Taking MeBr Alternatives to the Field

Strawberries—with a farmgate value of $580 million in 1997—are vital to California's agriculture economy. Along the central and southern coasts, the mild climate and sandy soils help produce high, quality yields and a long production season stretching from January through November. All this bounty comes with a high price tag—$20,000 per acre. With such high stakes, California strawberry growers are, understandably, reluctant to take risks.

And unproven alternatives to methyl bromide, the soil fumigant used by most strawberry growers, are risky. The technical and economic feasibility of an alternative needs to be demonstrated under production conditions before growers are likely to adopt it.

"Methyl bromide knocks out Verticillium wilt, Phytophthora root and crown rot, black root rot, and weeds, major problems for strawberry growers," says Tom Trout. "This fumigant allows the plant to develop a healthy root system, which in turn provides a healthy defense against pests and pathogens that attack the foliage and fruit. This decreases the amounts of pesticides and miticides needed." Trout is with USDA's Agricultural Research Service in Fresno, California.

But methyl bromide will no longer be available to U.S. growers after January 1, 2001. To help growers, ARS initiated a field validation project in 1995.

"Our aim was to determine if alternatives that appeared promising in experimental plots could be successfully scaled up to commercial production levels. Our objective is to work with growers to test potential alternatives under a wide range of conditions. This will give growers an opportunity to see how they can build their farming systems around changes that would need to be made should they adopt a particular alternative or combination," Trout explains.

The field validation project is divided into two parts: strawberries and perennial crops.

Strawberries

The California Strawberry Commission, in conjunction with the University of California, proposed the methyl bromide alternatives to be validated in the field. "We agreed with their decision that only chemical alternatives would be tested because there were no nonchemical alternatives ready for large-scale field trials on strawberries," Trout says.

Chemical alternatives selected include Telone C35, or 1,3–dichloropropene (1,3–D) + chloropicrin, and straight chloropicrin. Other ongoing research plot tests that support the validation project include variety trials, crop rotations, alternative bed configurations, and mulches. These include "high barrier" plastic films, additional chemical alternatives including metam sodium (Vapam) and Basamid, and tests to determine the minimum application rates to ensure effectiveness. Soil solarization is being tried in locations where solar heating is sufficient.

"Although we don't yet have all the data gathered and analyzed, indications are that alternative chemicals being field tested are viable. We can say that there's not likely to be a single alternative for methyl bromide, but perhaps a combination of alternatives and a different combination for different crops," Trout says.

In 1997, the first year of the grower demonstrations, five growers participated; seven are involved in the 1998 program. Site locations include three in Watsonville and one each in Santa Maria, Oxnard, Irvine, and San Diego.

Results so far indicate that some of the changes growers will face include small things such as types of plastic mulch to use. This is because the proposed alternatives tested in the field demonstration did not completely control weeds. Where growers are using transparent film, they may need to switch to opaque film with a particular methyl bromide alternative. In some cases, more time will be needed for the chemical to dissipate, which means there will be a longer waiting period between treatment and planting.

Trout says that both bed and flat fumigation were tested. In California, growers usually fumigate flat soil before forming beds. Bed fumigation lowers the amount of fumigant needed.

"This fall we'll be expanding the validation project by using the strawberry field's drip irrigation system to apply Telone C35. Growers now apply Telone, like methyl bromide, with an injection rig, which means the fumigant immediately turns into a gas," says Trout. "Using a liquid formulation means that water is the carrier and the fumigant is released more slowly. The advantage here is that we lower worker risk and we also lower emission rates." The new application method is being developed by the ARS Water Management Research Laboratory, headed by Trout at Fresno. This project is collaborative with Dow AgroSciences.

For strawberries, Trout says, Telone C35 seems the most viable fumigant alternative. However, there are restrictions on the chemical in California because of emissions and worker safety concerns. Because of air quality concerns, the State limits Telone II use to 5,000 gallons per township each year and requires a 300-foot buffer zone from any occupied residence.

Perennials

"We have a core group of ARS and University of California researchers, industry representatives, and growers," Trout says. "Our consensus at the beginning of the project was that there were not yet any alternatives ready for field-scale validations." This part of the field validation project has been more difficult because there is no single grower group like the Strawberry Commission for perennials.

ARS and UC collaborators have, however, set up several large-scale research plots on UC/ARS land. They're also testing alternatives on peaches, grapes, and nuts growing on grower-owned acreage.

"We're using chemicals and cultural practices in systems approaches as alternatives to methyl bromide," Trout says. "We also have some long-term fallow plots. It's very difficult to conduct short experiments with perennials. To get a real measure of effectiveness, we need 3 to 5 years. But, we don't have 5 years. So, we've solicited the help of two UC collaborators who have several years' experience working on the perennial replant problem."

Funded in part by ARS, collaborator Becky B. Westerdahl, extension nematologist with UC-Davis, is working on soil treatments for nurseries and commercial orchards. She has field trials in progress using two new products: Enzone and DiTera. Enzone, a liquid that breaks down to release carbon disulfide in the soil, can be used pre- and postplant. It is registered for use on California citrus, grapes, prunes, plums, peaches, and almonds. DiTera, a killed microbe, is registered for use on a number of California crops. Its active ingredient is a nematicidal toxin produced by a fungus.

Westerdahl is evaluating the effectiveness of Enzone on a commercial scale relative to tarped methyl bromide and an untreated control. The trial is on a prune replant site in Sutter County, California, which has a history of ring nematode and bacterial canker.

On a prune replant site in Tehama County, she is comparing nontarped, methyl-bromide-treated areas to untreated areas left fallow for 2 years following tree removal. The site has a history of ring nematode and bacterial canker. Two prune replant trials are in progress in Glenn County to determine a management strategy for ring nematodes on prunes. In one, Westerdahl is using the following treatments: untreated control, methyl bromide by injection, 1,3–D (Telone II) by injection, Enzone by flood, metam sodium (Vapam) by flood, and ozone by injection. The other trial compares the effectiveness of fallow to methyl bromide in a replant situation.

Westerdahl is also evaluating different timings and rates of postplant applications of Enzone and DiTera to control ring nematodes on prunes. In a test of DiTera's effectiveness against lesion and ring nematode in a commercial walnut orchard, she got a small—but significant—increase in yield compared to methyl bromide.

Another California collaborator, Michael V. McKenry (UC-Riverside) has many years' experience working with perennial crop replant problems. He has recently put together a database comparing 125 different treatment strategies with methyl bromide and is adjusting potential treatments to prevailing pest conditions. Crops involved are peach, plum, nectarine, almond, walnut, and grape. McKenry is now seeking growers willing to test his recommended practices.

"Judging from what we've seen so far, alternatives to methyl bromide will likely include a package of management practices and will be more management intensive. Identifying pathogens and pests requiring treatment will be much more important," Trout theorizes. "Nothing we've tried works as well as methyl bromide, but Telone C35 is promising and is commercially available. The formulation is experimental and probably is more costly than methyl bromide, with associated costs like weed control factored in."

Last Updated: July 24, 1998