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UC Master Gardeners of San Luis Obispo County

Sudden Oak Death

Sudden Oak Death (SOD)

By Kim Wilson, UC Master Gardener

EXTERNAL IMAGESudden Oak Death symptoms on oak trees

Sudden Oak Death (SOD) is an exotic disease caused by a fungus-like organism, Phytophthora ramorum. Many Phytophthora species are soil-dwelling root pathogens, but the causative agent of SOD is not a root pathogen. Phytophthora ramorum has not been detected in samples of California bay laurel trees as of 2024. San Luis Obispo County is considered not infested with this pathogen.

SOD Pathogen Diseases

  1. Fatal canker disease (Sudden Oak Death): Infects and kills several oak species including black oak, coast live oak, canyon live oak, and Shreve’s oak. It can also infect and kill tanoak. Infection occurs in the wood of branches and trunks. The pathogen enters through cracks in the bark and is not a root pathogen.
  2. Non-fatal foliar disease (Ramorum blight): Infects more than one hundred landscape and native plant species. The disease affects leaves and twigs, causing leaf spots or dieback, but usually does not kill the plant. These infected plants serve as breeding grounds for the pathogen and are the primary source of infection for oaks.

Disease Transmission

  1. The pathogen requires water to become infectious and produces spores only after rainfall. Spores are spread by wind to nearby susceptible plants.
  2. Disease activity increases during rainy years and slows during dry periods. It can spread approximately 100 yards per year, or several miles in very wet years.
  3. Foliar host plants act as vectors and are highly infectious. California bay laurel is the most important vector.
  4. Oak trees are not vectors, and the disease does not spread directly from oak to oak.
  5. Tanoak is unique because it can develop both foliar disease (allowing it to spread the pathogen) and canker disease, which can result in death.
  6. Wind and rain are the primary modes of transmission. Movement of people, animals, and soil is a minor contributor. Cleaning dirt from shoes, bike tires, and pruning equipment significantly reduces pathogen spread.

Identification of the Disease

  1. Identify whether susceptible plants are present at your location and learn the signs of infection.
  2. Determine risk at your location using the free SODmap mobile app. The app displays infection risk (high, moderate, low, or insufficient data) based on results from the SOD Blitz.

Prevention and Treatment

Prevention depends on understanding the distribution of host plants in your area. California bay laurel is the most important vector species.

  1. There is no effective treatment for infected oak trees.
  2. In high- or moderate-risk areas, consider selective removal of nearby bay laurel trees to create a buffer zone of 30–65 feet around high-value oaks. Buffer size depends on oak size and value.
  3. In high-risk locations or where infected host plants are known, healthy oaks may be protected with a yearly preventive phosphonate treatment applied between November 1 and December 15. This treatment does not cure infected trees.

How You Can Help

UC Berkeley and the California Native Plant Society train citizen volunteers each spring to collect symptomatic California bay laurel leaves and document tree locations. Results are announced in mid-October and provide data for the SODmap mobile app. Visit the UC Master Gardeners of San Luis Obispo website for SOD Blitz registration information.

References