Scientists say they have discovered a new way to kill California’s most virulent forest- and plant-depleting disease, and it turns out the technique is as old as steam cleaning.
Steam, in fact, is what researchers at Dominican University in San Rafael have been using to combat Phytophthora ramorum, a pathogen that causes sudden oak death.
Scientists from a variety of organizations including the U.S. Department of Agriculture, the California Department of Food and Agriculture, and Oregon State University demonstrated what appears to be an effective steam-sterilization technique Tuesday on a research plot of ornamental plants at Dominican.
“It’s certainly a new use for steam sterilization,” said Robert Leavitt, director of plant health for the Department of Food and Agriculture. “It looks like it is very promising, and it could be a big step forward as a long-term solution for ramorum blight in nurseries.”
The disease, called ramorum blight when found in nurseries and sudden oak death in the wild, has 107 susceptible host plants, including such common garden ornamentals as camellias. It was discovered in the woodlands of Mill Valley in 1995 and has spread to forests and wildlands in 14 California counties and Curry County, Ore.
The pathogen kills big oak trees and the smaller tan oaks, which have been nearly wiped out in parts of Big Sur, Jack London and China Camp state parks and the Marin Municipal Water District watershed lands near Mount Tamalpais.
For years, steam has been used to sterilize soil in greenhouses, but this is the first time it has been deployed on open fields of infected nursery plants. The technique involves piping steam through a hose underneath a tarp covering the infected plot. The researchers found that subjecting high-clay-content soil to at least 50 degrees Celsius (122 degrees Fahrenheit) for 30 minutes eradicated the deadly microbe.
The only other effective treatment for the disease that scientists know of is an expensive phosphite compound designed mainly for treating infected trees in people’s yards. The steam treatment could potentially be used to sterilize entire nurseries, pinching off the virulent killer at one of its sources, researchers said.
“It is still in the research stage, but we are working toward getting it to the commercial stage very quickly,” Leavitt said. “The concern is that ramorum will move from nurseries to homeowners and then from homeowners out into the wild, where it will spread across the country.
“Keeping it from spreading from nurseries is a giant step in protecting the whole U.S. from this disease.”