Two tanoak trees in Del Norte County tested positive for Phytophthora ramorum, a deadly plant pathogen known to cause sudden oak death, or SOD, which is a forest disease that increases oak and tanoak mortality.

Originally from Asia, the pathogen arrived to the west coast of California during the 1980s, according to campus professor of forest pathology Matteo Garbelotto. Since then, about 50 million trees have been killed on the West Coast, including in the Berkeley Hills.

According to Garbelotto, the pathogen spreads through leaf and stem infections. After signals were identified indicating the infection of the two tanoak trees in Del Norte, samples were sent to the UC Berkeley Forest Pathology and Mycology Lab.

Though the UC Berkeley Forest Pathology and Mycology Lab has verified that the two trees tested positive, Del Norte County is still waiting for a confirmation from the California Department of Food and Agriculture, or CDFA.

According to Garbelotto, detecting the pathogen early allows for its spread to be controlled, which can be achieved through removing infected trees and confining plant material to state grounds. Once the CDFA verifies the results, “the finding may affect any business that sells plant material that could potentially be infected by Phytophthora ramorum,” Garbelotto said in the email.

Garbelotto initially discovered the pathogen in 2000 with David Rizzo, professor and chair of plant pathology at UC Davis. According to Garbelotto, the pathogen arrived in the United States in the 1980s via an ornamental plant stock. He added that the infected trees  were “often asymptomatic.” 

Garbelotto also addressed the spread of the pathogen since its arrival.

“We believe that in 1986 the El Niño rains facilitated the escape of the organism from gardens into wild settings, and by 1994 it was starting to cause significant and visible mortality of tanoaks,” Garbelotto said. “By 2000 it was causing significant mortality of California coast live and California black oak.”

Through genetic fingerprinting, Garbelotto’s team was able to reconstruct the history of the pathogen, beginning with its introduction to the West Coast and spread between Big Sur and Northern Humboldt County.

The infection then moved through forests at a maximum rate of about three miles per year. It has since affected 16 California counties, according to Berkeley News.

Understanding the spread of Phytophthora ramorum has been aided by nearly 500 volunteer citizen scientists who participate in the SOD Blitz program annually, which identifies sites where trees have been infected.

“This finding was possible thanks to one of the largest Citizen Science programs in the world aimed at protecting trees,” Garbelotto said in the email. “A discovery like this made by a citizen science program shows how powerful Citizen Science can be and also shows California is a leader in this field.”