Acres U.S.A., page 17
January, 2000

Soil Solarization, Lasers, Mulch, Irrigation & Carbon Dioxide all Offer Alternatives to Methyl Bromide

The U.S. Congress this year postponed for four years a ban on the widely used fumigant, methyl bromide, a chemical implicated in the depletion of the Earthâs ozone layer. The congressional move allies U.S. law with the Montreal Protocol, which calls for methyl bromide use to be reduced in increments, beginning with 25 percent less in 1999, 50 percent less in 2001, 75 percent less in 2003, and then eliminated in 2005.

When the ban takes effect, methyl bromide will be replaced with a variety of alternatives.

The Alternatives

Alternative practices are being developed and investigated by an army of University of California scientists. The scientists have come up with a selection of safe alternatives to offer farmers.

Solarization

Soil solarization worked as well as methyl bromide in the first San Joaquin Valley field trials when the two treatments were compared in strawberries. It was found that both methyl bromide and soil solarization increased yields more than 30 percent over untreated plots.

Because solarization is much cheaper than methyl bromide treatment, farmers stand to profit $800 to $1,000 more per acre by tapping the sunâs energy instead of fumigating with the soon-to-be-banned chemical.

Farmers can treat the soil in the heat of the summer, just in time for the traditional fall planting, and enjoy what appears to be excellent control of weed seeds and soil-borne diseases.

Soil solarization is one of the most promising new techniques for killing weed seeds, nematodes and fungal pathogens in warm, inland valleys or desert areas. To solarize soil, farmers cover the ground with clear plastic and allow the sunâs energy to raise soil temperature to lethal levels. However, since there can be great differences in heat sensitivity among these organisms, scientists are conducting experiments to identify ãthermal deathä calculations for different species. ãWeâre developing guidelines that growers can use to predict when theyâve cooked the soil long enough,ä said Jim Stapleton, UC integrated pest management plant pathologist. Stapleton, IPM weed ecologist Tim Prather and hematologist Mike McKenry first tested the technique in large tanks under laboratory conditions and again under commercial conditions in the field. Their goal is to have an Internet-accessible database for growers, telling them how to kill off specific species of nematodes, fungal pathogens and weeds.

Soon, the nursery industry will have a new method certified by the California Department of Food and Agriculture for preparing soil. Scientists have developed a system for solarizing soil in nursery pots.

The pots are filled with planting mix, placed on a pallet and covered with a double layer of clear plastic held up like a tent by a frame or wire hoops. ãThe soil gets much hotter than in an open field,ä Stapleton said. ãIn the San Joaquin Valley, the soil temperature can go higher than 160 degrees.ä At that temperature, after just 30 minutes, the treatment is complete.

Certification, which is expected in March 1999, will allow growers to use the method in the production of nursery plants, which by law must be guaranteed not to harbor economically important nematodes.

Lasers & Radio Waves

Researchers have also developed two types of patented technology that are on the verge of commercial development. The first uses ultraviolet laser technology to zap food ÷ spoiling microorganisms. ãThe lasers disrupt the genetic material inside certain microbes, destroying their ability to grow, reproduce and cause food to rot,ä said research chemist Manuel Lagunas-Solar. A working prototype should be in place in a packing-house by the fall of 2000.

The second approach uses electro-magnetic energy in the form of radio waves to elevate soil temperatures just enough to kill nematodes, fungi and other pathogenic organisms. ãA relatively small increase in temperature does the job,ä Lagunas-Solar said. This technique could be field tested within the next few years with the integration of existing technology.

Mulch

Composted green waste, shredded pine bark and broken pecan shells were found to be just as effective at suppressing weeds in nursery container soil as pre-emergent herbicides in a recent two-year study.

The drawback with mulch seems to be in the application. Currently there isnât a good way to apply mulches mechanically for commercial production. The study also suggested that buried drip irrigation in 5-gallon containers provided better weed control than top irrigation.

Alternate Irrigation Practices

Oak root fungus is a huge problem in many California tree and vine crops. When it gets bad enough, farmers typically rip out the crop, fumigate the soil with methyl bromide and start over. Until recently pear growers believed they were immune to the problem. However, when farmers switched from flood to sprinkler irrigation 15 to 25 years ago in efforts to increase fruit size, the disease became more common in pear orchards. This lead researchers to investigate whether oak root fungus can be managed by modifying irrigation practices. ãWeâre trying to find a middle ground,ä said plant pathologist David Rizzo, who, with water relations scientist Ken Shackel, have set up an experimental pear orchard.

One device under study shoots out air at a high speed to blow dirt away from tree roots, allowing scientists to see the roots ÷ and the fungus on infected trees ÷ underground. Exposing the roots may alter the growing conditions enough to kill off the fungus.

Carbon Dioxide

Researcher are looking at several materials that could be substituted for methyl bromideâs use by packing houses to fumigate harvested fruits and vegetables exported to foreign countries and, sometimes, other states.

Combinations of carbon dioxide at cold-storage temperatures on pests found in table grapes ÷ western flower thrips, Pacific spider mites, omnivorous leafroller, and grape mealybug have been tested. The technique is showing promise and could eventually open international markets now closed to U.S. exports. The tests will continue on lettuce, asparagus and broccoli.

For more information, contact John Stumbos at the University of California ÷ Davis, (530) 754-9554, e-mail jdstumbos@ucdavis.edu.