Monitoring California streams for Phytophthora ramorum

Lead project scientist: Ashley Hawkins

“Stream monitoring” refers to the monitoring of open waterways (primarily streams and rivers) for the presence of Phytophthora ramorum, the causal agent of Sudden Oak Death (SOD), by means of baiting with susceptible plant tissue, usually Rhododendron leaves. Because this method potentially samples across entire watersheds and involves the cumulative exposure of bait leaves to infective propagules in the water over an extended period of time, results are generally more comprehensive and sensitive than terrestrial surveys alone. When P. ramorum is detected through stream monitoring techniques, terrestrial surveys are often conducted within the watershed in an attempt to locate the terrestrial source of inoculum.

For stream baiting, Rhododendron leaves are placed in mesh bags that float just below the surface of the water (see photo). These "bait bags" are left in a stream for one to three weeks, depending on water temperatures (the cooler the water, the longer the leaves can be left out). 

After one to three weeks, bait leaves are collected from streams and taken back to the lab. At this point, lesions are visible on the bait leaves, indicating that they have been infected by microbes in the stream (see photo). Cuttings are taken from the lesions and placed in petri plates containing a media that is selective for Phytophthora species. If P. ramorum spores have infected the bait leaves, they will grow out in the selective media and can be identified based on morphology. In some instances, molecular techniques are also used for determining if bait leaves are infected by P. ramorum. 

The California Stream Monitoring project was initiated in 2004 and stream surveys have been conducted annually each spring since, from February through June. The goal of this project is to monitor the presence and spread of P. ramorum throughout California, with a focus on areas at the edge of this pathogen's current range. For this reason, the distribution of waterways being monitored changes each year. The current focus is on frontiers of infestation in counties in which P. ramorum's distribution is still limited, particularly northern Humboldt County, and in uninfested counties that have a high risk of infection (e.g. Del Norte County). In 2016, stream monitoring sites were located in Del Norte, Humboldt, Monterey, and San Luis Obispo counties (see maps below).  

This project depends on the efforts of many collaborators throughout northern and central California. Collaborators play a pivotal role in selecting waterways to be monitored in their area, and in deploying and collecting bait leaves. Current organizations and groups participating in the California Stream Monitoring project include: the Bureau of Land Management, Cal-Fire, the Del Norte County Agriculture Commissioner’s Office, the Hoopa Valley Tribal Forestry Department, Humboldt County UC Cooperative Extension, Humboldt Redwood Company, the Mattole Restoration Council, Redwood National and State Parks, UC Berkeley Forest Pathology and Mycology Laboratory (Garbelotto Lab), and the Yurok Tribe Forestry Program. This project also relies on the hard work of our dedicated undergraduate assistants. 

Results from the 2016 stream monitoring season are illustrated in the maps below. 

 

Detection of P. ramorum in Del Norte and Humboldt Co. watersheds, as of June 2016. 

 

Detection of P. ramorum in Monterey and San Luis Obispo Co. watersheds, as of June 2016.