Tests under way in strawberries to replace methyl bromide According to USDA researchers, combinations of fumigants applied to strawberry fields through drip irrigation come close to performance of conventionally shanked and tarped methyl bromide fumigations.

Husein A. Ajwa, soil scientist at USDA's Water Management Research Laboratory at Fresno, Calif., says the chemicals have potential for preplant control of diseases, nematodes, and weeds.

They include chloropicrin, methylisothiocyanate, 1,3-dichloropropene (Telone), and metham sodium (Vapam), plus a pair of experimentals: propargyl bromide and methyl iodide. All are injected in drip systems as emulsified formulations.

Trials are under way to evaluate the amounts of water applied, combinations of compounds, rates, type and depth of drip tapes and methods. A typical installation is a pair of drip lines beneath raised beds, as used in commercial strawberry production.

Methyl bromide, long used as a fumigant for many crops, has been designated an ozone-depleter by international agreement and will be phased out by 2005.

Under the Montreal Protocol, which the U.S. and 38 other nations signed, only half the amount of the fumigant produced in 1991 will be available for the 2001 season.

Propose notifications Meanwhile, aside from the international phase-out, California's Department of Pesticide Regulation has proposed regulations requiring residential and other notifications and buffer zones where methyl bromide is to be applied. Growers complain these will cost them acreage and increase time required for fumigation.

USDA sources estimate lack of control of diseases and weeds would cause losses in production of as much as 50 percent in several crops, especially strawberries and tree and vine crops.

These developments have put USDA, University of California, and chemical company scientists to work under some urgency to find alternatives.

During the past three years, Ajwa and his colleagues at the Fresno lab have been studying methods of how to apply alternative compounds via drip irrigation on strawberries, which consume half the methyl bromide used for preplant fumigation, and stone fruit.

"The good thing about drip irrigation is it allows simultaneous, as well as sequential, addition of as many chemicals as you like," said Ajwa.

Ajwa is no stranger to drip irrigation, having grown up with it in his native Jordan before immigrating to the U.S. and earning a doctorate in soil science at Iowa State University.

Although the climates are quite similar, he said 30 percent of the vegetables, grapes and citrus in the Jordan Valley is under drip, compared to about 10 percent in the San Joaquin Valley.

Between 1996 and 1998 Ajwa investigated various drip systems and alternative compounds compared to untreated check plots of strawberries at Salinas and Watsonville.

Based on those findings, he reported that Telone C35 EC (a combination of 1,3-dichloropropene and chloropicrin) showed promise in controlling soil pathogens and weeds while producing strawberry yields nearly comparable to shanked treatments of methyl bromide and chloropicrin.

The drip method, Ajwa said, "can reduce costs since separate application equipment is not required. It is expected to be safer than present methods of shank injection since laborers are not required to be in the field during application."

The method depends, he added, on good irrigation systems and dependable injection equipment. He noted that selection of a specific drip tape depends on the soil type and its infiltration rate and the crop's water requirement.

Accurate calibration of injection equipment is essential. Since fumigant application through drip systems is done over a relatively long period of two to 10 hours, depending on the application rate and irrigation system, small differences in the injection rate make a large difference in the total amount of fumigant applied.

Ajwa is searching out the parameters for maximum efficacy, along with ways to reduce human exposure and environmental risks. Trials continue in Salinas and Watsonville, with plots at the USDA Agricultural Sciences Center at Parlier.

The Parlier plots have eight treatments for each compound and are fitted with detectors to measure how the gases from the compounds are distributed in the soil. Soil samples will be analyzed to trace microbial activity in fumigated soil.

Despite his optimism thus far, Ajwa concedes: "We don't have a silver bullet. Alternatives to methyl bromide will have to be combinations of practices and chemicals. For instance, we know that Telone C35 does a good job on everything but weeds, so we have to add Vapam to control weeds."

Adaptable to drip Strawberry production is readily adaptable to drip irrigation, he noted. Once raised beds are formed, drip lines can be installed and tested for uniformity and leaks.

Then the beds are covered with vinyl sheets and the chemicals are injected in the system, reaching down 15 to 18 inches. Two to three weeks later new plants are set out in holes punched in the sheeting, and the drip tape is used for irrigation the rest of the season.

"Estimates I have from industry sources are that this method costs about 75 percent of the typical $800 to $1,600 per acre of conventional methyl bromide fumigation. Remember, we are treating only the beds," said Ajwa.

In perennial crops, the subsurface drip lines would be buried in the tree row, and used only once for fumigation a month or two before planting. Micro-sprinklers on the surface would be used to irrigate afterward.

Ajwa is seeking grower cooperators who will be doing fumigation soon and would be willing to set aside a small plot for comparison between the conventional and drip irrigation methods. He can be reached at 559-453-3105.