The spread of sudden death of oak trees in Marina and 15 other counties in Northern California slowed significantly during the state’s drought, the researchers said.
“In 2021, the region had the lowest infection rate,” said Matteo Garbelotto, associate professor at the University of California, Berkeley.
The disease, first discovered in Mill Valley in 1995, kills tanoak, coastal live oak, California black oak, Shreve oak and live canyon oaks, among others. More than 3 million trees have died from the disease since 2000.
Garbelotto and David Rizzo, a professor at the University of California, Davis, identified a previously unknown species of late blight as the cause of the sudden death of the oak in 2000.
Since 2008, Garbelotto has been tracking the spread of the disease by using volunteers to collect samples from the Gulf of California laurels and tanoacs in their field. Bay laurels and tanoaki are sudden hosts of oak trees that spread disease to oak trees. Oak trees are not examined because they will require cutting the bark to check them.
Garbelotto said the study surveyed more than 14,000 trees in 16 counties this year. At Marina, 3.4% of the 777 surveyed trees were found to be infected with sudden oak death, up from 9.5% in 2020. This is the lowest percentage since 2018, when 2.5% of trees surveyed tested positive.
Across the region, 3.3% of surveyed trees were found infested, up from 7.4% last year.
Garbelotto warned that even though a higher percentage of samples test negative for the sudden death of the oak pathogen, this does not mean the disease is going away.
“Once a pathogen enters an area, it probably won’t go away due to a dry year,” Garbelotto said. “The causative agent has not disappeared. He’s in an inactive stage. “
Spore production, which causes severe outbreaks of plant disease, increases dramatically in warm, humid weather. Water spray and wind are believed to play a key role in the spread of spores from tree to tree.
With the world focused on a pandemic, the sudden death of an oak tree is receiving less attention than it once was, but Garbelotto said the threat to California’s forests and woodlands from the sudden death of an oak tree is comparable to climate change.
“Only about 30% of California’s forests that could have been colonized were colonized,” he said. “We are still at the beginning of our interaction with this disease.”
Of particular concern, in a survey this year, two trees in Del Norte County tested positive for a European oak sudden death strain.
Garbelotto said that the work of his laboratory and laboratories in Europe determined that this Phytophthora ramorum strain can infect trees more easily than his North American cousin. An attempt has been made to eradicate the European strain after it was first discovered in California last year.
Garbelotto compares the European sudden death oak strain to the generation of COVID-19 varieties such as omicron. He said that another common feature of the sudden death of an oak tree with coronavirus is that it spreads easily when its owners are close to each other.
“Sudden oak killing has become so common in California because the state’s forests are very dense,” he said. Extinguishing fires prevented natural thinning of forests.
Garbelotto says now is the right time for people to remove bay laurel if they are infected with the sudden death of an oak tree and are within 30 feet of the oak tree they want to protect.
Information on which Marine trees tested positive during the survey is available at SODblitz.org. Garbelotto’s lab has also initiated a new program to help arborists test oak trees to determine if they are infected.
Garbelotto says infected oak trees pose a fire hazard and pose a threat to life and property if dropped.
He said that although the oak may appear green, “if it is infected, it is probably already very dry, and if there is a fire, the oak will be very hot.
“If he is next to a building, he will very easily set that building on fire,” he said.
Mark Brown, chief executive of the Marin Wildfire Prevention Authority, said the oak’s lower sudden mortality rate would not do much to reduce the risk of a catastrophic bushfire at the Marina.
“Infected and dead trees still lie on the ground, providing fuel for the plant fire,” Brown said. “Once the contamination levels have subsided, it will take a long time for this dead and fallen material to decompose and no longer be a fire hazard.”
Brown said that while the sudden death of the oak was originally concentrated in the county watershed, it now appears to be happening across the county.
He said, “I don’t think there is any particular access point.”
