A Multi-Faceted Approach to Managing Powdery Mildew on Organic Table Grapes in Southwest Idaho

Final Report for FW08-322

Project Type: Farmer/Rancher
Funds awarded in 2008: $15,000.00
Projected End Date: 12/31/2009
Region: Western
State: Idaho
Principal Investigator:
Ariel Agenbroad
University of Idaho Extension
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Project Information


Powdery threatens organic growers’ ability to achieve high quality, marketable table grapes in southwest Idaho. Prevention is critical to the sustainability of this new industry, so University of Idaho Extension faculty and producer Michael Medes implemented and evaluated existing protocols developed by researchers in California, Washington and Oregon for effectiveness in organic Idaho vineyards. A multi-faceted approach that included trellis augmentation, early detection and changes to management strategies reduced mildew infection to undetectable levels. A comprehensive outreach effort has demonstrated results and delivered welcome recommendations to growers and horticultural professionals across the region.


Organic table grape production is a good match for southwest Idaho’s rapidly changing agricultural landscape. Population has increased over 30% since 2000. Farms are downsizing, and producers want successful alternative crops to diversify operations and generate income.

Between 1999 and 2007, Idaho vineyard acreage increased 85.8%. The climate is excellent for producing table grapes, and Idaho growers can potentially compete in regional and international markets by meeting high standards of quality and capitalizing on buyer preference for local and/or organic produce.

However, powdery mildew, Erysiphe necator (formerly Uncinula necator), threatens their ability to achieve premium, marketable fruit. Grape production is also labor intensive and can require considerable chemical inputs. As a result, conventionally grown table grapes often exhibit high levels of detectable pesticide residue.

Producer Michael Medes of Rocky Fence Vineyard grows organic table grapes on nine acres outside Emmett, Idaho and had secured local, regional and international buyers in 2007 when powdery mildew destroyed 92% of his fresh market crop.

Medes contacted University of Idaho Extension to help him research and manage this disease. Many growers in the region look to Extension to provide education about sustainable stewardship of soil, water, energy and wildlife resources and innovative, profitable crop enterprises and marketing. We applied to Western SARE for funding to assist him and the dozens of growers entering this new industry by assembling a set of sustainable grower management strategies, or “protocols,” appropriate for managing powdery mildew in southwest Idaho organic table grape production, with a focus on preventative measures.

Project Objectives:

This project will give Idaho producers and professionals the tools to better prevent and treat powdery mildew in organic table grapes through innovative trellising systems, early detection, disease forecasting and appropriate organic fungicide controls.

1. University of Idaho research and extension team, in collaboration with Michael Medes, will assemble a set of sustainable grower management strategies, or “protocols,” appropriate for southwest Idaho organic table grape production.

- Project team conducted a thorough examination of existing research and recommendations on prevention and management of powdery mildew and prescribed, implemented and evaluated an organic best practices management plan for Rocky Fence Vineyard for 2008 and 2009 seasons.

2. Project will focus on preventative measures such as grapevine trellis systems, early detection and climate-based disease forecasting and will use approved organic fungicide treatments when necessary.

- 1644 existing “T” vine trellis systems at Rocky Fence Vineyard were converted to expanded “Y” systems in early 2008.

- Over 42 individual cane, bud and/or leaf samples were collected and inspected or tested for presence of powdery mildew throughout the 2008 growing season.

- At least one dozen leaf and compost samples were collected and evaluated by Soil Foodweb Lab to determine microbial population and activity in 2009.

- Organic Materials Review Institute (OMRI)-approved wettable sulfur and lime sulfur, horticultural oil, compost tea and biologically-based fungicides were applied and evaluated for efficacy.

3. Protocols will be trialed at Rocky Fence Vineyard.

- Producer Michael Medes implemented the management plan, adapted the existing trellis system, purchased and applied all materials and documented progress with input and supervision from project coordinator and collaborators.

4. Tested protocols will establish University of Idaho Extension recommendations for powdery mildew management specific to organic table grape production in our region.

- Extension faculty team is now able to offer recommendations for proven cultural practices and organic spray regimens used in the prevention of powdery mildew.

5. Information will be distributed to producers and professionals through diverse, hands-on interactive outreach activities, reports and publications.

- Over 3,500 individual growers, horticultural professionals and others from 14 states and four territories have received information about this project since 2008 through varied outreach activities.

- Field days in summer 2008 and 2009 attracted over 60 guests to Rocky Fence Vineyard.

- Project poster presentations were displayed at nine events in ID and NM.

- Over 100 copies of a 2010 University of Idaho Extension DVD featuring the results of this project have been created and distributed to growers and professionals across the region.


Click linked name(s) to expand/collapse or show everyone's info
  • Esmaeil Fallahi
  • Tony McCammon
  • Michael Medes
  • Krishna Mohan


Materials and methods:

Work on this project began February 2008, as partners began selecting samples for evaluation for the overwintering fungus and choosing and marking formerly affected random vines to serve as sources for testing throughout the season. Cane, bud and leaf samples were tested during this time. In March, Medes applied a preventative dormant spray of lime sulfur (28% calcium polysulfide). Recommended rate is two to five gallons per 100 gallons of water, for 200 gallons dilute spray per acre. At this point Medes also began ordering and constructing the adapted trellis systems, converting his existing “T” trellises to a taller, wider “Y” style (Fig.1). “T” trellis systems have been the industry standard for over 60 years. However, new research indicates expanded “T” or alternate “Y” trellises lift and open the canopy, decrease humidity, improve ventilation and conserve time and energy, allowing quicker access to the fruit for inspection and treatment of pests and disease.

Bud break occurred around May 6, at which time no fungus was detected within the buds. Partners have since learned that in Idaho, low winter temperatures do not seem to allow the fungus to overwinter inside the buds.

Throughout the spring and summer, Ariel Agenbroad and Tony McCammon kept in close contact with Medes and with Dr. Esmaeil Fallahi and Dr. Krishna Mohan, testing weekly or bi-weekly for presence of the fungus. Medes was kept extremely busy researching and following a prescribed preventative spray schedule (Sulfur DF, 2-10 pounds per acre, applied every 7-14 days until temperatures reach 85°, then switching to a biological such as Serenade™ and following label directions), scouting the vineyard daily and finishing the trellis adaptation.

In September 2008, the project team held the first scheduled Field Day at Rocky Fence Vineyard. An introductory poster of the project and research objectives was presented, guests were served local and organic refreshments and guests toured the vineyard. Medes led the group on a comprehensive overview of the practices and techniques implemented during the 2008 season and answered dozens of questions.

In early 2009, we continued the work begun in February 2008, selecting vine samples for evaluation of overwintering fungus and choosing and marking vines to serve as sources for testing throughout the season. In mid-April, we recommended that a preventative spray of OMRI-approved dormant oil or Lime-sulfur be applied to the pruned canes. Medes worked to convert the last of the vineyard from “T” trellises to the recommended “Y” style.

Bud break occurred in early May, at which point Medes began applying preventative treatments of Sulfur DF every 14 days at the rate of six pounds/acre.

In August 2009, the project team hosted the second field day and tour at Rocky Fence Vineyard. Project collaborators shared data, captured photographs and evaluated attendees.

Some researchers and growers believe that the use of compost tea may show promise as a disease-suppressive technique. Medes used compost tea in the vineyard both in 2008 and 2009. In October 2009 and early January 2010, we sent treated leaf tissue and samples of compost to Soil Foodweb Lab in Corvallis, OR, to explore the powdery mildew-inhibiting effects, if any, present in these materials.

Throughout the duration of the project, collaborators continued to share results and early recommendations with individual growers, industry groups and professional Extension and horticulture organizations.

In summer 2010, the project team created a video project documenting results and recommendations to share with growers and horticulture professionals as a capstone piece of outreach.

Research results and discussion:

After making recommended changes to the existing trellis systems and following a prescribed preventative spray schedule, Medes began successfully harvesting and marketing clean, high quality Idaho table grapes to local, regional and international customers in August 2008. No trace of powdery mildew fungus was found in the vineyard that year.

In 2009, after following the same recommendations, no fungus was detected in the vineyard. Medes is now producing more than 6,000 pounds/acre of fresh table grapes.

Throughout both the 2008 and 2009 seasons, Ariel Agenbroad and Tony McCammon kept in close contact with Medes and sent samples to Krishna Mohan to test for presence of powdery mildew fungus. We had planned to collect local Idaho weather data to use with University of California Grape Powdery Mildew Risk Assessment Index (http://www.ipm.ucdavis.edu/calludt.cgi/GRAPEPMVIEW1 ), a modeling tool that correlates daily heat information with the onset of disease, calculates disease spread and then advises timing of treatments. However, in both years, well-timed preventative treatments and cooperative weather conditions meant that the vineyard was clean, and we were unable to detect the powdery mildew fungus visually or microscopically at any point. While this was a very good thing for Medes, it did not give us a chance to implement the disease modeling tool.

Soil Foodweb Lab soil scientists processing our leaf and compost samples found presence of bacterial and fungal life in the samples but did not think levels were high enough to effectively compete with powdery mildew fungus. Medes has since sourced more biologically-active composts and is experimenting with compost tea brewing techniques to further explore this as-yet-untested method.

Our project team is now fairly confident that powdery mildew epidemiology and organic management recommendations from University of California Davis are appropriate and applicable for Idaho vineyards. We believe being able to direct growers to existing, high quality information on table grape production that we have tested with good results, will aid us in our ability to help growers and contribute to the success of our expanding new industry.

We have also established stronger, more positive relationships between University of Idaho Extension and the table grape industry in Idaho, in particular the Snake River Table Grape Growers Association.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

In September 2008 and August 2009, we held field days/tours at Rocky Fence Vineyard, attended by over 60 participants. Medes led groups on a comprehensive overview of the practices and techniques implemented during the project and answered dozens of questions. We collected contact information from everyone attending the field days.

One hundred percent of tour participants surveyed in 2009 felt the field day was highly valuable, 100% were interested in attending future tours and 44% planned on pursuing organic certification for their operations.

Our introductory poster and handouts were presented at our two field days, two Extension organic certification workshops and the Idaho Horticulture Society meetings. This poster was also selected for display at the Sub-regional meeting of Western SARE in Spokane, WA in February 2009.

Our final results poster has been presented at the annual University of Idaho Pomology Field Day, the Snake River Table Grape Growers Association annual symposium, a Northwest Coalition for Alternatives to Pesticides 2010 farm tour and the New Mexico Organic Conference in February 2010.

Articles about this project have been published by Ag Weekly (http://www.agweekly.com/, September 2008), Bringing Home the Harvest (Rural Roots, Inc. Vol. 10, Issue 1, 2008), and University of Idaho College of Agriculture and Life Sciences News (http://www.extension.uidaho.edu/news.asp, August 2008 and 2009).
Articles in development have also been solicited by editors of Sustaining the Pacific Northwest (Washington State University) and The Farmer Exchange (Northwest Coalition for Alternatives to Pesticides).

Throughout the project we have been documenting our treatments, progress and results with photographs, interviews and video clips for reports, outreach materials and publications. In August 2010, the project team compiled our results and recommendations and created an educational video for use as our primary outreach publication. "Preventing Powdery Mildew in Table Grapes, a Multi-faceted, Organic Approach for Idaho," is a 20-minute video produced by Ariel Agenbroad and University of Idaho Video Productions for southwest Idaho commercial or hobby growers and horticulture professionals and is available by request, free of charge. We have made over 100 copies to date and are distributing the videos through our mailing list, at outreach events and through our industry partners. We hope to have the video available through our University of Idaho Extension publications catalog soon.

Education and Outreach Outcomes

Recommendations for education and outreach:

Future Recommendations

Table grapes are still a very new industry in Idaho, and we have discovered that powdery mildew is just one concern growers must manage along the path to sustainability. For Michael Medes and many of the growers we have met through this project, insect and weed management, soil fertility, storage and stable markets are all important concerns. One of the key benefits of this process has been the relationships created between University of Idaho Extension, individual growers and industry partners. We are confident that we will use collaborative approaches to addressing ongoing issues in the future.

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.