FRESNO, Calif. — For decades, California strawberry growers injected the potent pesticide methyl bromide into soil to kill bugs, weeds and plant diseases. But the chemical was slated to be phased out by international treaty, 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 conventional berry growers use to sanitize their fields. The restrictions are pushing California’s $2.3 billion strawberry industry to develop nonchemical alternatives to pesticides.

The industry and state have poured millions of dollars into research, but alternatives such as sterilizing soil with steam or growing berries in peat are not ready yet.

California supplies nearly 90 percent of the nation’s strawberries.

“We’re so limited in what we can do and the restrictions that are out there, it’s getting tighter and tighter,” said Rod Koda, who grows strawberries on 28 acres in Watsonville. “Some of the alternatives don’t show uniform results — a win-win one year and next year dead plants all over your field.”

Since the 1960s, California strawberry growers have fumigated their fields to control devastating soil-borne pests, increase yields and produce uniform and disease-free fruit. Fumigants are among the most dangerous pesticides, since their gaseous state enables them to drift from under the plastic tarps where they’re applied, said Sara Knight of Pesticide Action Network, which is asking regulators to end fumigation by 2020.