Sudden Oak Death’s Grip on California Forests

Oak trees have been in danger in California since the mid-1990’s, and researchers are now predicting a grim future for California’s emblematic tree. The killer, termed “Sudden Oak Death”, has killed an estimated 3 million trees since its introduction to California.

Quick Facts

  • Sudden Oak Death is caused by a fungus-like pathogen. It is fatal to many California-native oak species.
  • Since its introduction to California, the pathogen has killed an estimated 3 million trees.
  • Scientists have concluded that the pathogen’s spread in California is too great to completely eradicate it. Rather, management of the disease in localized areas is suggested to be our best option.


A Fatal Pathogen and Its Spread

Something weird began happening to California’s oak trees in the mid-1990’s.

Trees were plagued with a sudden onset of dying leaves and trunks that “bled” dark-colored sap, and locals concerned about the sudden tree deaths reported their observations to specialists.

The pathogen causing the symptoms was identified in 2001 as Phytophthora ramorum, a fungus-like species that infects plants. P. ramorum arrived to California by infested rhodendron plants. The fungus spread across the United States through shipments of nursery plants before it could be detected. It is spread through spores, which can be carried by rain, wind, and infected soil.

The symptoms exhibited by infected plants vary by plant species. Some plants can be infected with the pathogen and not have symptoms serious enough to be fatal, allowing the plants to continue living and spread spores from the pathogen as “carriers”. The California bay laurel is a well-known carrier, since its symptoms are limited to spots on some parts of the leaves and diebacks of twigs.

P. ramorum is known to cause death in a number of oak species, including coast live oak, Shreve oak, and California black oak. P. ramorum is also known to infect a relative of oaks, the tanoak. The main symptom of infected trees is a canker on the trunk, with blackish to reddish sap that “bleeds” from the sore. Infected trees take an estimated 2 years to die from the pathogen itself; however, other pathogens and insects readily infect the already-sick trees and speed up the tree’s death.

A Disease With Far Reaches in California

Since its introduction to California in the mid-1990’s, Sudden Oak Death has killed an estimated 3 million trees along the coast from Big Sur to Oregon. The effects of this disease also go beyond just the deaths of trees – trees that have died from Sudden Oak Death are dangerous fuels for California wildfires, a threat that has become more prominent as we face the effects of climate change.

Management practices have focused on the identification, quarantine, and removal of infected plants as a way to stop its spread. Public education initiatives have also focused on educating California residents on how to avoid spreading the disease, through practices such as sterilizing pruning tools and washing off equipment, tires, and shoes that have come in contact with infected soil.

Predicted spread of the sudden oak death epidemic through California, 1990–2030
An image from Meentemeyer et al. 2011 showing the predicted spread of Sudden Oak Death from 1990 – 2030, based on the assumption of similar weather conditions to those in California from 1990 – 2008.

Despite California’s efforts, a recent study (Meentemeyer et al. 2011) conducted by scientists from Cambridge, the University of California at Davis, and the University of North Carolina has concluded that completely eradicating the disease from California is not possible. The sheer number of trees infected makes it impossible to rid the state of the disease or stop its spread.

Instead, the study suggests that management should focus on the restoration and treatment of small, local forests as the most practical and cost-effective option. One such study focusing on removing dead and diseased trees is currently being conducted around Bolinas Ridge, giving us hope for the development of best management practices moving forward.


Author: Taylor Crisologo

Taylor studied biology at Cornell University, where she worked with the Cornell Lab of Ornithology on projects ranging from breeding herring gulls off the coast of Maine to dancing lyrebirds in Australia’s Blue Mountains. When she’s not researching great places to experience Bay Area nature, you can find her birding or reading a book at home with her husband and their two indoor cats (Max and Penelope).

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