Since the spring of 1994, scientists at the University of Florida have completed 14 studies on potential chemical alternatives to methyl bromide fumigation for tomato production in Florida. USDA's Agricultural Research Service and the Florida Fruit & Vegetable Association collaborated on the research venture.

The studies were done at University of Florida research facilities in Quincy, Bradenton, Gainesville and Immokolee.

"These sites were chosen to reflect regional variation in tomato production practices and were known to be infested with a number of economically important soilborne pests such as nematodes, fungi, bacteria and common weeds," says Joseph W. Noling. He is a nematologist at the Citrus Research and Education Center in Lake Alfred.

Alternative fumigant treatments evaluated included Enzone, Vapam (metam sodium), chloropicrin, Basamid (dazomet), Telone C-17 and methyl bromide in different concentrations.

"After we compared initial results obtained in 1994 from the four experimental sites, we knew that a separate, but complementary, herbicide treatment would have to be added to all of the fumigants we were evaluating if we wanted to control weeds and maintain tomato yields," Noling explains. "For all succeeding trials, we included 4 pounds of active ingredient per acre of the herbicide Tillam (Pebulate)."

According to Noling, the results of the 14 studies indicate that none of the alternatives came up to the overall performance of methyl bromide. Telone C-17, combined with Tillam, came closest to methyl bromide in maintaining yields and controlling nematodes and weeds. None of the alternative fumigants, when applied without Tillam, controlled yellow or purple nutsedge, one of the most troublesome weeds for Florida tomato growers.

"We need to be careful not to apply these results to any other crop production system where we now use methyl bromide. For example, using Tillam on Florida pepper has in some cases caused severe phytotoxicity," Noling cautions. "Additional research is needed to find a suitable alternative herbicide for crops that can't tolerate Tillam."

Since most of the studies did not reflect situations of high disease severity, Noling says, some may need to be reexamined. "Also, since all of the tested alternative chemicals require a longer period to break down in the soil, delays will occur in planting crops. Growers will need to be aware of the possibility of crop phytotoxicity and subsequent yield losses."

After only one trial, Enzone was dropped from the experiment because of poor yields, phytotoxic plant responses, low pest control and higher costs due to double-drip line delivery.

The Florida experiments also confirmed dazomet to be inferior in controlling nematodes, compared with other alternative chemicals like Telone C-17. "Under dazomet and metam sodium treatments, root gall ratings in most cases were no different from the untreated control. We applied dazomet at a 400-pound broadcast rate, which is considerably less than the maximum rate labeled for other crops. Because of its high cost and poor performance, we didn't think it prudent or economically viable to consider increasing application rates any further," Noling says. "Also, there were other treatments to explore that had higher efficacy at a lower cost." Dazomet is not currently registered for use on food crops in the United States.

Results from the 14 experiments identified Telone C-17 as the next best alternative to methyl bromide. With high pest pressures, all of the chemical alternatives tested—except Telone C-17 and chloropicrin combined with Tillam—resulted in considerably lower yields than those of the methyl bromide standard.

Telone C-17 also demonstrated excellent control of nematodes. Compared to chloropicrin, Telone C-17 (with Tillam) controlled nutsedge better and produced yields closer to those with methyl bromide, over a broad range of soilborne pest pressures.

But Telone C-17 is not without its problems. Noling comments that California cancelled its use in 1990 because of potential human health and safety concerns. Traces of Telone have been found in groundwater. However, after considerable field research, in 1994, California restored registration and use of Telone under limited acreage and strict enforcement of new application procedures.

"We recognize that Telone C-17, like methyl bromide, is not immune from environmental problems and future regulatory action which may affect its use in agriculture," Noling says. "Recent action by the U.S. Environmental Protection Agency ensures that the manufacturer of Telone will likely have to reduce maximum application rates on the label, limit geographical areas of use, and significantly increase requirements for personal protective safety equipment for field workers."

In addition to concerns like planting delays, potential phytotoxicity, cost, and possible corrosion of storage containers, there is the issue of commercial availability. DowElanco, the manufacturer of Telone, does not have the facilities to produce enough of the chemical to supply the potential demand of the agricultural community should the chemical be broadly adopted for use on an extensive scale.

"The development of any new chemical alternative to methyl bromide, especially one that will be extensively used, is likely to raise new and unexpected questions and problems that must be addressed before the alternative can be implemented," Noling comments.