Plant mold feature_image1
Professor Wolfgang Schweigkofler holds up a culture of Phytophthora tentaculata at Dominican University in Marin County. Experts there have spent years studying ways to fight Sudden Oak Death and are now turning their attention to P. tentaculata. (Daniel Potter/KQED)


Twenty years ago, scores of trees began visibly dying off around the Bay Area, in what turned out to be the advent of Sudden Oak Death. The cause was a microscopic parasite, Phytophthora ramorum.

Phytophthora comes from Greek and means “plant destroyer.” (It’s pronounced fie-TOF-thur-uh.) Of its many relatives, perhaps the best known is Phytophthora infestans, noted for causing the Irish Potato Famine. Though sometimes classified among fungi, they’re actually part of a distinct group known as “water molds.”

An ominous federal report five years ago warned of another Phytophthora species that had not arrived yet in North America. If it were to appear, the report said it “would likely cause severe economic impacts to the nursery trade, as well as environmental impacts on native species.”

Then in the fall of 2012, it showed up at a nursery in Monterey County. “We were like, what the heck is this?” says state plant pathologist Suzanne Latham.

She identified it through DNA testing as Phytophthora tentaculata. All the plants in the nursery were destroyed, Latham says, “and we thought we had an isolated detection.”

Then, about a year ago, P. tentaculata showed up again, this time outside the confines of a nursery.

An Insidious, Microscopic Hitchhiker

In a remote part of Alameda County, the San Francisco Public Utilities Commission is restoring native species across a vast, grassy terrain. It’s a mitigation project for several water systems the PUC is overhauling. At roughly an area of one hundred football fields, the massive project involved hauling in many thousands of plants that are native to California, at a cost of millions of dollars.

The plants included a shrub called toyon and a subshrub called sticky monkey flower. Both turned out to be hosts of P. tentaculata, which, unbeknownst to workers, was quietly hitchhiking into the site.

As a soil-born pathogen, tentaculata attacks and rots plant roots. Infected plants look “water-stressed,” meaning the parasite can masquerade as effects of the drought. It can spread by drifting in water, or with help from people: in contaminated potting soil, perhaps, or in dirt on workers’ boots or tools, or in the treads of truck tires.

That invasive species have the potential to sabotage restoration efforts was not news to the PUC. Greg Lyman, a habitat mitigation engineer, says the agency had taken pains to keep pests and pathogens out of the site, with a zero-tolerance approach to potential contamination.

Workers who rolled up with dirty equipment had to turn around and go power-wash it before they were allowed onsite.

A shrub infected with Phytophthora cactorum, which rots plant roots, leading to symptoms like yellowing leaves, stunting and necrosis. (Ted Swiecki/Phytosphere Research)
A shrub infected with Phytophthora cactorum, which rots plant roots, leading to symptoms like yellowing leaves, stunting and necrosis. (Ted Swiecki/Phytosphere Research)

The zero-tolerance policy meant inspections for nurseries, and throwing out batches of seeds that had even a fraction of a percent of extraneous weeds. To keep pests from creeping into the site, a contractor sterilized the logs used in landscaping inside a huge metal oven, heated with propane to more than 180 degrees. Baking a single batch of about five logs, Lyman says, typically required a full 24 hours.

Despite these many precautions, tentaculata and several other varieties of Phytophthora have now turned up at the site. How many kinds of plants these pathogens might eat and how much damage they might do is uncertain, but for Lyman, the nightmare scenario would be this: “We’ve introduced a pathogen into the watershed that could decimate a whole ecosystem.”

Before It Can Run Wild

Invasive species, once they’ve found a toehold in a new environment, can be difficult – if not impossible – to fully eradicate. The best time to try, experts say, is before they’ve had a chance to get established.

There were more than eight thousand outplantings of sticky monkey flower and toyon at the site in Alameda County. While not all of them were necessarily infected with tentaculata, every one of them had to go. Workers lopped each one off at ground level, with the roots still buried.

“Ripping them out would actually increase the risk of spread,” Lyman explains, “because as you pull them out you would leave some roots behind or you would accidentally spray roots and pathogens outside of controlled areas.”

Instead, the hope is to kill the mold where it lays, using a process called “solarization.” This entails putting sheets of plastic over the ground to trap the sun’s heat in the soil, warming it to 120 degrees Fahrenheit or higher, several inches down into the ground.

Photomicrograph of Phytophthora tentaculata, showing the filaments that grow through plant tissues, secrete toxins and enzymes and absorb nutrients. (Ted Swiecki/Phytosphere Research)
P. tentaculata filaments grow through plant tissues, secrete toxins and enzymes and absorb nutrients. (Ted Swiecki/Phytosphere Research)

Lyman says so far dealing with the infection has cost some $700,000, and while he acknowledges trying to halt the pathogen’s spread is a tall order, he insists “it’s not a lost cause. With education, and with changes in the practice, we can make a difference. We will make a difference.”

Nursery Shakeup

Tentaculata has also shown up at nurseries in Monterey, Santa Cruz, Placer and Butte counties, elevating concerns in the industry over the potential spread of tiny invaders. While Watershed Nursery in Richmond has kept it out, “everybody’s a little paranoid now,” says owner Diana Benner.

Over the last year, Benner and her co-owner, Laura Hanson, have added safeguards, like a spongy sort of doormat soaked in disinfectant, so people passing in and out of the chain-link gate won’t track in potential pathogens on the soles of their shoes.

They’ve replaced wooden potting tables with surfaces that are easier to disinfect. They now sterilize the many pots they reuse. They’re even working on a way to sterilize potting soil, rigging together a pair of metal trashcans with a smoker underneath for heat. A big part of the success or failure of these methods, Benner says, is vigilance.

“You can have all these things set up, and if your staff is not thinking about it constantly, it doesn’t matter,” she says. “So the biggest thing about it has been habits – habits, habits, habits.”

Benner also notes the nursery grows plants for restoration projects from seed, which is not believed to be a vector for passing along Phytophthora.

Plant pathologist Ted Swiecki says the danger of spreading exotic pathogens is familiar in places like Australia, where the species Phytophthora cinnamomi is widespread.

“They have huge education campaigns, they’ve been doing various types of treatments in different areas, they have quarantines, all kinds of sanitation stations, all kinds of efforts,” Swiecki says. “We don’t want to end up where they are. In a way, we’re in a version of that with Sudden Oak Death.”

But, while the microbe that causes Sudden Oak Death can infect many plants and kills only a handful of them, P. cinnamomi has “a host list of a couple thousand-plus species, which we could add to every day, because most of its hosts aren’t known. And it will kill most of those.”

With other exotic varieties of Phytophthora turning up around the Bay Area, Swiecki is urging action: he believes there’s still a chance for native plant nurseries and the restoration projects they supply to take heed, before things get worse.

“Every time we get a new Phytophthora species out there with limited information like tentaculata,” Swiecki says, “we don’t really know what it’s going to affect and how wide its host range is going to be. And when we have a combination of species out there, we really have a set of wild cards.”

  • Lola Themola

    One question i wish Potter had followed up on is the statement made by Latham: “All the plants in the nursery were destroyed, Latham says, ‘and we thought we had an isolated detection.’” Knowing a bit about tentaculata, i can say that it’s well known that controlling it is difficult. It can be brought from one site to another in a tiny bit of mud on a boot or a car tire. Surely State plant pathologists knew this. The very fact that it has been found here is proof of it’s ability to get around and to thrive. Many native plant nurseries in California are infected with it and spread it when a container plant is sold. I suspect the State was negligent in not testing virtually every relevant nursery in the state the minute they knew of its presence. The idea that we will eradicate this fungus from California is ludicrous. We will be fighting this for the foreseeable future. Methods of killing it in the field are arduous, long-term and expensive, and this fungus doesn’t just cause a bit of damage, it kills virtually every plant it comes in contact with. There will be the odd plant that withstands it, and those can be used for future germination, but all scientists understand that using such a reduced gene pool will have problems of its own.

    If only KQED would run to ground the underlying questions that appear in these kinds of articles. As media consumers, we deserve to have the full context and history of issues revealed for us.

  • James Gonsman

    When will government officials everywhere recognize that preservation of indigenous habitats is far more important than the profits that nursery owners and others garner from the importation of of exotic plants and animals and ban these activities.

  • Karen Mahoney

    The California Native Plant Society has been discussing and working together to raise awareness of good management practices. For more information/education, please watch these videos.

    If you would like to join the conversation or lend a hand with our preservation and conservation of native habitats, please consider joining us

    Video Links:
    All talks will be posted as they are available on the CNPS-SVC Youtube channel:

    Ted Swiecki’s talk (temporary URL):

    Publication: Nursery Industry Best Management Practices for Phytophthora ramorum

    Happy Gardening!

  • Heteromeles

    There’s another issue underlying this one. Where I am in San Diego, to my knowledge, there’s only one mycologist at the State Agriculture office for this county. One. When we get into potential epidemics like this, California is massively understaffed to do anything about it.

    It’s not that I think that state or federal government should be taking care of all of it. Rather, the point is that our ability to detect, inform, educate, and act is massively underfunded. We are very vulnerable to plant pathogens, including crop killers.
    This is at a time when College plant pathology departments are downsizing or disappearing all across the country, because it’s not as profitable as biomedical research and there’s not a demand for students or research in the field. While as a long-time CNPS’er, I hate to see the damage caused by invading Phytophthora to native plants, this is about a lot more than natives.

  • Joswald94123

    I would like to get a message to the researchers, botanists, etc. that were mentioned in this story. There may be another disinfection approach that may work better than what they have been using so far. It is a material that was discovered at LSU several years ago but has never been commercialized. Fortunately, it is quite easy to make by simply mixing the correct ratio of standard chlorine bleach and hydrogen peroxide.I would like to get this info to all of them since this material that I have dubber “OxBow? is the most effective way to wipe out mold.


Daniel Potter

Daniel Potter is a reporter for KQED Science. Before that, he worked at Nashville Public Radio for six years. He’s gathered tape for The New York Times, contributed to a growing list of podcasts, and done national features for NPR on everything from bats to meningitis. He tweets at @hellodanpo.

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