Phytosphere Research

Assessing Methods to Protect Susceptible Oak and Tanoak Stands from Sudden Oak Death

Paper presented at the Fourth Sudden Oak Death Science Symposium, Santa Cruz (Scotts Valley), CA, 15-18 June 2009 Management/Treatment session

T. J. Swiecki and E. A. Bernhardt
Phytosphere Research, Vacaville CA

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Landowners and managers have been seeking ways to protect susceptible oak species and tanoak from sudden oak death (SOD) caused by Phytophthora ramorum.  Because disease epidemiology differs between tanoaks and susceptible oaks, we are testing different control strategies appropriate for different forest types.  The disease management studies described here are intended to test whether various management techniques will be effective when applied in the field at the stand scale.

In tanoak stands, we are testing applications of potassium phosphite (Agri-FosTM) in combination with removal of small diameter understory tanoak, if present, in contiguous blocks of trees ranging from about 250 m2 to 1.3 ha.  In all plots, we have used bark spray applications with rates scaled to stem cross-sectional area.  We also shifted the application zone higher on the stem (up to 6 m height) to enhance uptake.  At one location, stem injection applications have been added for comparison purposes.

Our initial study plots were established in December 2005 in Sonoma County; the most recent and largest plots were established in San Mateo County in 2007 and 2008.  To date, SOD-affected trees are lacking or occur at very low percentages in all but the oldest treated plots, where no significant differences due to treatment have yet been detected.  Data collected in spring 2008 will be reported for the symposium.

In oak stands, where California bay is the primary source of P. ramorum inoculum, we are testing removal of bay near susceptible coast live, California Black, and Shreve oak as a disease management technique.  We have established plots to monitor disease development and progress in treated and control oaks and regrowth of bay in treated areas. 

We established plots to study bay removal in localized zones around individual oaks (minimum bay foliage to oak trunk clearance=2.5 m) at five locations in Sonoma, Napa, and Solano Counties in 2007.  Bay neighborhoods around oaks and oak disease status were assessed prior to treatment.  We identified matched pairs of trees with similar bay neighborhoods and then performed bay removal around one member of each pair, leaving the untreated member as a control.  Bay neighborhoods were reassessed immediately after treatment and annually thereafter.

In two locations in San Mateo County we have initiated studies of area-wide removal of all bay within patches as large as 3 ha.  Control plots without bay removal were established adjacent to the treated plots.  Bay neighborhoods and disease status for a sample of the oaks in each plot were assessed prior to treatment.  Bay neighborhoods in the treated plot were reassessed shortly after bay removal was completed.  At one location, bay removal is being tested in conjunction with potassium phosphite treatment to protect a group of large coast live oaks.  Comparison groups using only bay removal or no treatment are included.  These studies will be discussed in the context of adaptive management efforts that can be initiated to prevent or minimize SOD impacts in selected stands of oaks and tanoaks.

This research was supported by cost-share grants from the USDA Forest Service, Pacific Southwest Research Station and Forest Health Protection Program, State and Private Forestry Section, San Francisco Public Utilities Commission, and Midpeninsula Regional Open Space District.