Understanding and Managing Sudden Oak Death in California

2.2.2.  During: Managing Disease in Areas Currently Impacted by SOD

Once P. ramorum has become established in an area, the main focus of disease management changes from preventing pathogen introduction to limiting disease impacts. In P. ramorum-infested areas where SOD impacts are likely to continue to increase over time (the During phase), typical management goals are:

1. Minimize P. ramorum infection and mortality of SOD canker hosts.

2. Minimize hazards associated with SOD-affected trees.

Management considerations related to the first goal are described in this section; those related to the second goal are described in the After section below. Management emphasis may shift from the first to the second goal as the number of SOD-infected trees increases.

Information needs—

What is the incidence of SOD in the affected stand?—

More SOD management options are available when disease incidence is still low than when most at-risk trees have become infected. Only some of the at-risk trees typically become infected during each favorable infection period and little or no new disease may develop in dry years (fig. 1-16). Management actions implemented during lulls in the epidemic can protect asymptomatic trees from becoming infected during later disease pulses.

When assessing SOD impact within stands, note both the presence of cankers and the severity of the infection (percentage of the trunk circumference that is affected, presence of secondary organisms). SOD may go into remission (see Changes in canker appearance over time) in some oaks that only have one to several small cankers. Such trees may be candidates for future disease suppression activities. The degree to which the trunk is girdled by cankers and the presence of secondary invaders (beetles, A. thouarsianum) in cankered areas also affects the likelihood that an infected tree may die and/or fail. Observers also need to take past weather conditions into account when making SOD assessments (see Sidebar 2-3—Time Considerations and SOD) to avoid underestimating infection levels.

Management actions—

The primary methods for minimizing P. ramorum infection are listed in table 2-4 and described in detail in Part 3. Many of these management methods work best when combined in an integrated fashion. For example, using chemical applications to increase the resistance of trees to infection will be more effective if coupled with measures that reduce the amount of inoculum to which the tree is exposed.

Selecting individual trees for treatment—

Some SOD management treatments are implemented on a tree-by-tree basis. To maximize cost effectiveness, relatively intensive individual treatments should preferentially be used on high value trees that have a high risk of becoming infected (table 2-5).

Table 2-5—Factors associated with increased SOD risk for individual trees

Category Factors associated with higher risk
Favorable microclimate conditions for infection N or E facing slopes, shaded drainages, greater canopy shading, fog drip
Amount of P. ramorum spores to which the tree is exposed little or no clearance from bay, high local bay and/or tanoak density, extensive poison oak climbing through canopy1
Host tree size and condition larger stem diameter, greater bark thickness1, high tree vigor1, actively expanding (brown) bark fissures1, low water stress1

1Correlated with SOD risk for coast live oak.

Monitoring treatment efficacy—

Annual reassessments of disease symptoms of treated and nontreated areas will show how disease is progressing at a location, how well treatments are working, and whether additional or follow-up actions may be needed.

An efficient way to conduct long term monitoring is to repeatedly assess individual trees so that trends over time can be tracked. Trees can be identified by using permanent tree tags in combination with GPS coordinates or other spatial information.