Why Organic?

Food & Health

Organic & Kids

Find Organic Foods

CCOF Fact Sheets/Flyers

Resources

Organic Strawberry Production

Growing Strawberries in California
History of the Study
The Challenge
Findings
Additional Resources

Growing Strawberries in California

California is the largest strawberry producer in the United States, accounting for over 80% of the nation’s fresh strawberries on 50% of the total acreage in production. This discrepancy is attributable to California’s ability to grow four times the amount per acre as other states in the US. To achieve these yields and to control the pests that plague strawberries, the majority of California growers have become increasingly dependent on chemical inputs. Organic strawberry production, which excludes the use of toxic synthetic fertilizers and pesticides, and requires soil building and biological pest control, accounts for 4% of the state’s total production. Consumer demand is rapidly out growing the supply, and there is the potential for farmers to make large profits. But what is holding back farmers from meeting this growing demand? A high level of pest pressure and a lack of research; “Conventional farmers have decades worth of research to draw on, while organic growers have very little scientific data to rely on,” stated Carol Shennan previous director of the UCSC CASFS, but there are a number of researchers hoping to change this.

History of the Study
A group of researchers at the Center for Agroecology and Sustainable Food Systems, are currently trying to provide organic strawberry farmers with the knowledge they need. Organic strawberry producers are struggling to address the challenges that have been mitigated by harmful chemicals and petroleum based fertilizers in conventional strawberry production. Three of the most pressing challenges facing organic strawberry growers are soil fertility, soil borne diseases, and pests.

From left to right Sean Swezey, Steve Gliessman, Joji Muramoto, Carol Shennan and Jim Cochran

From Left: Sean Swezey, Steve Gliessman, Joji Muramoto, Carol Shennan, and Jim Cochran

The team is comprised of Joji Muramoto, Steve Gliessman, Steve Koike, Carol Shennan, Sean Swezey, Carolee Bull and Karen Klonsky, and an organic grower, Dan Schmida (Sandpiper Farms), and the land owner, Robert Stephens, completed a study 5 year study with the cooperation of Elkhorn Ranch. They researched fertility management to optimize fertility input use and soil borne disease management without use of chemical fumigants.

The Challenge

Maintaining high levels of soil nitrogen fertility throughout the extended growing seasons of strawberry production is extremely difficult. Strawberry is a nitrogen sensitive crop and yield and quality of the fruit is strongly affected by the levels of nitrogen in the soil. Soil nitrogen fertility is maintained and enhanced by the application of nutrients to the soil. In conventional production this requires large amounts of synthetic fertilizers. In organic production it requires large amounts of compost and cover, and some growers supplement these inputs with commercial forms of organic fertilizers. The application of these organic fertilizers often precedes the rainy season in Central California resulting in large amount of nitrogen run off, leaching into the water table, and polluting streams and groundwater.

Strawberry field at Elkhorn Slough

Organic Strawberry Field at Elkhorn Ranch

The study sought a way to reduce the negative environmental impact of excessively applied pre-plant nitrogen in organic strawberry production and at the same time maintaining the fruit yield, and saving the growers’ money. Their study showed that to limit nitrogen loss during the rainy season to a given target level, the maximum amount of nitrogen from compost and pelleted organic fertilizer that can be applied may be determined from residual inorganic nitrogen content in the soil.

Another challenge to commercial strawberry production is soil born diseases, particularly, Verticillium dahliae. V. dahliae is a plant pathogen that causes strawberry wilt resulting wilt in the leaves and eventual death. In conventional production strawberry wilt has been controlled using methyl bromide an acutely toxic soil fumigant. Methyl bromide is also used to control weeds, other soil-borne diseases, nematodes, and soil-dwelling insects. In an organic system where the use of methyl bromide and chemical fumigants is prohibited, organic strawberry producers are forced to rotate the strawberries off the land after 3 to 5 years of production and either plant alternative crops or leave the land fallow. This alternative crop or fallow period often causes an economic loss and inhibits many growers ability to remain in production.

Findings

The team used integrated ecological practices to control the threat of soil borne diseases and in particular, Verticillium dahliae. They applied broccoli and mustard residues, two crops that could suppress V. dahliae population in soil. Interestingly numbers of V. dahliae present in test soils decreased after application of broccoli residues regardless of the treatments but no such consistent reduction was observed after the application of mustard residues. Some weed species found within the strawberry fieldsalso host V. dahliae.. Their presence stressed the importance of weed management integration into soil-borne disease management.

The team showed the benefits of diverse strawberry/vegetable rotations and integrated ecological practices on the agroecological systems health. The integrated ecological practices included; compost application, biofumigation using mustard cover crop and broccoli residues, use of plastic mulch, systematic application of nutrients, crop rotations with vegetables that do not host Verticillium dahliae and use of relatively resistant strawberry varieties. The impact of these practices of the field on both overall agroecosystem health and soil health was monitored over 5 years using crop yields, disease incidence, the loss of nitrogen during the rainy season, an economic analysis, soil microbial diversity, available phosphorous and other nutrients.

To find out more about the researchers findings check out their EcoFarm Workshop.

Additional Resources

The team is hoping to dispense their findings to the farmers that can really use it and have set up a series of workshops in September 2008. Combining new data from other projects, the workshop will provide up-to-date information on fertility, pest, weed, and disease management as well as economics in organic strawberry production on the Coastal Central California. Contact Mark Bolda at UCCE, Watsonville, for more information on the workshop at mpbolda@ucdavis.edu.

Keep up to date with the group's latest findings at the Center for Agroecology and Sustainable Food Systems' home page.

Learn more about organic strawberry production at the National Sustainable Agriculture Information Service's website and integrated pest management on organic strawberries at the UC IPM Online website.