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 midway through our evaluations of compost. We have characterized the soils based on microbial and physiochemical properties, presented in an attached handout, and continue to evaluate the effect of compost on field soil.  In fall 2013, we established six field trials in three distinct geographic regions evaluating compost and in some cases cultivars. We have concurrent greenhouse and growth chamber trials evaluating root health and disease suppression. Surveys are currently underway and results will be available in the next report. The expansion of the Western SARE-funded project has been due to a large grant awarded from the National Strawberry Sustainability Initiative. 

Outlined below is the 2013 progress report in response to each objective outlined in the initial proposal. 

Objectives/Performance Targets

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

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

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

Our previous approach was not providing meaningful results; therefore, we altered our methodology.  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 are grown in compost-amended soil infested with V. dahliae, then removed and the roots are cleaned and plated on semi-selective medium. Individual infection points can be identified by growth of the fungus from the root. The number of infections per unit root length will provide a sensitive measure of how effectively compost suppresses the activity of V. dahliae in soil. Berry growers have found the RaBeT system unrealistic; therefore, we eliminated it from our evaluations.

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

Fluorescein diacetate (FDA) hydrolysis as a measure of microbial activity, the composition of microbial communities determined through DNA sequencing, and basic soil tests are the main methods that we are using to understand the influence of each compost on the soil and how this may correlate to yield. In fall 2013, we set up six field trials evaluating the four compost treatments. We conducted FDA hydrolysis on 176 samples to measure microbial activity. We also extracted DNA from all the samples in order to conduct microbial community analysis to gain insight into the composition of bacterial communities associated with each compost. We are also including some soil testing to better understand soil nutrient availability over time. 

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

Differences between the composts are currently seen through FDA hydrolysis and soil tests, but we will not have insight to the significance of these differences before the end of the current field season and yield data collection.

4. Improve outreach and extension efficiency.

a. Use Q-methodology to generate grower-identified needs and priorities
b. Conduct a social network analysis to map pathways of knowledge transfer

Surveys are being prepared for dissemination this month. We have solicited feedback from those working in strawberries at University of California (UC) Cooperative Extension, USDA, and UC at Davis and at Santa Cruz, as well as a few in the industry. I will administer the survey during several meetings that occur in January and February, which is the slow season in strawberry production. 

Accomplishments/Milestones

We were granted an award from the National Strawberry Sustainability Initiative for research entitled, “Sustainable Strawberry Production in the Absence of Soil Fumigation,” which greatly bolstered the project funded by Western SARE. We are excited about the scale and depth of information we can now generate with the combined funds.

A second major milestone was the establishment of six field trials this fall.  In each trial, four commercial composts of different source material were added, and either one or two strawberry cultivars were planted. This was a major effort to both identify the sites and establish the trials. They will continue for one year, and we will be evaluating the effects of compost on strawberry production.

Impacts and Contributions/Outcomes

We were able to collect informative data from this first year field trial. Mean relative growth rate was collected from February to July, during which plant canopy diameter was measured to track growth by plants in the compost treatments. Canopy growth is typically correlated with yield, and we saw a benefit of compost treatments early in the season. However, no difference was seen in mean relative growth rate by July, and the slower growing plants sustained higher yield later in the season. This led us to wonder why the high yielding plants early in the season tapered off. To evaluate this, we will be looking at the microbial communities over the growing season, as well as root growth and soil nutrient availability. For this field trial, total bacterial and fungal colonies were monitored as a peek into the microbial differences between treatments. 

We had a grower field day at Driscoll’s research location in May where the trial was included on the tour.  I also attended the Russell Ranch Field Day hosted by the Agricultural Sustainability Institute at Davis where I presented a poster. And lastly, I attended the annual meeting for the American Phytopathological Society in Austin, TX where I presented a poster on the Western SARE-funded research. 

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.

• Poster presented at the Annual Phytopathological Society Meeting Austin, TX 2013
• Research summary of field trial 2012-2013

Collaborators: