As time ticks away, steadily decreasing the number of crop seasons left before 2001, the search for alternatives to methyl bromide intensifies. Each year, Florida growers produce fresh-market tomatoes worth about $450 million on 50,000 acres, relying almost exclusively on methyl bromide to rid their soil of pests and diseases. Researchers at the University of Florida have stepped up their efforts to find pest control strategies that are as effective as methyl bromide, yet environmentally sound and economically feasible for growers.

"Controlling weeds, especially purple nutsedge, is one of the most important limitations of fumigant alternatives for Florida growers who now plant tomatoes on raised, polyethylene mulched beds of methyl bromide-treated soil," explains James P. Gilreath. A weed scientist with the University of Florida's Gulf Coast Research & Education Center in Bradenton, Gilreath and colleagues have been experimenting with the herbicide Tillam (or pebulate). Although Tillam is an old product, commercial tomato growers in Florida have little experience with it because methyl bromide gives such good weed control.

Aided by University of Florida researchers John P. Jones and Joseph W. Noling, Gilreath has been working with Tillam since 1994. Research collaborators include USDA's Agricultural Research Service, the Florida Fruit and Vegetable Association, the Florida Tomato Committee and the U.S. Environmental Protection Agency.

"Although we haven't found anything as effective as methyl bromide, we got good results when we used Tillam to control the nutsedge and chloropicrin for soilborne diseases," Gilreath reports. "We found that Tillam controls nutsedge in both mulched and nonmulched tomatoes. However, applying a herbicide can require additional equipment and more time in field and bed preparation."

Also, how the Tillam is applied is vitally important. "The herbicide must be thoroughly incorporated in the soil within minutes after it is applied. Using bedding disks does not provide adequate mixing of the herbicide with the soil and could result in crop injury," he states.

Although the Tillam label specifies double incorporation by disking at right angles, research has determined that unidirectional incorporation by disk, field cultivator, or rototiller followed by preparation of a raised bed gives comparable results. Gilreath says that thoroughly mixing Tillam into the soil will improve its effectiveness and reduce its phytotoxicity. Also, the soil should be moist and free of clods and large pieces of plant debris. A fumigant should then be applied as soon as possible after incorporating Tillam.

It is generally recommended that power-driven cultivation equipment or a tandem disk be used to apply Tillam preplant, followed by a spike-tooth harrow, with disking performed twice at right angles. "In experiments with mulched tomatoes, we got good nutsedge control by using a rototiller to incorporate Tillam, then injecting Telone C-17 (1,3-dichloropropene plus 17 percent chloropicrin). In fact, this treatment worked as well as methyl bromide applied alone in many of our experiments," Gilreath reports.

But most tomato growers don't have rototillers to incorporate Tillam into the soil. Although bed preparation equipment that includes a rototiller in the mouth of the bedder is available, the rototiller requires more horsepower and, thus, a larger tractor than that needed for a typical bedder. Gilreath says that readily available cultivation equipment typically consists of a disk and a field cultivator, or an S-tine harrow. This harrow has small sweeps and rolling cages on the rear for breaking up soil crusts and trash. Numerous field ditches for irrigation and drainage make right angle disking virtually impossible.

Given time constraints, especially between the fall and spring seasons in west central Florida, land preparation must be done quickly with a minimum amount of equipment, according to Gilreath. "Viability of any alternative fumigant would depend on an easy, or minimal, change in growers' equipment and operations," he says.

Therefore, in the spring of 1996, he and colleagues selected a test site in Bradenton that was heavily infested with purple nutsedge and incorporated Tillam into the soil with one pass of a disk or field cultivator in one direction and prepared the beds with a typical bedder. They compared this plot to one where they used a tractor-powered rototiller prior to bed preparation and added applications of Telone C-17.

In fine, sandy soil with 0.78 percent organic matter and 7.1 pH, they applied Tillam at 4 pounds of active ingredient per acre, and incorporated the herbicide 6 inches deep with a 5-ft disk, a 6-ft field cultivator and a tractor-powered rototiller immediately after application to the soil surface. Telone C-17 was applied at 35 gallons per acre.

"We found no difference in nutsedge control in plots that were tilled with a rototiller, disk or field cultivator. When we added a fumigant, nutsedge control was better initially, but by late season there was no difference in the amount of nutsedge growing in fumigated and nonfumigated plots," Gilreath says. "Plant vigor and fruit production were greater in fumigated plots, but about the same in plots where Tillam had been incorporated by rototiller, disk or field cultivator."

Trials on commercial tomato farms have demonstrated good weed control when Tillam was applied and incorporated with a S-tine harrow, followed by bed formation with bedding disks or disk hillers and a bed press. However, nutsedge was not a problem weed on these farms. Gilreath says that the acid test is under way this spring on a commercial tomato farm that has a history of nutsedge problems. "We hope results there will reflect our research findings," he says.

Growers are concerned with a number of factors related to fumigant alternatives to methyl bromide. Besides efficacy, the ease with which a new procedure can be adapted is significantly important, as is the cost of equipment replacement and any inherent changes. Different fumigants have different price tags, but all of the currently available alternatives require the use of a herbicide like Tillam. At approximately $31.25 per acre, the cost for Tillam isn't restrictive. Although proper application increases the cost factor due to equipment and manpower requirements, the cost is still nominal.

Gilreath says that the package treatment of Telone C-17 and Tillam costs about the same as methyl bromide on an equivalent rate basis, but it costs more to apply these materials, and none of the alternatives are as foolproof as methyl bromide.

Research has shown that Tillam works as well with chloropicrin and other fumigants as it does with Telone C-17. However, Telone C-17 provides more broad-spectrum pest control than the other alternatives.

"Whatever alternative we choose most likely will have to be combined with a herbicide, though few herbicides are approved for use on other vegetables. We really don't know what growers of cucumbers, watermelons, peppers, eggplants and a host of other crops will do to control nutsedge and other troublesome weeds," says Gilreath.

One thing is clear, the future of soil-borne pest control without methyl bromide will be one of a more prescriptive nature. The grower will have to know and target pests in order to select what is best for individual situations.

Says Gilreath, "We are entering the age of integrated pest management in soil fumigation. It will be a challenge, but farmers face those every day. Hopefully, research will lead the way."