Compost-Induced Disease Suppressive Soils for Control of Verticillium Wilt of Strawberry

Compost-Induced Disease Suppressive Soils for Control of Verticillium Wilt of Strawberry

Summary

We propose to investigate the use of compost for management of Verticillium wilt in the California Central Coast strawberry growing region. With four local composting facilities, we will evaluate the suppressive effects of composts in greenhouse and grower-field trials on certified organic land and conventionally managed soils. Compost and soils will be characterized by microbial and physiochemical properties to identify correlations with suppressiveness.

The industry-wide shift in strawberry production generates a tremendous need for knowledge transfer and grower support. Accordingly, as a complement to the proposed biological research, we will employ social network analysis and grower-identified needs assessment to identify pathways of knowledge transfer among strawberry growers and to better understand grower perceptions of their goals, needs, and management styles to best develop MB-alternative outreach.

We are near completion of our evaluation of compost. Field trials for the 2013-2014 growing season were completed in November 2014 and most controlled studies are near completion. We have characterized the composts based physiochemical properties, as pure material as well as nutrient release in a growers’ field over a season. We continue to work on defining the microbial community profile for each compost under field conditions and expect to complete this component in the next couple of months.  Starting in January 2014, we began and completed our grower surveys, “Questionnaire on Managing Soilborne Diseases in Strawberries,” by mail, online, and at meetings. The expansion of the Western SARE-funded project has been due to a large grant awarded by the National Strawberry Sustainability Initiative, July 2013-July 2014.

Objectives/Performance Targets

• • Evaluate locally available composts for suppression of Verticillium wilt caused by Verticillium dahliae

• • Test compost in certified organic and conventionally managed field soil in greenhouse and field trials

• • Test compost in soilless substrate, used in the “Raised Bed Trough (RaBeT)” system

Suppression of Verticillium dahliae by compost will most likely occur through biological control in the soil and in the rhizosphere. Predation, parasitism, antibiosis, and competition are all means of suppressing pathogens, which interfere with a pathogen’s success at infecting the root cortex. Thus, the impact of a soil amendment will best be measured by the extent to which it influences the frequency with which roots become infected. To address this question, strawberries were grown in compost-amended soil infested with V. dahliae, then removed and the roots cleaned and plated on semi-selective medium. Individual infection points were identified by growth of the fungus from the root. The number of infections per unit root length provides a sensitive measure of how effectively compost suppresses the activity of V. dahliae in soil. Over four trials, the methods and results have been refined.

We expanded our disease suppression assays to evaluate control of Pythium ultimum, a soil dwelling pathogen involved in the disease complex known as black root rot. To test this, we used cucumbers as indicators of effective suppression since they are highly susceptible to P. ultimum and can be grown and show symptoms quickly. Soil amended with and without each compost was spiked with P. ultimum and sown with cucumber seeds. After seven days under controlled conditions, seedlings were counted for percent germination and those that germinated were measured in length. We have conducted this trial four times and are continuing to refine our methods and results.    

Berry growers have found the RaBeT system unrealistic; therefore, we eliminated it from our evaluations.

• • Identify factors in compost, physical and/or microbial, that strongly correlate with disease suppression to enhance consistency and reproducibility of compost applications.

We are evaluating compost characteristics in three main ways: fluorescein diacetate (FDA) hydrolysis as a measure of microbial activity, composition of microbial communities determined through DNA sequencing, and basic soil fertility tests of both pure compost samples and under field conditions. In fall of 2013, we set up six field trials evaluating the four compost amendments. We conducted FDA hydrolysis on 176 samples to measure microbial activity. In 2014, we extracted DNA from soil samples amended with each of the four composts at two time points: planting and mid-season (six months later). DNA sequencing and analysis for identification of bacterial taxa are currently underway and will be completed in the next few months. Over 2013-2014, we monitored basic soil fertility at each field, with three to five sample periods over the growing season in order to understand soil nutrient availability over time of each compost under each soil type and location.

• • Develop recommendations for compost production and application to induce disease suppressive soils.

We are in the processes of analyzing all the data collected during the 2013-2014 growing season, such as yield data from each of the five field trial locations. Our research program did not include evaluations on compost production methods for disease suppression, but rather evaluations of four composts of differing material of origin for disease suppression. As such, we will be developing a summary of our findings which will include the effect of each compost on nutrient release, suppression of Pythium ultimum and Verticillium dahliae, plant productivity, plant growth and root development.

• • Improve outreach and extension efficiency.

1. 1. Use Q-methodology to generate grower-identified needs and priorities

1. 1. Conduct a social network analysis to map pathways of knowledge transfer

Starting at the University of California Cooperative Extension Annual Strawberry Production Meeting held on the central coast in January 2014, we began administering the “Questionnaire on Managing Soilborne Diseases in Strawberries.” We attended a total of three meetings in January and February, released an online version of the questionnaire on 7/18/14, and completed a large mailing on 10/7/14 with a return request. Due to limits on time and resources, we eliminated the resource-demanding Q-methodology and simplified our inquiry to the described approach. Survey analysis is currently underway.

Accomplishments/Milestones

The award we were granted from the National Strawberry Sustainability Initiative for research entitled, “Sustainable Strawberry Production in the Absence of Soil Fumigation,” greatly bolstered the project funded by Western SARE. Combined funding supported a team of four part-time research assistants, in addition to several field crew who harvested the berries for three to six months, all of whom were critical to collecting and generating large data sets. We are currently synthesizing all of the data collected during this past year and will be ready to present them in our final report.

A second major milestone was completion of the surveys. Usingthree outreach approaches – mail, online and meetings – we recovered a total of 61 surveys from California central coast growers out of a total of about 168 growers. The online version cast a wider net, yielding 216 responses from growers across California, as well as out of state and internationally.

A third milestone was the establishment of our website. This greatly helped provide a platform for information extension, especially to our grower collaborators. Yield data and results from our controlled environment studies were posted as they became available. The web site also presents general information on strawberry production in California, fumigation phase out, our research, and our funders.

A fourth milestone was completion of our five field trials! This was a large undertaking and required significant organization and manpower to collect all of the data and manage the sites.

Impacts and Contributions/Outcomes

Field trials during 2013-2014 provided significant amounts of data and observations, which will largely shape our understanding of how each of these composts affects strawberries. Differential effects by compost are present but vary based on the indicator, such as canopy size, yield, root development, disease suppression, etc.

I was active in networking and information dissemination and attended several meetings this year. I presented a poster on the Western SARE-funded research at the annual meeting of the American Phytopathological Society in Minneapolis, Minnesota. As an invited speaker, I shared this research at two meetings: (1) California EPA and Department of Pesticide Regulation (DPR), and CalRecycle brown bag lunch and (2) UCCE Sacramento Strawberry Meeting. As a guest, I attended the California Compost Symposium, March 28, 2014 at California Polytechnic State University (CalPoly), Soil Health Symposium hosted by California DPR at UC Davis June 17, 2014.  This upcoming February 3-6, I will be giving an oral presentation on much of this work at the North American Strawberry Symposium in Ventura, California.

This past August, an outbreak of Verticillum wilt prompted a series of growers to question whether compost was the source of inoculum introduction. Working closing with both the composting and grower community, I was contacted and asked to respond to the growers who were preparing their fields for the next growing season and wondering if they should apply compost. With my advisor, we worked together to develop a short brief describing the sources of Verticillium inoculum and minimal risk of compost being a major contributor, which was primarily circulated through farm advisor Mark Bolda’s active blog, my website, and personal connections. I have attached the article in this report.

Through this work, I have spent quite a bit of time in the field and prompted several key people to think more about compost and my trials and to join me in evaluating the plots. This includes individuals engaged in both conventional and organic strawberry production.

• Sacramento Strawberry Grower Meeting Handout
• Verticillium wilt and Compost Brief
• American Phytopathological Society Annual Meeting Poster
• Questionnaire on Managing Soilborne Diseases in Strawberries

Collaborators: