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

Safeguard your valuable trees. Learn what causes Sudden Oak Death and put a prevention plan in place with help from Arborwell Professional Tree Management. Contact us at to request an arborist consultation for your San Diego, Sacramento, or San Francisco Bay Area property today.