Cultural Management of Onion Thrips and Iris yellow Spot Virus

Cultural Management of Onion Thrips and Iris yellow Spot Virus

Summary

Iris yellow spot virus (IYSV) and onion thrips threaten sustainable, profitable onion production in Utah and the western U.S. Onion growers rely on high-risk insecticides for thrips management, which has increased insecticide resistance and may increase the severity of IYSV outbreaks. Very little is known about how to effectively manage IYSV. In collaboration with growers, we propose to survey the Utah onion industry to ascertain the effectiveness of IYSV and thrips management strategies used on commercial onion farms. Two additional two-year, replicated field studies will evaluate plant nitrogen status, trap crops and crop rotation on the incidence of thrips and IYSV.

Objectives/Performance Targets

In order to meet the multiple goals of increased environmental and economic sustainability, enhanced quality of life and worker safety, and increased farmland diversification in onion cropping systems, we propose the following six objectives:

1) Work with local growers to determine onion thrips and IYSV pressure in primary Utah onion growing areas and correlate IYSV pressure to common farm practices;

2) Determine the effects of crop and pest management strategies on thrips’ survival and population size;

3) Evaluate nitrogen (N) inputs, N leaching potential, alternative fertilizers, trap crops and rotation on thrips, IYSV, onion yields and storage quality;

4) Conduct grower workshops and field days on control options of onion thrips and IYSV;

5) Conduct economic cost-benefit analysis of proposed changes to management of onion thrips and IYSV; and

6) Disseminate results through extension bulletins, the internet, trade journals and scientific literature.

Accomplishments/Milestones

1) Work with local growers to determine onion thrips and IYSV pressure in primary Utah onion growing areas and correlate IYSV pressure to common farm practices.

Sixteen commercial onion fields were surveyed over summer 2009 for IYSV, onion thrips and soil and plant N.

Thrips Evaluation: Onion thrips population densities were monitored in 16 commercial onion fields in Box Elder, Davis and Weber Counties, following the same procedures used in 2008. Onion thrips data remains to be analyzed and summarized.

IYSV Diagnosis: Kent Evans retired from the project in summer 2009, due to leaving Utah State University and taking a position elsewhere. He will be replaced in August 2010 by a new Plant Pathologist with extensive experience with IYSV. While he trained the students in the analysis prior to leaving, directly supervised the first month of sampling, and is still providing us with feedback and advice, his presence in the lab directly supervising students was sorely missed and may have resulted in more mistakes occurring toward the end of the 2009 season than we would have hoped.

Nevertheless, we successfully tested more than 4800 samples for IYSV during the 2009 season. As in 2008, ELISA testing detected the presence of IYSV early in the season in all fields while visual rating did not detect the disease until August. Symptoms were generally mild in 2009 compared with 2008, with only four fields showing any significant IYSV symptoms by late August. Preliminary correlations indicate a positive relationship between thrips numbers and IYSV incidence and a positive correlation between N application rate and IYSV incidence.

Soil and Plant N: Soils and onions were sampled on a monthly basis from June through September 2009. Soil, tissue N, onion dry weight, leaf area and leaf chlorophyll were tested as in 2008. Onions were collected from all onion fields for storage trials. Pre- and post- storage data were collected and onions graded according to size. We are behind in analyzing our final soil N extracts due to equipment failure. A new N auto-analyzer has now been purchased and the process of analyzing the last of the frozen samples from 2009 will begin shortly. On-farm N use was only marginally correlated with yield in 2009. Preliminary correlations have revealed further relationships between thrips, IYSV and soil and tissue N, which we will analyze in greater detail this fall.

Grower Management Practices: Management practices and site information for all the fields in the 2008 and 2009 survey have been collected. Further in-depth interviews with growers whose fields stood out for some reason are in process. Once the soil N samples have been analyzed, formal data analysis of the entire survey will begin later this fall. One issue that emerged in the 2009 data set is that, unlike 2008, there was a large variation in onion planting date. This was due to a very cold and wet spring that made it very difficult for our growers to plant in a timely fashion. This has led to a confounding of onion size with planting date and N application rate. We are in the process of consulting with a statistician on how best to resolve this. Using regression analysis based on growth stage rather than calendar date is a likely avenue we will pursue.

2) Determine the effects of crop and pest management strategies on thrips’ survival and population size.

Aerial adult trapping data, species of thrips collected in the different fields and egg count and emergence data have been completed for the 2009 season as in 2008. Final correlations with all management practices will be run this fall.

3) Evaluate nitrogen (N) inputs, N leaching potential, alternative fertilizers, trap crops and rotation on thrips, IYSV, onion yields and storage quality.

Replicated Trial 1: Data collection in 2009 on onion thrips, soil nitrogen and quality, and onion growth and quality proceeded as described in our proposal. The six treatments were analyzed monthly for soil nitrate and ammonium, dehydrogenase activity, pH, electrical conductivity, total carbon and total nitrogen. In May and July, following application of the soil biostimulant MoreLife, the soil samples also were analyzed for soil respiration and microbial biomass. The onion plants were sampled monthly for total biomass, total tissue nitrogen, leaf count and leaf tissue area. Chlorophyl measurements were also taken in August. Onion harvest data was collected to include number of onions per area, onion size and total weight. Pre- and post-storage data were collected and onions graded according to size. Additionally, random sampling of 10 onions per plot were used to assess quality such as onion size category, number of scales per bulb and any occurrence of doubles.

Initial results from 2009 indicate that dehydrogenase enzyme activity, soil respiration, and microbial biomass were reduced in the high N treatments (359lb N / acre). Higher N rate also increased motile thrips (adults and larvae) densities above those found in lower N plots (119 lb/acre). Thrips densities were intermediate in the low N + MoreLife plots. We speculate that the higher rates of N increase the attractiveness of onion plants to thrips. Corn planted before onions reduced thrips densities as compared to wheat. Corn uses more N for growth than wheat. We speculate that thrips were less attracted to onions following rotation with corn because of lower N levels in the soil.

ELISA testing for IYSV was conducted for all treatments in June and August. The July sampling was missed in this trial due to miscommunication with the students. The June data was largely negative; however, we did find a trend that was almost statistically significant of greater IYSV in the higher N plots.

Soil N leaching was assessed in all plots at monthly intervals from May through October 2009 using suction cup lysemeters placed at a depth of 2 and 4 feet. No leaching occurred after June and was sporadic at 4 feet. Monitoring of the lysemeters resumed in March 2010, however, insufficient suction pressure could be placed on them, presumably due to changes in soil contact with the lysemeter over the winter. Winter N leaching in the plots was therefore determined through analysis of soil extracts sampled to a depth of 3 feet. Suction cup lysemeters were placed in all new plots to a depth of 2 feet in May 2010 and soil leachate was collected on a monthly basis as in 2009.

This trial is currently being replicated as described above for a second year, with a slight change to the IYSV ELISA testing schedule. Our new plant pathologist, Claudia Nischwitz, will not start work on the project until early August. However, based on our previous two years of data, we feel justified in postponing this analysis until that time. Detailed testing of all treatments for IYSV using ELISA will take place in August and September 2010.

Replicated Trial 2: Replicated plots of carrot, lacy phacelia and buckwheat were established at Morgan Reeder’s farm in spring 2009 and sampled every two weeks for thrips and monthly for soil and onion tissue N, leaf count, leaf area and leaf chlorophyll. The field was sampled for IYSV using ELISA testing in June and August.

All three trap crops were more attractive to onion thrips adults early in the season (June) when onions were in the two-to-four true-leaf stage. Phacelia was more attractive to thrips than buckwheat and carrot. From mid-July to August, thrips were repelled by trap crops or thrips preferred solid stands of onions. Overall, the fewest adult thrips were found in buckwheat plots and in the onion row adjacent to buckwheat. Counts of on-plant population densities of thrips eggs and larvae have been completed, but data remains to be analyzed.

This trial is currently being repeated on a second farm operated by Wade Norman. Based on our survey, this farm typically has greater thrips pressure than Morgan Reeder’s farm. The trap crops have been established and data collection is under way. As described above, analysis of the effect of trap crops on IYSV incidence will be determined in August and September when our new plant pathologist arrives.

4) Conduct grower workshops and field days on control options of onion thrips and IYSV.

Field days, workshops and grower surveys: A summer onion field day was conducted on August 11, 2009, at which we presented updates on the early findings generated by the project. IYSV field identification and thrips sampling techniques were also demonstrated. Approximately 60 onion growers and extension personnel attended, including out-of-state guests. The event was very well received.

In addition to the field day, two evening walks were held. The first evening walk on June 25 was conducted on Morgan Reeder’s farm and focused on the trap crop study. Twelve growers participated. The second evening walk was held on September 1 and focused on thrips and IYSV identification, as well as a discussion of cultural management of onion thrips and Iris Yellow Spot Virus. While these evening walks had relative low attendance, they were very well-received, and we feel that they were particularly valuable due to the high degree of interaction that occurred between project leaders and growers and between individual growers. We are repeating these evening walks this year with the first one scheduled for June 30.

A winter meeting was held on February 16 2010 which was attended by 45 growers and extension personnel. Three presentations were given highlighting early results from this project including the on-farm survey, early findings on the effects of soil N and trap crops on onion thrips, and early results on the effects of high and reduced N on onion yields, quality and soil processes.

Due to low participation in our pre-project survey, we repeated the same survey at the winter meeting. Results will no longer reflect a true pre-project baseline but will nevertheless prove helpful in evaluating grower interest in and uptake of our findings.

5) Conduct economic cost-benefit analysis of proposed changes to management of onion thrips and IYSV.

This objective will be conducted once data collection on the field surveys and field trials is complete.

6) Disseminate results through extension bulletins, the internet, trade journals and scientific literature.

Updates on the project were presented at the National Allium Meeting in Austin, TX in December 2009. A summary of our research findings as presented in the 2009 winter onion meeting was published in the trade magazine Onion World. Two abstracts on the project findings were submitted and accepted; one to the American Society of Horticultural Sciences meeting in Palm Desert, CA in August and a second to the American Society of Agronomy in Long Beach, CA in November.

Impacts and Contributions/Outcomes

Forty-five onion growers, industry leaders and service providers from around the western U.S. attended the 2010 Utah Onion School on February 16 in Brigham City, UT. Participants heard from a variety of presenters who outlined on-going research conducted at Utah State University. Topics identified by the onion growers as being of primary interest were those studies that focused on the relationship between farm nitrogen use and onion thrips. The grower/participant evaluation collected at the end of the sessions noted that most growers are paying more attention to crop and soil nitrogen levels since these have been shown to influence thrips populations. Dr. Reeve’s and her graduate student, Kristie Buckland’s, findings demonstrate that farms and replicated plots with lower N additions tend to have fewer problems with onion thrips and this could reduce the development of Iris Yellow Spot Virus. Growers noted that reducing N applications will save on thrips sprays, thus there is both cost savings and improvements in sustainability. Some growers reported that the idea of reducing N applications to control thrips goes against the common theory and practice that healthy vigorous onion growth (more N applied) and regular thrips sprays are the best ways to reduce IYSV. However, the USU research results and several farmers practicing reduced N applications strongly show this not to be completely true. At present, about 20% of the onion acreage in Utah is grown using more appropriate N applications with almost no reduction in productivity, improved storage quality and fewer pesticide applications. These topics were further discussed at the June 30, 2010 twilight field tour and will be the focus of the summer onion tour scheduled for August 10.

Collaborators: