The days of methyl bromide use are quickly coming to an end, with stocks of the fumigant running out.

With that looming on the horizon, researchers have been searching for alternatives that will achieve similar results.

Alternate fumigants are available, and more are in development. But many growers say the magic bullet has yet to be discovered. This has some researchers turning their attention to heat.

Heating the ground to a temperature high enough to kill pathogens is nothing new, but technological developments and cleaner, cheaper fuel sources might allow heat to become a viable fumigation alternative, researchers say.

Let the sun shine in

One of the basic methods of using heat is solarization. Growers cover the ground with clear plastic and seal the edges—similar to what they do when using a fumigant. Then Mother Nature takes over as the sun heats up the soil.

Bob McSorley, a University of Florida nematology professor based in Gainesville, says this inexpensive and environmentally friendly method can heat the soil to 110 degrees Fahrenheit 3 inches deep or 100 degrees 6 inches deep where many nematodes and weed seeds reside.

The plastic should remain in place for six weeks. Florida’s crop rotations lend themselves to the technique since fields are already fallow during the summer’s peak temperatures.

Direct sunlight is required daily during that six-week period, but McSorley says a few hours of cloudy, gray skies are acceptable.

Sunshine State Carnations in Hobe Sound has participated in solarization studies with the university and has been very happy with its ability to kill winter weeds, McSorley says.

Plastic transparency, rather than thickness, has proven more important. The plastic should be clear and ideally 2 mil thick or less. Equally important is the material’s durability. It needs to hold up during the summer’s heat so it can be removed easily after treatment.

Soil type doesn’t appear to be an issue—solarization has worked on sandy soil around the state, limestone near Homestead and clay near Quincy, he says.

The method has two main downfalls.

Nematodes can live below the 3- to 6-inch depth that solarization reaches, and the method only is effective for four months compared with methyl bromide’s eight to nine months.

Getting steamed up

Steve Fennimore, a University of California Cooperative Extension specialist based in Salinas, is experimenting with steam-treating fields with a sterilizer. The machine has a boiler that heats steam to more than 300 degrees F. On the front of the machine is a 100- by 74-inch panel on which 99 10-inch spikes have been mounted on the bottom.

The spikes are pushed into the ground to inject the steam. Each area of the field is treated for about five minutes.

Sandy or light soils are the easiest to treat, but Fennimore also has had success in clay loam in Watsonville, Calif. The steam treatment has effectively killed weed seeds, pathogens, nematodes and more in just minutes, he says.

“This machine…there is something special about it,” Fennimore says. “It is surprising how fast it heats the soil. Within two minutes, it will take 60-degree soil and heat it to 200 degrees. It is like a microwave.”

The quick-acting treatment allows growers to steam in the morning and plant that afternoon when the ground has cooled.

The treatment does have some disadvantages, Fennimore says.

The costs to operate the steam machine from Lynden, Wash.,-based Oxbo International Corp., inluding labor and fuel to generate the steam and run the machine, are $4,200 per acre. Methyl bromide in California costs $2,700 to $3,000 per acre, he says.

Applying steam to raised beds rather than entire fields could cut expenses by 28 percent, and researchers are trying to reduce costs to less than $3,000 per acre, Fennimore says.

Future studies will include examining propane as a cleaner, less-expensive fuel, as well as ways to speed the injection process.

Similarly, Fennimore is working with spot treatments where growers apply steam directly to the planting site rather than to the entire field. He and his colleagues engineered a system using an auger outfitted with a hose that cost $12,700. The boiler used to heat the steam costs another $37,000. He envisions growers renting the steam generator or having a custom applicator perform the work.

Photo by Steve Fennimore, University of California
Steam treating only the planting site with a specially designed auger fitted with a hose could be more economical than treating an entire field. The auger set-up costs about $12,700, plus an additional $37,000 for the boiler. It would be suited to larger-scale growers or custom applicators.

In December, he treated about 200 holes heating the planting site to 170 to 180 degrees F in just four minutes. Fennimore says this method shows promise for orchard replanting situations.

“Most plants we plant [in holes] that are steam-treated really thrive,” he says. “Flower growers have been using steam for decades and say steam grows some pretty flowers. It is not a snake oil. It works. It is true to say steam is slow and expensive, but how expensive and how slow is relative. I think we can make it cheaper and faster.”

A combination of methods

This is the second year that Erin Rosskopf, microbiologist and weed scientist with the U.S. Horticultural Research Laboratory in Fort Pierce, is testing propane-powered steam treatments. The steam is just part of an integrated approach that she and her colleagues are using.

First, solarization is used to kill nematodes and weed seeds in the upper layer of the soil. Then plastic perforated drain tiles buried 12 inches deep deliver steam to the lower soil layers.

She is testing this method with cut flower producers, who need the deeper controls and who have 1 to 20 acres near urban areas where buffer zones preclude fumigation.

An engineering group at the lab has been refining the boiler and steam process, and Rosskopf says a more portable and efficient boiler system using propane has been designed.

She estimates the initial investment for this treatment is less than $1,000, which includes the cost to bury the tiles that were trenched in. The tiles can be used for several years.

In addition to the steam research, Rosskopf and her colleagues—Nancy Burelle and David Butler—are working on anaerobic soil disinfestation.

This involves forming a typical raised planting bed into which composted chicken broiler litter and a carbon source have been incorporated.

In her studies, Rosskopf has used waste molasses from sugarcane production. Then the raised beds are tarped for three weeks with plastic similar to that used in solarization.

“We put water into the bed to create an anaerobic condition,” Rosskopf says. “What that does is facilitate changes in the microbial soil community. ...Combined, this eliminates pathogens, kills many grass weed seeds and has a negative impact on nematode populations.

“We have been doing this in a pepper/eggplant double crop. I am hesitant to say it provides broad-spectrum weed control, particularly control of nutsedge, but the weed control for grasses is excellent.”

Yields in the small-plot work have been equivalent to those achieved in plots treated with methyl bromide. In fact, she says it appears to provide better yield increases over time.

The deciding factor

Convincing growers to use heat rather than chemicals will be the largest obstacle to overcome, McSorley says. Ted Campbell, executive director of the Florida Strawberry Growers Association, Plant City, says he thinks the concept is a bit of a stretch.

“We already cover our fields with plastic, and we have horrible nematode problems,” he says. “The sting nematode is nasty, and we haven’t even found another fumigant to take it out. What it requires is more chemicals and more applications to do a less effective job, and that isn’t a good answer.”

Campbell says the association and growers will be interested in new research, though, because they are anxious to solve this problem.

“It is life or death to the industry,” he says.

Contact Citrus + Vegetable Magazine at vlboyd@att.net or (209) 571-0414.