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 were initiated during 2013, Year 1 of the project. We surveyed 26 commercial onion fields and found relationships among incidence of Iris yellow spot virus (IYSV), onion thrips densities late in the growing season, and the presence of weeds on field edges that harbor thrips and the virus. We found no relationship between onion bulb diameter in late summer and thrips densities early-, mid- or late in the season across the 26 fields. This finding supports previous results from Utah that onion bulb size is not correlated to thrips densities and is contrary to the general belief of producers and other published studies.

In early season samples of thrips on weeds along field edges, four weeds were found to support all life stages of onion thrips, and three of these tested positive for IYSV. Flixweed, field bindweed and common mallow supported thrips reproduction and had the virus in their tissues. These three weeds are perennial or biennial and have strong potential to serve as green bridge hosts (plants that can harbor the pests over the winter to provide sources of infestation the following year). Shepherd’s purse is a winter annual and supported thrips reproduction, but IYSV was not found in its tissues. Prickly lettuce, dandelion and foxtail barley appear to be less important green bridge hosts, although they did support low levels of thrips reproduction, and prickly lettuce and dandelion were positive for IYSV.

All results are preliminary, and sample processing for the study on relationships among nitrogen fertility rates and attraction to thrips is still in progress. Development of the crop risk model and the weed-thrips-virus interaction study will begin in 2014. A pre-project survey of producer crop management practices was conducted in 2013, and follow-up surveys will be conducted in 2014 and 2015. Data on producer practices will be linked to crop performance and pest pressure data to form the database for development of the risk model, and changes in grower crop management behaviors over the course of the project will be used to measure impacts of the project.

Objectives/Performance Targets

2013 was the first year of the project. Project team members met on April 24 to plan research activities for the year, and again on September 16 to discuss accomplishments, budgets and plans for 2014 and to review graduate student applicants for the MS position that will begin in the second year of the project. The project team will meet in January 2014 to plan the year’s research and outreach activities and confirm the hire of a MS student.

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.

In 2013, we expanded a previous dataset on response of onion growth and yield to parameters related to pest pressure, field conditions and the adjacent agricultural landscape. This dataset will serve as the foundation for an interactive onion economic risk model to be developed in Objective 2. With the assistance of producers, we identified 26 commercial onion fields in three counties (Box Elder, Davis and Weber)to include in the 2013 survey. When fields were first visited to establish plots in May to early June, a map was prepared for each field to identify crops in adjacent fields (alfalfa, wheat, corn, pasture, other) and target weed species that can serve as overwintering hosts of thrips and IYSV. The list of target weeds was developed based on previous research that identified weeds on which thrips and IYSV were found. Six to eight plants of each target weed were collected, returned to the laboratory and processed for thrips and IYSV. Twelve plots were established in each onion field; three onion plants were sampled in each plot two or three times (early-, mid- and late-season) for plant growth (leaf number, dry weight and bulb diameter), thrips densities (in situ and whole plant wash samples on late-season dates only), and Iris yellow spot virus (IYSV; percentage of infected plants). Soil and onion tissue samples were collected in June and August from each field to test for nutrients, pH and microbial biomass and activity (soil only). Soil and onion tissue data analysis is in progress. Bulb yield and quality were measured once in each field after onion plant tops had tipped over, but before bulbs were removed from the field. Each cooperating grower was interviewed to obtain crop production and pest management practices for each field that was surveyed.

Objective 2. Develop a crop risk model based on onion production and pest management parameters associated with profitable yield and quality.

In Objective 1 we are developing a database of producer practices, onion growth and yield, onion plant and soil nutrient and health status, and incidence of thrips and IYSV for the 26 fields sampled during 2013. This dataset will be added to previous data collected in surveys of 31 fields in northern Utah during 2008-09. More data points will be added in Year 2 of the current project. This entire data set will be used to develop the crop risk model beginning in 2014.

Objective 3. Determine how management of key weed hosts influences incidence of onion thrips and IYSV in onion.

The research plot design was finalized, soil prepared for onion beds in the study field and nitrogen applied to soil beds to prepare for planting onions in spring 2014. This research will begin in the 2014 growing season with the assistance of a MS student.

Objective 4. Determine linkages among reduced N fertilization rate, onion tissue N, phenolic defense compounds and thrips populations.

The performance target is to determine mechanism(s) for why onion thrips and IYSV pressure is lower in fields with reduced levels of nitrogen. These effects have been observed in previous studies. These results will be used to further inform the risk model being developed in Objective 2. In 2013, thrips densities were sampled from a research field with plots managed with standard and reduced fertilizer rates. The plots were also sampled for onion tissue and soil nutrients as well as soil microbial biomass and activity. Thrips samples are still being processed and data analysis is ongoing. Onion tissue from the June sampling was stored at -80°C and will be freeze-dried and sent to Dr. Larry Phelan at Ohio State University for measurement of secondary compounds and amino acids.

Objective 5. Develop and deliver outreach products to the onion industry and research, extension, and regulatory communities of interest; and assess project impacts.

The project was just initiated in spring 2013, so no outreach products have been developed yet. Dr. Daniel Drost will share research results with the multi-state coordinating committee, W2008: “Biology and management of Iris yellow spot virus, other diseases, and thrips in onion” on December 12, 2013 in Denver, CO. This meeting will be attended by onion researchers and extension specialists from across the U.S. Project results will be presented at the annual Utah Onion Association winter meeting on February 11, 2014 in Brigham City, UT. Arrangements are being made to videotape fields with differences in weed diversity and abundance along field edges, and different scenarios of neighboring crops during 2014 to be used in an educational video on crop and field management practices that influence thrips and IYSV incidence.

Accomplishments/Milestones

Objective 1: Utah onion producers collaborated on selecting fields to survey and in providing background data on field management practices. Producers will provide final yield results, both pre- and post-storage when data is available.

2013 field survey results: In most commercial fields that were surveyed, thrips densities increased over the season. However, in some fields with more frequent insecticide applications, thrips numbers were lower in late- than mid-season samples (Fig. 1). Thrips counts in the whole plant wash samples on the late-season date were two to five times higher than in the in situ counts on the same date (sampled different plants in the same plot), demonstrating the greater precision of the whole plant wash method for extracting thrips from plants (Fig. 1). However, the whole plant wash method is more labor-intensive, and thus more costly. There was not a relationship between bulb diameter on the late-season sample date and onion thrips densities in early-, mid- or late-season samples (Fig. 2) supporting the team’s findings in a previous Western SARE-funded project: bulb size is not correlated to thrips densities. This is counter to other published studies and what most producers believe about the potential for thrips to reduce onion yields.

To evaluate the effects of different grower practices and field locations on IYSV infection in onions planted on similar dates, seven fields were selected for more intensive sampling. Fifty plants per field were collected in early July and mid-August for IYSV testing. All plants in July were negative for the virus. The abundance of mature prickly lettuce and common mallow, two weeds known to host thrips and the virus, were measured along the field edges. The intensity of IYSV in the field late in the season was related to the abundance of the two weed species and to late-season thrips densities (Table 1).

Three biennial or perennial weeds – flixweed, field bindweed and common mallow were found to support thrips reproduction and tested positive for IYSV; thus, they are all good candidates for green bridge plants (over-wintering reservoirs of thrips and the virus) (Fig. 3). Shepherd’s purse is a winter annual and supported thrips adults, eggs and larvae; thus, it could serve as a source of thrips for onion fields in the spring, although none of the plants tested positive for IYSV. Prickly lettuce was positive for the virus but had lower thrips densities. Foxtail barley and dandelion were poor thrips hosts; thus, their suitability as green bridge plants appears to be less.

Objectives 2, 3, and 4: No accomplishments or milestones to report yet.

Objective 5: Results of field surveys will be discussed in a roundtable session to be held at the annual Utah Onion Association winter meeting in February 2014. We will gather grower feedback and identify approximately 30 more fields to survey in 2014. The in-person surveys of Utah onion producers conducted in spring 2013 will be used in lieu of a mail-out survey. The project team felt that we could gather more detailed information from a larger proportion of the industry through the in-person surveys. These data will be used as our baseline to evaluate changes in grower behaviors and adoption of crop and pest management practices from the beginning to the end of the project.

• 2013 Results – Figures and Table

Impacts and Contributions/Outcomes

None to report yet.

Collaborators: