With this month's federal ban on most uses of methyl bromide, University of Florida scientists are searching for alternatives to the widely used soil fumigant that is essential for the production of fruits, vegetables and ornamentals in Florida and the nation.

The fumigant, which controls soil pests and weeds, was banned by the Environmental Protection Agency in response to the Montreal Protocol international treaty because the chemical harms the Earth's ozone layer, reducing its ability to protect the planet from radiation. Only a few emergency uses of the fumigant are still permitted.

"Finding a replacement that will be as cost-effective as methyl bromide is proving to be difficult, but we do have some promising new materials and approaches to help growers protect their crops and the environment," said Jim Gilreath, a professor of horticultural sciences at UF's Institute of Food and Agricultural Sciences. 

He said many fumigants such as Chloropicrin, Telone C-35, Inline and Vapam are being evaluated, alone and in combination. Recent tests showed that a combination of Telone C-35 and Tillam herbicide produced good results on tomatoes, but the manufacturer of Tillam went out of business, and the product is not currently registered for use.

"As we continue to look for alternatives for methyl bromide, we have found that the use of new virtually impermeable film and metalized film covers on plant beds allow growers to reduce their use of the fumigant by as much as 50 percent," he said. "When it comes to holding or keeping fumigants in the soil, these high-barrier mulch films are far superior to conventional polyethylene mulch films that have been used by growers for the past 30 years."

Gilreath, who leads the soil fumigation research program at UF's Gulf Coast Research and Education Center in Wimauma, said his research findings were quickly adopted by growers this year. Almost every tomato farm in Florida is now using metalized film on some part of its acreage along with the one-half rate of methyl bromide.

"In fact, several of the larger tomato producers shifted all of their acreage to metalized film in the Southeast - from Florida to the Delmarva peninsula in Virginia - based on results from our research," Gilreath said. "Growers have been very satisfied with the results and plan to continue using the highly retentive films."

He said the savings in the cost of methyl bromide - along with more equitable distribution of limited supplies of the fumigant and reduced impact on the ozone layer - have made this approach very successful.

Gilreath, who works in cooperation with Joe Noling, a professor of nematology at UF's Citrus Research and Education Center in Lake Alfred, and Dan Chellemi, a pathologist at the U.S. Department of Agriculture's Horticultural Research Laboratory in Fort Pierce, said they began testing different types of films about eight years ago in an effort to reduce the need for methyl bromide. 

"Two years ago, we began working with metalized films because they also reduce the movement of silverleaf whiteflies and viruses they transmit," he said. "We tested the metalized films with methyl bromide and a product called Inline, which is Telone C-35 mixed with an emulsifier so that it can be injected through drip irrigation lines in the plant beds."

In each case, application of methyl bromide and Inline in conjunction with metalized film greatly increased retention of the fumigant in the plant beds, providing effective control of soil-borne pests, especially hard-to-control weeds such as nutsedge, he said.

Effective nutsedge control was obtained with 175 pounds of methyl bromide per acre under metalized film, which was superior to that obtained with 350 pounds of fumigant per acre under conventional polyethylene film.

"While it is possible to use either metalized or virtually impermeable film to reduce methyl bromide application rates by one-half, success involves more than just laying the film over the plant bed and reducing the amount of the fumigant," Gilreath said. "Success requires close monitoring of the fumigant delivery system to make sure that the gas is applied uniformly in the bed through all three gas knives."

He said nonuniform application guarantees poor fumigant performance at any rate, and the downside results can be even more dramatic with reduced rates of methyl bromide. Before trying rate reductions, growers should modify their fumigation equipment to allow better control over uniformity of flow, which can mean the difference between success and failure.

Meanwhile, until an effective replacement for methyl bromide is found, growers are also relying on critical use exemptions granted by the United Nations Environmental Programme on a year-to-year basis. The Florida Fruit and Vegetable Association is currently seeking exemptions for producers through the 2006 and 2007 growing seasons.

By Chuck Woods