Nowhere is the search for alternatives to methyl bromide more intense than in California's Pacific coastal valleys, from Monterey south to San Diego. For this is where about 80 percent of the strawberries grown in the United States come from. Here growers plant more than 23,000 acres of strawberries, valued at about $500 million a year.

"The progress we can report so far is not so much about a successful organic alternative as it is about the need for more information on how to work with previously fumigated soils," he says. "We found that organic growing techniques did not do well in soils that had been fumigated in successive years, cultivated, left unfumigated, and planted with the disease-susceptible strawberry varieties commonly used in California."

This scenario spells disaster for farmers regardless of how much they want to adopt nonchemical growing practices.

Sances tested alternative chemical treatments as well as promising organic mixtures of broccoli, high rates of compost, and then applied mycorrhizal bacteria to treated soil. The conventional alternatives obtained high fruit yields, he says, but the organic material could not regenerate the soil to the extent needed to suppress root pathogens and achieve commercially acceptable yields.

"Soil reconditioning takes time. Unfortunately, California strawberry growers cannot afford to take a 40 to 50 percent cut in yields until the soil can build up resistance to pathogens," Sances explains.

Repeated treatments with fumigants kill harmful, disease-causing organisms in the soil. But they also eliminate beneficial microorganisms and species complexity, and cause the soil to lose some of its ability to retain nutrients and water and resist erosion. For these reasons, Sances recommends adding additional organic matter and beneficial microorganisms to both fumigated and organic-supplemented fields.

Local strawberry growers cooperate in Sances' project to incorporate broccoli residues, mushroom compost, and a combination of both into the soil. They're also involved with testing three chemical alternativesTelone/chloropicrin, Basamid, and metam sodiumwith methyl bromide/chloropicrin (75/25) as a standard, and an unfumigated control. For these studies, 25 tons of both broccoli and organic compost were applied per acre.

"In fields treated with organic residues, weeding costs were five times more than in those chemically treated. The significant difference in weeds in these fields suggested that the organic matter probably added weed seeds to the system," Sances reports. "The cost to control weeds in unfumigated soil is a significant factor for organic strawberry growers. We have had to use black plastic mulch to limit weed growth and keep weed control costs to a manageable level. However, yields are not as high or as early as with conventional clear plastic."

As would be expected, yields were highest from the methyl bromide/chloropicrin, Telone/chloropicrin, and Basamid treatments. By season's end, soil treated with organic amendments fared only slightly better than untreated soil, but diversity of the beneficial microorganisms increased. The problem with a first-year, single-application organic strategy is that yield was already reduced and root pathogens were already present in plant tissues late in the season when numerous beneficial organisms were active. Sances explains that strawberries, as well as most annual crops, need protection early in the season when plants are developing, flowering, and ready to bear fruit. Conventional fumigants work well because they're effective immediately after application early in the season, when root protection is most important.

"We've grown three successive cover crops on soil that is being used for the 1997 season in addition to adding high rates of compost and mycorrhizal innoculants. We planted strawberries when the levels of early-season beneficial microorganisms were highest, giving organic soil treatments the same advantage as chemical alternatives," says Sances.

Elaine Ingham, Oregon State University soil ecologist, collaborated on this research. For the 1995 field studies, they used a site that for 9 years had been fumigated repeatedly with methyl bromide/chloropicrin and planted with strawberries. Since these chemicals would have left very few beneficial microorganisms in the soil, it's not unusual that even high rates of compost and broccoli mulch didn't bring yields up to par.

Then in 1996, they tried the same treatments in soil that had lain fallow for 3 years and had never been planted to strawberries. Results were more encouraging: organic amendments produced yields that were only slightly lower than the methyl bromide/chloropicrin standard.

For the next growing season, Sances is also using vigorous, disease-free strawberry plants produced in nurseries without methyl bromide/chloropicrin. Called "plugs," these plants are grown in artificial potting mix instead of fumigated soil. Plugs have been used by Florida growers quite successfully for a few years.

Since he has used plugs for just one season, Sances says data are preliminary, but results indicate that they will work well in an organic production system. Yields from nonfumigated and composted soil were comparable to those from bare-root plants grown with methyl bromide, but more data are needed.

"We've gathered some valuable information from this research, but most importantly, we now know what doesn't work as an alternative way to grow strawberries on California's central coast," Sances says. "We now know that what we end up with will be a combination of organic and chemical methods to produce the nation's strawberries in the next century."