/resource-center/insects-mites-and-diseases/what-causes-sudden-oak-death/

What Causes Sudden Oak Death?

What causes Sudden Oak Death in trees and plant life? Sudden Oak Death, or SOD, is caused by the pathogenic fungus Phytophthora ramorum. This waterborne mold pathogen infiltrates plants and trees through contaminated irrigation water, wind-blown rain, infected plants, and contaminated pots and soil mixes. This fungus is also associated with other tree wellness issues like Ramorum leaf blight, Ramorum dieback, and Phytophthora canker.

Sudden oak death affects more than oak trees

Since its discovery in the San Francisco Bay Area in the mid-90s, SOD has killed millions of oak trees from central California to southern Oregon, including live oaks, black oaks, Shreve oaks, tanoaks, and others. Despite its name, this especially dangerous pathogen affects more than 100 species of plants, shrubs, and trees, blocking their ability to transport water and nutrients. Highly susceptible vegetation includes:

  • Oaks and tanoaks
  • Kalmia/mountain laurel
  • Umbellularia/California bay laurel
  • Rhododendron
  • Viburnum
  • Camellia
  • Pieris

Symptoms of Sudden Oak Death

SOD is difficult to differentiate from other pathogens. Our knowledgeable arborists know the signs, which display as bark cankers/calluses on tree trunks and woody twigs and branches. Other symptoms of SOD are seeping black/reddish ooze on bark, twig/shoot dieback, and leaf spots on shrubs and plants. Cankers are the most serious sign. These lesions can make trees already weakened by SOD more susceptible to attack by bark beetles, which can quickly lead to death and the need for tree removal and replacement.

How to Treat Sudden Oak Death

There is no cure for SOD. Preventing its spread with meticulous tree management is the best defense, as plants showing only minor symptoms can quickly spread this deadly disease. Our tree care professionals control the spread of SOD by:

  • Identifying infected trees nearby in the same watershed
  • Reducing the number of neighboring plants that could host the disease, so they cannot touch healthy trees
  • Applying preventative treatments to the tree base in the fall and spring to suppress the spread of SOD fungus
  • Proactively treating affected trees and plant life to support their health and improve overall resistance to disease

Contact us today to schedule a consultation with an expert arborist.

FAQs

  • The earliest signs of Sudden Oak Death usually appear as bark cankers on the trunk, often with a seeping black or reddish ooze. Other symptoms include twig and shoot dieback and leaf spots on nearby shrubs. Because SOD closely resembles other pathogens, an ISA Certified Arborist® should confirm the diagnosis before you decide on a treatment plan.

  • No, there is no cure for Sudden Oak Death once a tree is infected. The most effective approach is prevention, since even plants with minor symptoms can spread the disease quickly. Preventative treatments applied to the tree base in fall and spring help suppress the fungus, and an ISA Certified Arborist® can build a plan to protect the healthy trees nearby.

  • Sudden Oak Death spreads through the waterborne pathogen Phytophthora ramorum, which travels in contaminated irrigation water, wind-blown rain, infected plants, and contaminated pots or soil mixes. Because it moves so easily within a shared watershed, identifying and managing infected trees early is essential. Reducing neighboring host plants also keeps the pathogen from reaching healthy trees.

  • No. Despite its name, Sudden Oak Death affects more than 100 species of plants, shrubs, and trees. While oaks and tanoaks are highly susceptible, the pathogen also infects rhododendron, camellia, viburnum, mountain laurel, California bay laurel, and Pieris. On many of these plants it causes related issues such as Ramorum leaf blight rather than killing the tree outright.

  • Not always. Removal depends on the severity of the infection and the risk to surrounding trees, since cankers can weaken a tree and invite bark beetles that hasten decline. An ISA Certified Arborist® can assess whether targeted treatment, selective removal of host plants, or full removal and replacement best protects the rest of your landscape.