Plant Pathogen Threatens California Oak Trees

By Staci DeKunder

We are often overwhelmed by headlines addressing the latest disease outbreak facing the human population. In recent years, we have read with great concern about Ebola, measles, Avian flu, etc. Unfortunately, there is a similar outbreak facing oak trees in California. Sudden Oak Death is responsible for the death of over one million California oak and tanoak trees. And as it turns out, a microscopic pathogen called Phytophthora ramorum (P. ramorum) is behind the disease.

Matteo Garbelotto was one of the first two scientists to discover P. ramorum in 1995. Over 20 years later, scientists understand much more about how this tree killer operates and how it came in contact with the oaks.   P. ramorum thrive in humid environments, and can spread from plant to plant via wind, rain or with help from humans. Some plants are susceptible to the pathogen (like the California oak and the tanoak), and others are merely host carriers (California bay laurel, rhododendron and camellia). When a susceptible plant is infected, the pathogen attacks the tree’s bark, finding pathways into the tree. From there, it blocks the plants ability to circulate water and nutrients. This results in a fast demise for the tree, with symptoms of brown leaves and sap leaking from the bark.   If the pathogen finds a ‘host ‘plant, the plant is not harmed, but the pathogen can easily be transmitted to a nearby susceptible plant.   This is an issue both in nurseries and in the forest.   A simple Process Map can be created to depict how the pathogen wreaks its havoc on the trees.

As with most situations, understanding the problem is an important step to identifying solutions. Prior to discovering the pathogen P. ramorum, scientists were baffled by the bleeding trees. They initially suspected insects, but could find no visible wounds or damage typical of insects. Creating a Cause Map can help analysis the cause-and-effect relationships that are responsible for an impact to the goals. Asking ‘why’ questions beginning with the affected goal helps us to learn about the causes of an event. In this case, the environmental goal was impacted by the death of millions of trees. The hard work of Garbelotto and his fellow scientists showed that the trees were dying because they were exposed to the pathogen P. ramorum AND the fact that the trees were susceptible to its affects. The plants were exposed to the pathogen because the pathogen was carried from nearby plants. This was due to the fact that there were infected plants were located close by AND the presence of a mode of transportation. This mode of transportation could have been wind, rain and / or human transport. The human transport could be a result of people accidentally moving infected plants or soil.   There are infected plants close by because certain plants act as a ‘breeding ground’ for the pathogen AND because the pathogen was accidentally imported to the United States via host plants via the ornamental plant trade in the 1980’s. (Click on “Download PDF” above to see a Process Map and Cause Map of this issue.)

Fortunately, there are several identified solutions that can help minimize the impact of this pathogen. Using the Cause Mapping process, these solutions can be tagged to the specific causes that they impact. Then, a table of solutions can be created so that the owners (and due dates if applicable) can be tracked.   Five solutions are shown on Cause Map to help save the oak trees including: federally regulating the movement of host plants, using caution when moving plants and soil in infected areas, removing some host plants in infected areas, a phosphite spray which can be applied to infected trees and a smartphone application that can help educate and expand the current understanding of infected areas.