in onion leaves and soil, phenolic defense compounds, and thrips populations (Objective 4), onion plots treated with low and high rates of N fertilizer were sampled in May through August. As in 2013, 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) N rates. Completion of soil and onion tissue samples from 2013 and 2014 is in-process.

Development of outreach products and assessment of project impacts (Objective 5) is underway. 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. In the summer, a camera mounted on a drone quadcopter was flown over onion fields to collect video 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 will be premiered at the 2015 Utah Onion Association (UOA) conference. In 2014, five presentations on project results were delivered to 100 growers and industry professionals at the UOA conference in February and field tour in August. Presentations were delivered to 73 researchers and educators at two entomology conferences and to 330 growers, industry leaders, and researchers at two national onion industry conferences and a national onion research meeting. 

UOA meeting attendees were evaluated for impacts in 2014 (20 responses received): 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 during the next year. At a grower advisory roundtable discussion (February) and UOA board meeting (November), growers voiced continued support for research on the interactions between crop and pest management factors and stated it is highly relevant to their operations. They requested cooperator reports on crop health and pest findings in their fields and indicated priorities for future research: water conserving irrigation and optimal fertilizer management.

Objectives/Performance Targets

2014 was Year 2 of the project. Project team members met on March 3 and October 28, 2014 to plan and coordinate research and outreach activities and discuss results and budgets. We hired a Master’s student, Andrew Swain, to address Objective 3. All project investigators and Andrew participated in the meetings.

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. We have completed the data collection for this objective. We sampled a total of 57 fields in thee onion-producing counties of northern Utah in Years 1 and 2 for soil and tissue nutrient content, soil health, thrips populations, IYSV incidence, onion plant growth, and bulb yield and quality. Sample processing is still in-process. During Year 3 we will compile all of the data along with grower crop and pest management practice information with data from a previous onion field survey conducted in 2008 and 2009 (and also funded by Western SARE). This enlarged database will inform the crop risk model that is under development. 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 Western SARE 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, 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 MS student Andrew Swain. Andrew and advisors carried out the first run of this experiment at the USU Horticultural Crops Research Farm in Kaysville, UT during 2014. Sample processing is still in progress. One lesson learned in 2014 was that un-mowed resident weeds grew too tall and may have blocked access of thrips to onion plots. In 2015, we propose to use a hedge-trimmer or shears to maintain weed height at no more than 3-4 feet. A second lesson learned was that we need a larger sample size of weeds and onions in 2015 to adequately measure virus incidence. The soil and onion beds have been prepared at the research farm, which will be planted in spring 2015 for the second run of this experiment.

Objective 4: Determine linkages among reduced N fertilization rate, onion tissue N, phenolic defense compounds, and thrips populations. This is the second year of this study. Samples were collected for soil properties in May and July of each 2013 and 2014. Available soil nitrogen and microbial biomass and activity was measured. Measurement of soil pH, EC, and macro- and micro-nutrients is still in process. Onion secondary compounds, free amino, and total N was measured on freeze dried samples from the 2013 growing season. The 2014 onion tissue samples are still being analyzed. In both years, 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) N rates. The thrips results support the initial hypothesis that elevated N application rates will increase the risk for thrips and IYSV.

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: created 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 Year 3 and finish the risk model database and predictive tool by the end of the project period.

Accomplishments/Milestones

Objective 1.  We continued the grower-collaborative survey of commercial onion fields in the three primary onion producing counties of northern Utah: Box Elder, Davis, and Weber. In 2013, we surveyed 26 fields on 11 farms, and in 2014, we surveyed 31 fields on 13 farms. In the early-season, we measured densities of thrips and incidence of Iris yellow spot virus in weeds on field edges. Our goal is to determine which species of weeds are good overwintering hosts, or “green bridges”, for thrips and the virus. Flixweed, common mallow, field bindweed, and to a lesser extent, prickly lettuce, had moderate to high densities of onion thrips adults and larvae on plants (Figs. 1 and 2) and thrips eggs inserted into leaves (data not shown). Our results indicate that these weeds 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. Thrips and the virus were also sampled in the onion crop near field edges and in the field interior in the June, July, and August to compare pest occurrence in onions near and further away from weedy ditch banks and adjacent crops. Onion fields were sampled for soil properties in June and August. Available soil nitrogen and microbial biomass and activity was measured. Measurement of soil pH, EC, and macro and micro nutrients is still in process. Samples for onion bulb weight and size were collected in September after growers had lifted plants in the fields. In 2014, in situ counts of thrips peaked in most fields in July (mid-season) and then declined in August (late-season) (Fig. 3). Whole plant wash counts of thrips are still being completed. There was a field edge effect on thrips densities: more thrips were found on onion plants 10-20 feet from the field border than in the interior, 250-260 ft away from the border (Fig. 4). We have observed this edge effect in other studies. These results support our findings that weeds and adjacent crops are sources of onion thrips. The number of onion leaves per plant did not differ by location in the field but was greatest in the late-season and least in the early-season (Fig. 5). Onion bulb diameter in August was greater in the interior of fields than on the edges (Fig. 6). This could be an effect of fewer thrips on plants in the field interior; however, as in 2013, we did not find a significant effect of thrips densities on bulb size.

Objectives 2 and 3: No milestones to report yet.

Objective 4.  As in 2013, we observed increased thrips densities on onion plants with higher rates of nitrogen (Fig. 7).  In 2014, we also observed greater hatch of thrips eggs (a measure of viability) on plants fertilized with 350 lb as compared to 120 lb N per acre (Fig. 8). These results along with those in Objective 1 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 N. Data analysis of chemical composition of onion plants remains to be completed.

Objective 5.  Pre- and early-project data on industry perceptions and impacts have been collected and summarized. A grower roundtable was held in February 2014 and will be held again at the Utah Onion Association meeting in February 2015. We will ask growers and industry representatives for their feedback on project results and accomplishments to-date and seek guidance on priorities for future research and extension work. Post-project impact data will be collected in late 2015 and/or early 2016.

• 2014 Annual Report Accomplishments Figures

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 will be premiered at the Utah Onion Association meeting in February 2015. The videos will be available to the public on YouTube. The risk model tool is under development. The dataset to support the model will be completed in 2015, and the user interface tool, a fillable spreadsheet, will be released for beta testing during 2015.

Collaborators: