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 US. 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 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

Fifteen commercial onion fields were surveyed over the summer of 2008 for IYSV, onion thrips and soil and plant N. A further 16 fields are currently being surveyed this summer.

Thrips Evaluation: Thrips evaluation of grower fields in 2008 is completed. The second year of data collection is currently under way. Based on the 2008 data, thrips densities peaked in mid to late July in transplanted fields and in late July to late August in seeded fields. There was significant variation in thrips population densities among the 15 fields sampled. Fields could be pooled into two major groups based on thrips abundance: Low to moderate thrips densities (7 fields); adult thrips counts were between 10-20 and larval counts were 5-120 thrips per plant. High thrips densities (8 fields); adult onion thrips counts were up to 50 and larval counts were up to 370 thrips per plant. Densities of predatory thrips were low throughout the season. Onion fields with lower onion thrips densities generally had higher numbers of predatory thrips.

IYSV Diagnosis: We tested more than 4500 samples for IYSV during the 2008 season, (some weeds and mostly onions), to determine the host range, incidence, and severity of IYSV in and around onion fields in the survey. The second year of data collection is currently under way. Based on the 2008 findings, testing using ELISA detected the presence of IYSV early in the season in all fields while visual rating did not detect the disease until August. Significant variability in IYSV severity was detected among fields. Two new weedy hosts for IYSV were discovered and confirmed using molecular fingerprinting. Preliminary correlations indicate a positive relationship between thrips numbers and IYSV incidence.

Soil and Plant N: Soils and onions were sampled on a monthly basis from June through September and soil and tissue N tested. Onions were collected from all seeded onion fields for storage trials. Pre- and post-storage data were collected and onions graded according to size. Soil and plant tissue N sampling is under way for the 2009 field survey. Data show very high variability in soil N sampled from fields with many fields having high residual (up to 150 Units N) at the end of the growing season. Nitrogen use was not correlated with yield in 2008. Preliminary correlations found only weak relationships between thrips, IYSV and soil and tissue N.

Grower Management Practices: Management practices and site information for all the fields in the 2008 survey have been collected. Final correlations will be run once the 2009 season data have been collected.

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 2008 season. Data collection for the 2009 season is underway. Final correlations with all management practices will be run once the 2009 season data have been collected.

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: Replicated plots of wheat and corn were established at the Kaysville Experiment Station and various N fertility and biological stimulant treatments (MoreLife) applied in October 2008 according to our proposal. A second application of N fertilizers and MoreLife were applied in mid March 2009 and the onions seeded. Master’s student Kristie Buckland started her program funded by this project in April 2009. She has been collecting monthly soil plant and N data, soil microbial activity data, and N leaching data from all the plots. Thrips and IYSV evaluations are also under way in these plots. Additional replicated plots of wheat and corn have been planted adjacent to the onion plots in preparation for the second year of the experiment in 2010.

Replicated Trial 2: Replicated plots of carrot, lacy phacelia and buckwheat were established at Morgan Reeder’s Farm in the spring of 2009. Monthly data collection as described above is under way for the 2009 season. This experiment will be repeated in 2010.

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

Field days and workshops and grower surveys: A summer onion field day was conducted on August 12th at which we introduced the project and presented the idea of a whole farm approach to onion thrips and IYSV management. IYSV field identification and thrips sampling techniques were demonstrated. Approximately 55 onion growers and extension personnel attended including out-of-state guests. The event was very well received.

A second winter meeting was held on February 17, 2009, which was attended by 42 growers and extension personnel on a very snowy day. Presentations were given on IYSV, the onion thrips survey and nitrogen and a whole farm approach to IYSV management. Post-meeting evaluations ranked the research being conducted on this grant as the most interesting and valuable information presented. A research meeting was held with Morgan Reeder in April 2009 to discuss the 2009 field season and trap crop trial.

One hundred copies of a pre-project survey were mailed out to growers and industry personnel in late January 2009, however, only 14 were returned despite a reminder note being sent out. For the post-project survey at the end of the project we intend to try emailing an electronic version of the survey along with the paper copy and offering entry into a prize draw for all returned completed surveys.

The 2009 summer field day is planned for August 11, 2009, and two evening field walks are planned for the last week in June and July.

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.

Posters introducing the project were presented at 1) the Western SARE regional meeting in Cheyenne, Wyoming, 2) at the National Allium Meeting in Savannah, Georgia, and 3) at the Pacific Branch Entomological Society of America Conference in San Diego, California. Dr. Alston gave an invited presentation titled ‘Onion thrips: contributions of egg survival and adult dispersal to populations on plants’ at the Rocky Mountain Agribusiness Association 56th Annual Convention and Trade Show, January 8, Denver Colorado. Dr. Kent Evens was invited to give a presentation on this work at the ID Plant Pathology meeting titled ‘IYSV incidence and severity in Utah onions, 6-7 November 2008, Jerome, ID and at the Treasure Valley Onion meetings titled ‘IYSV incidence and severity in Utah onions, 3 February 2009, Ontario, Oregon. Two introductory fact sheets ‘Iris Yellow Spot Virus in Onions’ (PLP-010-PR) and ‘Onion Thrips’ (ENT-117-08PR) were published in March and September 2008 and two short papers have been published in Plant Disease titled ‘Natural Infection of Iris yellow spot virus in Twoscale Saltbush (Atriplex micrantha) Growing in Utah’ April 2009 Vol 93:460 and ‘Green Foxtail (Setaria viridis), A Naturally Infected Grass Host of Iris yellow spot virus in Utah’ June 2009, Vol 93:6, announcing the discovery of two new weedy hosts for IYSV. One of these hosts was a grass and represents the first time a grass has been confirmed as a host for IYSV.

Impacts and Contributions/Outcomes

The post-meeting survey completed by attendees of the 2009 winter regional onion meeting listed the reports of the research coming out of this project as the most interesting and useful information presented. Onion growers are struggling in Utah. Our 2008 farm survey showed that not a single field surveyed was free of IYSV. Growers indicated that even though they were harvesting good yields, the onions were rotting in storage, a problem associated with IYSV infection. Several growers indicated on meeting evaluation forms they would now be paying closer attention to nitrogen applications and soil and weed management. One of the two new weedy hosts of the virus (Green Foxtail) is a grass and represents the first time a grass has been confirmed as a host for IYSV. This is a very important discovery that has come directly out of this research. Recommendations generated as a result of this work will be invaluable in helping Western onion growers become more economically and environmentally sustainable.

Collaborators: