The disease under the microscope.
Oak ambrosia beetles are very small and aggressively colonize stressed trees.

Sudden Oak Death Complex in Detail

What is Sudden Oak Death?

SOD is a complex of three different organisms which can ultimately lead to tree mortality. One is a pathogen called Phytophthora ramorum, which destroys the vascular system of the tree. The second is a group of three beetles; a bark beetle and two species of ambrosia beetles. The third is a wood decay fungus called Hypoxylon thouarsianum.

Phytophthora ramorum

Phytophthora (“Plant destroyer”) ramorum is often referred to as a fungus. In reality, it is part of the group from the Phylum Oomycota. There is the aquatic group, the so-called water molds and then there are the terrestrial Oomycetes. P. ramorum belongs to the terrestrial group. It still requires water to reproduce. The sexual reproduction of Phytophthora is characteristic of brown algae. The hyphae (filamentous root-like structure) closely resemble the same structure found in fungus. For this reason, long ago, it was put into this group.

Oak Ambrosia Beetle (Monarthrum scutellare, M.dentiger)

Beetle HitsOne sixteenth to one eighth inch long, these beetles are capable of causing incredible damage and tree mortality. As the beetles bore into trees they produce sawdust.

A Tree Destroyed by Sudden Oak DeathThey are attracted to the infected areas below the bark of trees. They bore through the bark into the sapwood and cambium where they reproduce. As ambrosia beetles bore into trees to create reproduction galleries, they further weaken the tree and girdling often occurs, which can lead to catastrophic failure.


Hypoxylon Up CloseThe third organism is an opportunistic decay fungus called Hypoxylon thouarsianum. Weakened trees are often colonized by this pathogen. It enters a tree through wounds, works its way into the cambium tissue and just like the beetles and Phytophthora ramorum, it can girdle a tree. In October 2003, the fungicide Agrifos was approved for treatment of this disease. The material is applied either by trunk drench or by injection into the vascular system.

How Do Trees Become Infected?

Ganoderma Has Hit These Bay Laurel LeavesP. ramorum can spread several different ways. Rain splash from the leaves of nearby bay laurel and tan oak trees has been determined to be the most common way it is spread onto other trees and shrubs. The pathogen reproduces on the water film of the mentioned trees during rain events, and then due to wind or the weight of the water film, the infected water splashes onto the trunks and leaves of other trees and shrubs.

Irrigation water from infected streams, mud from shoes or bike tires and probably forest animals are said to spread the pathogen as well.

Although Sudden Oak Death is known primarily as an oak problem, many other trees are affected. Virtually all trees and shrubs native to California are on the list of host plants. See the California Oak Mortality Task Force Hosts & Associated Plants list.

How You Can Protect Your Trees

A proactive approach is the only way to preserve your oak trees from this disease. A site visit by a well educated and experienced arborist will help to determine the best treatment approach. There are two basic components to a treatment program.

  1. Pathogen Source Management
  2. Pathogen Treatment

Source management is determined by the proximity of either bay laurel trees or tan oak trees. These trees are the main source of pathogen spores. It may make sense to remove some of these trees or there is the option of selective pruning. A site visit by an expert arborist will help determine your options.

Pathogen treatment of selected trees using the chemicals Agrifos/Pentrabark is recommended by the University of California for tree preservation. The material Potassium Phosphonate has been used for decades to treat similar species of Phytophthora throughout the world. There are two ways to introduce the chemical to a tree.

  1. Trunk Drench
  2. Trunk Injection

Trunk drench: The chemical combination is mixed together and then applied onto the trunks of trees. Great care must be taken to protect the applicator. Some believe, and we agree, that this material is highly toxic if it contacts skin and is absorbed. Tree Solutions believes that there is not enough understanding of side effects or warning about them.

James Neve Working On A TreeTrunk injection: The phosphonate compound is correctly mixed and then injected directly into the vascular system of a tree. This is the fastest way to introduce the chemical into a tree and our recommendation for already infected trees. The process of injection  produces small wounds to trees. Oak trees are great compartmentalizers and it has been shown by research that these trees can withstand repeated injections. UC Berkeley Matteo Garbelloto lab is endorsing this application technique , alternating every other year with the trunk drench method of application.

We have treated infected trees, and many of these trees are still alive. Read about best practices on our Sudden Oak Death Treatment Options page.

Also, see our updated recommendations and information from the 2009 SOD Science Symposium.


Certified Arborist WE-6717A

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