US (CA): Strawberry growers seek alternative to pesticides

For decades, California strawberry growers like Rod Koda injected the potent pesticide methyl bromide into soil to kill bugs, weeds and plant diseases before planting strawberries. But the chemical was slated to be phased out by international treaty because it depletes the Earth's ozone layer. And later its replacement methyl iodide was pulled off the market after numerous public protests.

Now, California regulators have proposed stricter rules to protect the public from a third fumigant that Koda and other conventional berry growers use to sanitize their fields. The restrictions are pushing California's $2.3 billion strawberry industry toward developing non-chemical alternatives to pesticides.

Methyl iodide was pulled from the US market by its Japanese manufacturer last year after criticism from environmentalists and scientists who said the chemical may cause cancer. And only a small portion of growers are still allowed to use methyl bromide before it's completely disallowed. Environmentalists and farmworkers say those rules are not strict enough for chloropicrin, an eye and lung irritant once used in chemical warfare.

Many growers are already experimenting with growing strawberries without fumigation. On part of his land, Koda mixed a carbon source such as rice bran into the soil, places a tarp over the field and saturates the beds with water to trigger growth of bacteria. The bacteria rid the soil of Verticillium, one of the most persistent berry diseases, at similar levels that fumigation does, said University of California, Santa Cruz researcher Carol Shennan. But the method — called anaerobic soil disinfestation — is more time consuming, does not yet control for other strawberry diseases, and there isn't enough rice bran for all the growers.

Another option is growing strawberries in non-soil substances, filling the beds with coconut husk fiber or even pine bark — but such soil-less media are low in nutrients and require use of fertilizers. Soil pathogens can also be killed off with heat generated by a steam machine — researchers have already built and tested the prototype. This method may, in turn, require more use of herbicides to control unwanted weeds not killed off by the steam. "People said for years that growing strawberries without fumigation couldn't be done," said Steven Fennimore, a researcher with the University of California, Davis. "But to a limited extent it can be done, the technology is there."

Source: abcnews.go.com