By Peter Fimrite
It is the forgotten killer when compared to our increasingly frequent climate calamities, but the virulent pathogen known as sudden oak death remains active and is spreading death so fast it could destroy California’s coastal forest ecosystem, UC Berkeley scientists reported Thursday.
The deadly microbe has now established itself throughout the Bay Area and has spread along the coast from Monterey to Humboldt County, according to a study of 16,227 trees in 16 counties in Northern California.
Millions of coast live oak and tan oak trees have withered and died over the past quarter century, leaving acres of kindling for wildfires, but the outbreak this year was one of the worst. Oak trees have historically been abundant in California and southwestern Oregon, with hundreds of millions of them stretching all the way to Baja California.
The rate of trees infected almost doubled in 2019 — from 3.5% to 5.9% — and was 10 times higher in some places compared with the 2018 survey, said Matteo Garbelotto, the director of the UC Berkeley Forest Pathology and Mycology Laboratory, which tested leaf samples taken by 422 volunteers.
Infections were found in all the well-known hotbeds, like Marin and Sonoma counties, the East Bay, Big Sur and the Santa Cruz mountains. But the 12th annual survey detected more of the pathogen this year in virtually every location. That’s mainly because the disease spreads faster in the kind of wet weather that hit California last winter, Garbelotto said.
“There was a significant increase in infection rates over last year, but that’s not totally surprising because we had a lot more rainfall,” Garbelotto said. “But it was a surprise to see them all at once. It’s telling us we are entering a different phase of the disease, where the organism isn’t really establishing itself in new areas, but is showing itself more when weather conditions are favorable.”
Sudden oak death is an exotic disease that was discovered in Mill Valley in 1995. It now exists in forests and wildlands in 14 California counties and in Curry County, Ore., just across the state border.
It kills oak trees, including California’s signature tree — the live oak — and there are 107 susceptible host plants, including such common garden ornamentals as camellias and rhododendrons. Although some hosts are sickened, they do not always die from the fungus-like ailment. Instead, these plants, bushes and trees help spread the deadly spores.
In 2001, forest pathologists in Oregon discovered what was killing trees in Curry County in southwest Oregon – a devastating disease known as sudden oak death. Almost 20 years later, sudden oak death hasn’t spread beyond the county’s borders.
Although the initial goal was eradication, limiting sudden oak death’s spread proved to be a success, said Everett Hansen, a now-retired Oregon State University professor who helped spearhead the effort to contain the spread of the disease.
“From day one, we invoked legal machinery to mandate the destruction of diseased trees,” Hansen said. “Every time we found a diseased tree we cut it down as fast as we could. We were going full bore. So, we went through all these years without any published data to suggest what we were doing was working.”
In a new study published in the journal Forest Pathology, Hansen and colleagues at the Oregon Department of Forestry and U.S. Forest Service highlight the successes of the two-decade effort to manage and reduce the spread of sudden oak death in Oregon.
In 2001, federal, state and local agencies marshaled their resources to manage the outbreak. They quarantined areas where the trees had been infected and cut down and burned sick trees.
“We focused on local treatments instead of landscapes, sometimes by design but sometimes out of necessity,” said Hansen, lead author on the study. “If it was one tree and we cut it down and all the surrounding trees for a mile we might well have eradicated it. But we never had that chance. We didn’t have enough chainsaws.”
However, the researchers found that these treatments did demonstrably reduce the infestation. They concluded that eradication of sudden oak death is difficult — the pathogen that causes the disease may survive in soil for several years – but not impossible.
By Stephen Koranda
Plants sold at more than 60 stores in Kansas were infected with a disease that kills oak trees. The Kansas Department of Agriculture said sudden oak death was confirmed in hundreds of rhododendrons in the state, and the agency is asking Kansans to destroy any potentially infected plants.
The infected rhododendrons came from a nursery in Oklahoma and were sold in 10 states, including Kansas and Missouri. In Kansas, the plants were sold during April, May and June at 60 Walmart stores and the Home Depot in Pittsburg.
This is the first time sudden oak death has been found in the state. It’s a disease that has heavily damaged some forests on the west coast, where it was discovered in the mid-1990s.
“It is just devastating when it gets to oaks,” Kansas State University Professor Cheryl Boyer said in an interview. “It will kill the whole mature trees very, very quickly.”
Ryan Armbrust, with the Kansas Forest Service, noted in a statement that many of the state’s oak trees are from varieties that are less susceptible.
“But there are millions of red, black, pin, shumard, blackjack, shingle and other oaks that could be impacted should this disease gain a foothold in the state,” he said.
By Guy Kovner
Sudden oak death has killed up to 50 million trees and 100 million more may be infeceted, according to new research.
A dry winter curtailed the presence of a deadly forest pathogen this year in Sonoma County and 13 other Northern and Central California counties, but experts still expect the oak-killing disease to spread and warned landowners to be vigilant.
Since the mid-1990s, sudden oak death has killed up to 50 million trees from Big Sur to southwest Oregon and is entrenched in the woodlands, spreading rapidly after wet winters and slower during dry years.
“It’s constant, it’s emerging,” said Richard Cobb, an assistant professor of forest health at Cal Poly San Luis Obispo. “It’s probably going to get a lot worse.”
Cobb said Monday he’s about to publish his estimate of tree mortality, 90 percent of which are tanoaks and most of the rest coast live oaks. Another 100 million trees may be infected by the insidious pathogen that typically takes one or two years to produce symptoms in the infected trees, he said.
The pathogen can be spread by human footprints and nursery plants, but in nature it rides on water droplets blown from the leaves of bay laurel trees, a host species that abounds among the oak and tanoak trees susceptible to the disease.
“We know there’s a lot of disease out there,” said Matteo Garbelotto, director of the forest pathology and mycology laboratory at UC Berkeley, which has organized annual sudden oak death surveys, known as the SOD Blitz, since 2008.
This year’s survey found the estimated rate of infection — based on lab analysis of leaves collected from bay laurels and tanoaks — at 3.5 percent throughout the 14-county region, a marked decline from 12.8 percent last year.
Sonoma County, divided into three areas, also showed sharp declines, which Garbelotto said were anticipated because the 2017 survey, conducted in the wake of two straight wet winters, found the highest infection rate ever recorded in 11 years.
But the survey conducted in May found another consequence — the presence of oaks showing symptoms of infection had increased to 12.2 percent throughout the region, up from 9.4 percent last year.
Next year’s rate should be higher, Garbelotto said, coming two years after the 2017 rains and matching the time it takes for symptoms, such as bleeding cankers in oak tree bark, to appear.