Phytophthora ramorum
Detection of "P. ramorum" in watercourses has emerged as the earliest of early detection methods. This technique employs pear or rhododendron baits suspended in the watercourse using ropes, buckets, mesh bags, or other similar devices. If plants in the watershed are infected with "P. ramorum", zoospores of the pathogen (as well as other "Phytophthora" spp.) are likely present in adjacent waterways. Under conducive weather conditions, the zoospores are attracted to the baits and infect them, causing lesions that can be isolated to culture the pathogen or analyzed via PCR assay. This method has detected "P. ramorum" at scales ranging from small, intermittent seasonal drainages to the Garcia, Chetco, and South Fork Eel Rivers in California and Oregon (144, 352, and 689 mi2 drainage areas, respectively). It can detect the existence of infected plants in watersheds before any mortality from the infections becomes evident. Of course, it cannot detect the exact locations of those infected plants: at the first sign of "P. ramorum" propagules in the stream, crews must scour the watershed using all available means to find symptomatic vegetation.
Phytophthora ramorum
Early detection of "P. ramorum" on a landscape scale begins with the observation of symptoms on individual plants (and/or detecting "P. ramorum" propagules in watercourses; see below). Systematic ground-based monitoring has been difficult within the range of "P. ramorum" because most infected trees stand on a complex mosaic of lands with various ownerships. In some areas, targeted ground-based surveys have been conducted in areas of heavy recreation or visitor use such as parks, trailheads, and boat ramps. In California, when conducting ground-based detection, looking for symptoms on bay laurel is the most effective strategy, since "P. ramorum" infection of true oaks and tanoaks is almost always highly associated with bay laurel, the main epidemiological springboard for the pathogen. Moreover, on many sites in California (though not all), "P. ramorum" can typically be detected from infected bay laurel tissues via culturing techniques year-round; this is not the case for most other hosts, nor is it the case in Oregon, where tanoak is the most reliable host.