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

Project Overview

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

Annual Reports

Information Products


  • Fruits: grapes


  • Crop Production: cover crops, foliar feeding, organic fertilizers, application rate management
  • Education and Training: demonstration, display, extension, farmer to farmer, mentoring, networking, on-farm/ranch research, technical assistance
  • Farm Business Management: new enterprise development, marketing management, risk management, value added
  • Natural Resources/Environment: biodiversity
  • Pest Management: biological control, biorational pesticides, botanical pesticides, chemical control, cultural control, economic threshold, field monitoring/scouting, flame, integrated pest management, mulches - living, physical control, prevention, sanitation, weather monitoring
  • Soil Management: organic matter, soil quality/health
  • Sustainable Communities: new business opportunities, partnerships, public participation, urban/rural integration, sustainability measures


    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.

    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.