by Jeremy Wheeler
November 20, 2002

Sudden Oak Death Approaches Mattole Watershed

Sudden Oak Death (SOD) is a plant disease epidemic that is rapidly approaching the Mattole watershed from its epicenter around the San Francisco Bay Area. Though it has not yet been found within the Mattole River watershed, it has recently been confirmed in California bay laurel trees in the Redway area, adding Humboldt to the list of 12 infected counties. The cause of the disease is a fungus-like microorganism known as Phytophthora ramorum. This organism is closely related to the pathogen responsible for Ireland's potato blight, as well as huge forest die-offs in western Australia.
In California, tens of thousands of oak trees have been killed by the disease since it was first noticed in Marin County in 1995. SOD now affects every costal California county from Monterey to Humboldt. Equally alarming is the rapid increase in the number of plant species infected. SOD has now been found in more than a dozen non-oak native species including huckleberry, madrone, California bay laurel, and most recently douglas-fir and redwood.
The disease behaves differently among the plant species it infects. To date, SOD has proven lethal in tanoak, black oak, and coast live oak. Hardest hit in the epicenter of the outbreak has been tanoak, causing massive die-offs mainly in Marin, Santa Cruz, and Sonoma Counties. In other species, effects have been less dramatic, ranging from spots appearing on leaves to the death of new shoots and leaves.

Quarantines Now In Effect

In an attempt to control the spread of SOD, both state and federal agencies have put quarantines into effect in all 12 infected counties, including Humboldt and Mendocino. This means that a permit, known as a “compliance agreement,” is now required to transport plant materials from any of the 16 known susceptible species (see sidebar). In the case of redwood and douglas-fir, only the needles and branches less than one inch in diameter are currently quarantined.
For the other species, the quarantine applies to all parts of the plant. Even if there is no apparent indication of infection on a given property, it is required that any who would transport material from these plants off-property have the area inspected to confirm absence of the disease. Where no SOD is detected, the county agricultural commissioner will issue a “free-from” certificate, which serves as a compliance agreement and is valid for one year. If SOD is found and confirmed on site, a more involved process is required to obtain the compliance agreement. In the case of timber operations regulated by the California Department of Forestry, the compliance agreement is issued as part of the harvest plan process. According to Mark Stanley of the interagency SOD Taskforce cite when he said this, these regulations are still evolving with the rapidly changing understanding and spread of the disease; it is likely that they will change.

Signs and Symptoms

The first thing to understand about positively identifying SOD in the field is that it is generally not possible-- the only way to be certain is to have a plant sample cultured in a lab. There are many diseases or infestations that cause similar symptoms, and symptoms vary depending on the plant species infected.
In oaks, symptoms often begin with the appearance of oozing cankers on the trunk of the tree. These cankers vary greatly in size and in the amount of discharge they emit. The tree at this stage is weakened to the point of being highly susceptible to bark beetles infestations, which very well may lead to its demise.
In non-oak species, symptoms vary from the appearance of dead spots on leaves, in the case of California bay laurel and madrone, to drooping and dying branch tips and dying needles in redwood and douglas-fir.

Prevention Best Medicine

There is currently no effective treatment known for the disease. Therefore, preventing further spread is especially important. If spending time in an infected area, take care to avoid accidental transport of plant materials, by thoroughly sweeping the back of one’s truck, for example. It is possible for the spores of the fungus to be carried in mud or water, so it is important to rinse muddy tires and boots before leaving an infected area.
For more information, including identification guides with color photographs, check the web site www.suddenoakdeath.org. At this site there is also a place to submit information on suspected infections.


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Table of Contents for Mattole Restoration Newsletter, Issue 19 - Winter/Spring 2002-03