Sudden Oak Death: Overview
Sudden Oak Death, Phytophthora ramorum, Phytophthora ramorum Blight
Tree Solutions arborists have been involved in the recognition and treatment of Sudden Oak Death (SOD) disease since the early period of discovery, 1995-1998. Early on, we offered our assistance to the research effort in order to obtain a thorough understanding of this disease. We have treated thousands of trees and continue our efforts to help preserve one of our precious resources.
Sudden Oak Death mainly affects trees and shrubs along the Pacific coast beginning in central California from the Big Sur coast extending northward into Curry County in Oregon. Although the pathogen mainly exists in coastal areas, it also extends inland. The disease is present in other parts of the United States and Europe, our focus is the California coastal areas from Monterey to Santa Clara counties. A comprehensive list of affected trees is available from the California Oak Mortality Task Force website.
The tree disease known as Sudden Oak Death was first reported by arborists, in Marin County in 1995. At that time it was primarily observed in tanbark oak trees. A Phytophthora infection was suspected because of the characteristic pattern of discoloration in the bark and sapwood of infected trees, but laboratory tests obtained by the UC Cooperative Extension and other sources consistently came back negative for any known disease organism. The primary sign of the disease recognized at that time was a rapid wilting and browning of the foliage and death of the tree, hence the name Sudden Oak Death or SOD.
The phenomenon became known as “SOD Syndrome” because the affected trees showed signs of multiple associated disease and insect pest invasions. One of the principle factors in the rapid decline of affected trees was heavy infestation of oak bark beetles and/or ambrosia beetles; tiny borers that are known invaders of stressed oak trees. Because of the lack of definitive identification of a causal disease organism (pathogen), and because of the increasing association of beetle activity with SOD, emphasis on control during the late 1990s was focused on beetle control.
By 1998 and 1999, SOD in Coast Live Oak trees became prevalent and was reported in Sonoma, Santa Cruz and other counties. The loss of live oaks became a much more serious event than the loss of the less-valued tanbark oaks.
SOD task forces were formed and serious research began. Finally in 2000, a UC forest pathologist, Dr. Matteo Garbelotto, implemented an advanced laboratory process that resulted in the positive identification of the apparent causal pathogen for SOD as Phytophthora ramorum.
Many concerned arborists from several counties offered their assistance to the research teams. Tree Solutions president Jim Neve and arborist associate Don Cox were involved in the early process of discovery and field work supporting research on SOD.
Don Cox reported one of the first cases of SOD in Marin County in 1995 and participated in collaboration among scientists and tree care professionals with the first “oak task force”. He alerted scientists to new areas of infection and organized work parties for the dissection of infected trees in the field and collection of samples for lab analysis.
Jim Neve volunteered his arboricultural and pest control expertise for field work and assistance to the research teams, collecting samples and utilizing new injection technique with systemic fungicides, including potassium phosphonate, the treatment that would become the current standard for control of SOD.
Species of Phytophtora (Sudden Oak Death)
There are over 100 known species and mating types of Phytophthora to date. Phytophthora ramorum is the terrestrial form of the water mold pathogen. It is in the class of Oomycetes from the Kingdom Stramenopila. Literally translated from Greek origin; Phyto,-phyte (plant) and phthora (destruction) or “plant destroyer”. It thrives in cool moist environments.
Most of us were first introduced to this pathogen through history recollections of the Irish Potato Famine. The pathogen Phytophthora infestans is responsible for the death and migration of thousands of Irish citizens when it destroyed a large percentage of their staple food source, the potato.
The pathogen destroys the plant growing tissue called cambium as well as the vascular system phloem and xylem cells which are responsible for tree life. The presence of the pathogen in this tissue causes girdling, which eventually leads to blight.
Reddish brown sap oozing from the tree trunk is a wound response to the pathogen. It usually takes the form of droplets. It is one of the visual diagnostic indicators of disease presence. A thorough site and tree examination and assessment as well as an experienced understanding of the disease complex is very important for diagnosis, several other pathogens produce similar bleeding symptoms.
Removing the bark over an infection area reveals dark zonal lines and diseased tissue within the zonal borders.
An oak may be infected up to one year or more before symptoms appear. In our experience, once a bleeding lesion appears, tree mortality of coast live oaks can follow approximately two years later. We have also experienced trees infected by Phytophthora ramorum and ambrosia beetles to the point of girdling of over 50% of a tree circumference that stay green and healthy appearing after four years of treatment. This in field observation proves that there are no hard and fast rules that apply to each and every tree.
Beetles invade diseased or stressed trees. It is their job in the forest to seek and destroy these trees. It is said that weakened trees emit a chemical, signaling stress, thus attracting the beetles. Beetles are attracted to the canker area. Just like the pathogen, these beetles girdle infected trees. However, they girdle at a much faster rate than the pathogen. If one is trying to preserve and protect a tree, it is important to protect from beetle attack as well as the pathogen itself.
Sawdust material (frass) is produced by one of three possible beetle species. Red frass is from the western oak bark beetle (Pseudopityophthorus pubipennis), and white frass is produced by ambrosia beetles (Monarthrum dentiger and M. scutellare).
The combination of Potassium Phosphonate and a surfactant which is sprayed onto the trunk as a drench, or P. Phosphonate alone injected into the tree vascular system was approved for treatment in October of 2003. It is the only approved treatment to date. It is not a cure and it is best to treat trees before infection occurs. The material has both an indirect and direct effect on the pathogen. It primarily helps a tree to boost its defense system which helps fight off infection.