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Alternatives for fumigation tested

The first time crops and orchards are planted in virgin soil, they grow beautifully. But the plants themselves change the biology of the soil, setting up subsequent plantings for severe stress. In the past, the problem has been solved by moving to new farmland, rotating crops, fallowing, and heating soil under plastic tarps. The most effective treatment is soil fumigation.

Key Points

• The DPR is proposing to approve methyl iodide, but with several restrictions.

• Injecting superheated steam has potential, but is expensive and slow.

• With walnuts: kill old trees, leave field fallow a year and plant resistant rootstock.


Now the Department of Pesticide Regulation is proposing to approve methyl iodide. However, it would impose several restrictions not required in 46 other states that have already approved its use. Even so, methyl bromide is considered so dangerous that chemists use only small amounts in their labs. Thus, University of California scientists are working to find alternatives:

Superheated steam

Injecting superheated steam may replace chemical fumigants, says UC Cooperative Extension specialist Steve Fennimore. Working with a company in Kingsburg, Calif., Fennimore imported an Italian machine that is used for greenhouse basil plantings. The machine, manufactured by Ferrari Constructione, has a 100-by-74-inch platform fitted with 99 10-inch spikes that inject steam into the ground. “Within 2 minutes, it will take 60-degree soil in the surface eight inches and heat it to 200 degrees,” Fennimore said.

However treating the field is expensive. The machine must crawl 8 feet at a time up and down the field, pushing the spikes into the ground and leaving them there for 6 minutes. With one steam machine, it takes about 30 hours per acre.

Fennimore calculated that the steam machine labor and fuel costs are $3,848 per acre. Chemical fumigation costs $2,700 to $3,000 per acre. Applying steam to raised beds rather than the entire field could cut steam expenses to about $3,000 per acre.

Air quality is also a concern; the Italian machine runs on diesel fuel. “We’re trying to develop a practical system, and we’re interested in switching to propane,” says Fennimore.

Depth is another issue. The 10-inch depth of the steam injectors wouldn’t be sufficient for trees whose root zones reach 4 to 6 feet. UC Riverside nematologist Mike McKenry is studying steam application not on the whole field, but in holes that give steam access to the full root zone. He is using a steam generator powered by propane, with an injector on a long hose.

“We will pull the steam generator on a tractor; there’s an auger on the front of it. We will dig down 4 feet and we will treat with steam prior to the planting of the tree, McKenry says.

Starve and switch

McKenry is also studying an alternative he has named “starve and switch.” While orchards grow, the chemicals excreted by tree roots alter the flora and fauna in the soil. The resulting soil environment hinders the growth of new trees when the orchard is replaced, a process McKenry calls “the rejection component.” The rejection component, McKenry says, inhibits the tree growth for at least a year after replanting the tree, but the tree grows as if planted in fumigated soil. But the farmer must wait longer for an investment return.

“We [can] kill the old roots with a systemic herbicide such as Roundup and fallow the field for a year, so the microbes in the soil starve,” McKenry explains. “Then we switch to a rootstock with tolerance to the rejection component.”

New rootstocks

Replanting walnut orchards without fumigation is difficult since there are no nematode-resistant rootstocks. But there is hope. One tree from China — Juglans cathayensis No. 21 — has walnut breeders excited. It has natural resistance to root-knot and root-lesion nematodes. For the first time, a rootstock may be available in walnuts to try “starve and switch,” in which trees are killed with Garlon herbicide, the field left fallow for a year and then a tree on resistant rootstock replanted.

McKenry has tried grafting an English walnut to the rootstock, and it took the graft; however, at the graft union, there is a lack of compatibility, resulting in a weak connection that could break when trees are shaken for harvest. “This rootstock will have to go back to [UC] Davis and be crossed with English walnut to bring improved graft affinity to the graft union,” McKenry says.

Warnert writes for the University of California Department of Agriculture and Natural Resources.

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DIFFERENCE: The healthy-looking strawberry plant was grown in soil treated with methyl bromide alternatives. The other is from untreated and unfumigated soil, and is infested with verticillium wilt.

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FUMIGATION ALTERNATIVES: Tim Brown (left) of Pajaro Valley Greenhouses Inc., Watsonville, Calif., and ARS agricultural engineer Tom Trout discuss methyl bromide alternatives in a Monterey County field.

This article published in the August, 2010 edition of CALIFORNIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.