According to information available from the California Oak Mortality Task Force and communication with Yana Valachovic (who was around and involved then), infected California bay laurel trees were first confirmed, and Humboldt thus added to the list of quarantined counties, in July, 2002. This first infestation was found in a few backyards in Redway.
In 2004, the first appreciable P. ramorum-caused tanoak mortality was mapped by the US Forest Service Aerial Pest Detection unit; about 125 acres contained tanoak mortality at that time, distributed patchily over an area of about 9 square miles in the Redway vicinity. A cumulative total of nearly 27,000 acres with SOD-related tanoak mortality have now been mapped in Humboldt County via aerial detection (with regular ground-check efforts to confirm pathogen presence by lab test). The majority of this mortality, is scattered across a contiguous area of about 360 square miles.
This comment probably has too much detail for the timeline, but I find it useful to understand how quickly the pathogen can spread through areas with a fairly regular distribution of forest hosts (in this case, primarily California bay laurel and tanoak). Of course, while it seems plausible that this southern Humboldt area could have been infested as a result of natural spread after an initial introduction of an infected nursery plant(s) to Redway, we cannot discount a possibility that multiple events of infected-plant introductions have occurred and resulted in the pattern of forest infestation we see today.