Researchers Develop Organic Disease Controls for Strawberry Growers

Jane Sooby's picture

Fungi love strawberries for the same reasons humans do: they are sweet, juicy and delicious fruits. Verticillium wilt fungus is particularly fond of strawberries, and a persistent threat to California’s $2.6 billion strawberry industry. But a team of Organic Farming Research Foundation (OFRF) research partners at the University of California Santa Cruz (UCSC) have developed an effective, organic system for fighting this soil-borne threat.

The UCSC team’s findings are gaining attention even from conventional growers, as the toxic fumigant that the industry relies upon is being phased out, with few alternatives available.

Growing strawberries without chemicals is a challenge, one reason that organic strawberries are sold for a premium price. Few rotational crops are as profitable as strawberries, creating a strong disincentive for conventional growers to rotate.  Conventional monocrop systems rely on chemical fumigants to rid the soil of pathogens and weed seeds that inevitably build up when a single crop species is grown year after year. Verticillium wilt poses a continual threat to strawberry production as it has a wide range of host crops, and can survive in the soil for more than five years even when no host crops are grown.

Because of pest and disease challenges, there was a time that organic strawberry production was thought to not be possible, but organic farmers have developed strawberry growing strategies that use border insectary plantings to attract beneficial insects and multi-year rotations to reduce weed and disease pressure. On the Central Coast of California, rotating strawberries with broccoli is common due to earlier research that showed rotation with mustard family crops helped reduce Verticillium wilt pressure on strawberries.   

As part of an ongoing effort to improve organic strawberry production systems, researchers at UCSC received special fruit grant funding from OFRF in 2010 to study a management system that has been found to reduce strawberry disease levels in the soil for two years even when lettuce, a host crop for the disease, was planted after strawberries.

The system has a complicated name—anaerobic soil disinfestation (ASD)—but it is based on simple concepts.  A carbon source is added to the soil, which is then held in an anaerobic (no oxygen) state for a period of weeks by irrigating soil to saturation level and tarping the beds. The carbon (in this study, rice bran) stimulates a bloom of microorganism growth, but lack of oxygen in the soil kills off them off. When the tarps are removed, the system becomes aerobic again and beneficial microorganisms recolonize the soil, and reduce the incidence of disease for the next two seasons.

This system was originally developed simultaneously in Japan and the Netherlands. In the U.S., UCSC researchers Carol Shennan and Joji Muramoto have been instrumental in studying how it works. Research on this system is also being done in Washington state and Florida. Hundreds of Japanese farmers use ASD with the result that fumigant use in Japan has decreased. Different feedstocks tested to stimulate soil microbial growth have included ethanol, molasses, grass clippings, and composted steer manure.

Because compounds found in mustard family plants have been found to suppress disease organisms in soils, the Santa Cruz team added the use of mustard cake as a disease suppressant and fertility source into the study. This work was done in collaboration with Farm Fuel, Inc., a local company that grows and harvests mustard seed as a biodiesel crop, then packages the leftover mustard cake as a fertility and disease-suppressing input.

The UCSC research, recently presented to the OFRF Board of Directors during their annual meeting, showed that ASD reduced levels of Verticillium in the soil for two years and increased strawberry yields compared to untreated controls. An economic analysis of the different treatments showed that a rotation of broccoli—ASD + mustard cake—strawberries—lettuce had the highest net returns due to good yields of strawberry and lettuce even with the highest treatment costs.

Dr. Muramoto told OFRF directors that the $28,000 grant from OFRF that initiated the soil disinfestation study in 2009 has since leveraged another $2.8 million in funding from government grants, allowing the study to expand and continue.

For more detail on this study, please refer to the full report online.

Net Returns Above Land Costs ($ per acre).

 

Cover crop

Broccoli

Cauliflower

Treatment

Strawberry

Lettuce

Total

Broccoli – ASD

-331

2,075

0

-2,379

20,326

3,114

22,806

Cauliflower – ASD

-331

0

326

-2,379

24,637

3,509

25,762

Fallow – ASD

-331

0

0

-2,379

21,944

3,993

23,227

Broccoli – MC

-331

2,492

0

-2,543

15,675

2,818

18,111

Cauliflower -MC

-331

0

492

-2,543

20,894

3,127

21,640

Fallow – MC

-331

0

0

-2,543

12,755

2,604

12,486

Broccoli - ASD+MC

-331

2,492

0

-4,895

29,328

3,725

30,319

Cauliflower - ASD+MC

-331

0

576

-4,895

20,658

2,963

18,971

Fallow - ASD+MC

-331

0

0

-4,895

25,683

3,950

24,407

Broccoli – UTC

-331

2,742

0

0

17,494

2,156

22,716

Cauliflower - UTC

-331

0

910

0

14,893

2,781

18,253

Fallow – UTC

-331

0

0

0

12,885

2,475

15,029

                 

ASD = anaerobic soil disinfestation

MC = mustard cake

UTC = untreated control

. The media recently exposed the state of California’s pesticide regulatory program as allowing unsafe levels of toxic fumigant use for conventional strawberry production.

. It’s hard to argue against the economic clout of California’s strawberry industry: with sales of $2.6 billion per year, it is California’s 6th most valuable fruit crop. Conventional production’s reliance on deadly chemicals, however, may not pencil out in the long run.

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