Sudden Oak Death Plagues Humboldt’s Forests

Humboldt County is known for its beautiful forests, but sudden oak death threatens its trees

Sudden oak death is the common name for a disease that started infecting trees 20 years ago and has since killed over a million trees—including trees in Humboldt County.

The University of California Cooperative Extension explained that the disease is caused by a pathogen called Phytophthora ramorum.

“It is caused by a microscopic fungus-like organism, Phytophthora ramorum, a lethal, canker-causing pathogen of certain oaks and tan oak trees,” UCCE wrote.

Susan Marshall, a wildland soils professor at Humboldt State University, is involved in two grant programs that deal with pathogens like sudden oak death. Marshall is connected to Christopher Lee, an HSU alumni with a Ph.D in forestry from the University of Missouri, who now works as a forest pathologist at the California Department of Forestry and Fire Protection.

“It gets into the vascular tissue that is just underneath the bark, and it kills that tissue,” Lee said. “If it does that in several different places around the circumference of the tree, then it will eventually kill a band of tissue all the way around.”

If the Phytophthora ramorum does kill the tissue all around the tree, the tree is effectively girdled. Generally, live tissue transports water and nutrients up the tree, but if those pathways are blocked lower down, everywhere above the infection dies. The organism infects the tree’s circulatory system and can spread to the nutrient tissue and water-conducting tissue, xylem and phloem, essentially starving the tree and clogging it up. A full ring is a sure death sentence.

Marshall and Lee described Phytophthora ramorum as being like a fungi or brown algae, with characteristics similar to closely related plant pathogens. Specifically, they are in the class Oomycota, which are a distinct line of fungus-like eukaryotic microorganisms. They are fungus-like because they have a long, branching net-like structure like the hyphae of fungi. They are algae-like because they descend from the same phylum, Heterokontophyta, as many algae.

Phytophthora ramorum is not the only pathogen that affects trees, but it is the most visible and the most deadly, devastating thousands of acres of forest. In California, sudden oak death has been most prominent in and around Sonoma County, according to reporting by the Times Standard.

Humboldt County’s dense forests of tan oak, the main host for the disease, is at especially high risk of tree death. Humboldt’s weather and climate are an unfortunately-inviting environment for sudden oak death. The dense oak forests in the area means both greater humidity and a shorter distance for a pathogen to travel.

“As far as these diseases go, it would probably be worse under a warmer and wetter sort of scenario,” Lee said.

The recent fires around California also have an indirect connection to Phytophthoras. The loss of vegetation limits the way pathogens spread due to a wildly new arrangement. It is good to note heat from fire can sometimes help slow a pathogen’s spread by eradicating an area where the pathogen had a large presence. Lee noted that if the root system of a tree isn’t fully dead, however, Phytophthoras may have a chance of surviving in its host.

The main goals of the programs Marshall is involved with are to identify the disease more rapidly and figure out how to slow its spread.

There isn’t a specific way to control a disease like this, but Marshall said rapid testing of plants in nursery stock may catch Phytophthora ramorum before it can infect new hosts.

“Every year that [we] can buy that [sudden oak death] doesn’t leapfrog into some other county and cause quarantines and regulations on those counties is a little bit of economic damage that they’ve staved off,” Lee said.

Sudden oak death has only affected one percent of Humboldt’s trees, but its impacts in California and Oregon demand researchers like the ones Marshall and Lee are involved in will be continuing to study it and find newer and faster ways to help manage the remaining forests along the coast.