In areas that are not currently infested with P. ramorum, the main management objective is to prevent introduction of the pathogen. In noninfested areas, the first question to address is whether P. ramorum could become established, given the climate and vegetation type. If it is likely that P. ramorum could invade the area, the next question is how soon this could happen. Answers to these questions will indicate the degree of threat posed by P. ramorum. As the threat level increases, more intensive precautionary measures may be justified.
Phytophthora ramorum may be present as a foliar disease on California bay, tanoak, and possibly other hosts (see 1.2 SOD Disease Cycle in California Forests) for years before SOD cankers appear. Inspection, sampling, and testing of California bay leaves and/or tanoak twigs can be used to help determine whether P. ramorum is already established on foliar hosts in the area (see Sidebar 1-5—Diagnosing SOD). If infections on foliar hosts are present, see 2.2.2 During: Managing disease in areas currently impacted by SOD .
The related pathogens P. nemorosa and P. pseudosyringae can produce symptoms on California bay leaves and canker hosts that are nearly identical to those caused by P. ramorum. Identification of the pathogen via a lab test is needed to positively confirm that symptoms are due to P. ramorum. Sampling should be conducted by persons that have been trained in SOD recognition and sampling protocols. The COMTF website (http://suddenoakdeath.org) provides a list of individuals that have recently completed a training session.
If the distance from an infested area to a non-infested management unit is less than about 5 km, the area has a moderate to high risk of becoming infested in the short term (1-5 years). Spread of the pathogen can occur if prevailing storm winds blow from infested areas toward the noninfested area or if watercourses flow in that direction. Stands that include large patches of California bay or tanoak have a higher risk of becoming infested with P. ramorum than stands where these species are uncommon or lacking. More intensive management actions are needed to counter the disease threat in stands that have a higher risk of P. ramorum infestation (table 2-4).
If the current distance from an infested area to a noninfested area is 5-10 km or more, the unit has a low to moderate risk of becoming infested in the short term via natural processes. Human activities pose the greatest risk of introducing P. ramorum through movement of infested plant material or soil. High risk areas include areas near roads, trails, and buildings. Watercourses that pass though an infested area may also carry inoculum that could be splashed on to susceptible foliar hosts.
Maps of Phytophthora ramorum distribution are available from the OakMapper website (http://www.oakmapper.org/). OakMapper provides locations of confirmed P. ramorum isolations, from several official sources as well as unverified community submissions. The official confirmed detections in OakMapper show many of the areas known to be infested, but does not completely describe the distribution of P. ramorum at a high level of detail. Within most infested areas, relatively few samples are submitted for official confirmations, especially after P. ramorum has been confirmed in a general area. Also, most routine P. ramorum detections by SOD researchers are not in the OakMapper database. Maps (SOD Blitz, SODMAP) available from the Forest Pathology and Mycology Laboratory at UC Berkeley (http://www.cnr.berkeley.edu/garbelotto/english/sodblitz.php) may show detailed results for some areas. More detailed information about the distribution of P. ramorum in your local area not yet in maps, may be available from your local County Agricultural Commissioner and UC Cooperative Extension offices. and the Forest Pathology and Mycology Laboratory at UC Berkeley.
Introduction of P. ramorum to an area does not guarantee that it will become established and cause disease. Disease will not develop if suitable hosts are not present or if the environmental conditions are unfavorable for pathogen survival, reproduction, and infection (see 1.3. Factors That Influence SOD Development).
The density and distribution of California bay and tanoak are good indicators of SOD disease risk. Areas that lack both of these species are at low risk for SOD even if susceptible oak species are present and climate conditions are favorable. In oak stands, SOD risk increases as the amount of California bay in the stand increases (see 1.2.2 SOD in Oak / California Bay Forests). Risk is also higher in stands where California bay is distributed throughout the area so that the pathogen can easily spread from tree to tree. Similarly, SOD risk in tanoak stands is greater where tanoak stands are denser and more continuous. California bay presence within tanoak stands increases overall SOD risk and is commonly associated with earlier and more rapid disease progress. However, SOD risk can be high in tanoak stands with little or no California bay.
For a given stand composition, overall SOD risk increases as the climate and microclimate become more favorable for P. ramorum (see 1.3.1, Environmental Conditions). Disease risk will be higher in areas that receive higher levels of spring rainfall and have longer wet periods (due to high humidity, fog, shading, slope and aspect). Relative disease risk is lower in drier areas, or where wet conditions occur more sporadically during a given wet season or from year to year.
Maps developed by various research groups which predict the risk of P. ramorum infection based primarily on hosts and climate variables are shown in Kliejunas 2010. These models generally indicate that the greatest risk of SOD establishment is in the coastal counties of Monterey north to Del Norte County. Risk extends eastward into adjacent counties where suitable hosts are found. These models are best used to predict SOD risk at a regional scale. They may drastically over- or underpredict disease risk at the scale of a specific stand or parcel because the vegetation maps used in the models are generally not detailed enough to reflect the local scale.
The relative importance and usefulness of management actions in noninfested areas vary with the overall SOD threat (table 2-4). In noninfested stands close to an area where P. ramorum is present, monitor canker and foliar hosts at least annually for evidence of SOD. Looking for foliar symptoms on California bay or tanoak provides an earlier indication of P. ramorum spread than does observing stem cankers or SOD-killed trees. Detailed descriptions of specific management actions are discussed in Part 3.
Table 2-4—Relative importance/usefulness of management actions to prevent new SOD infections in areas where Phytophthora ramorum does not currently occur
| Relative importance of management actions | ||||
|---|---|---|---|---|
| Risk of SOD due to species mix and climate conditions | Proximity to known P. ramorum | Exclusion | Bay removal | Chemical control |
Low |
Distant (5-10+ km) |
+++ |
— |
— |
|
Midrange |
+++ |
+ |
— |
|
Close (<1 km) |
++ |
+++ |
++ |
Moderate |
Distant (5-10+ km) |
+++ |
+ |
— |
|
Midrange |
+++ |
++ |
+ |
|
Close (<1 km) |
+ |
+++ |
++ |
High |
Distant (5-10+ km) |
+++ |
++ |
— |
|
Midrange |
+++ |
+++ |
++ |
|
Close (<1 km) |
+ |
+++ |
+++ |
+++ = high,
++ = moderate
+ = low,
— = very low