Responding to an invasive pathogen: Tackling sudden oak death on Bay Area watershed lands

Background/Question/Methods

When alarming numbers of dead coast live oak, (Quercus agrifolia) appeared on 23,000 acres of protected watershed lands above San Francisco Public Utilities Commission (SFPUC) reservoirs on the San Francisco Peninsula (Peninsula Watershed) (San Mateo County), managers realized they needed to prevent further ecological damage from the sudden oak death pathogen, Phytophthora ramorum. To sustain forests that serve as water collection and storage for 2.6 million customers, a collaborative adaptive management project was initiated in 2008 to understand the ecology and impacts of  this fungus-like microorganism (Kingdom Stramenopila) and apply the findings to sustain tanoak (Notholithocarpus densiflorus) and coastal evergreen forests.  We asked: How could spread and establishment of the sudden oak death pathogen be prevented by management intervention? 

In 2008, a network of 16 plots was established within the Peninsula Watershed to track infection and tree mortality. Additionally, in a mature, healthy tanoak stand, we applied preventive potassium phosphite spray annually to 233 trees, and monitored 273 adjacent trees as a control.  To protect coast live oak, we removed California bay laurel (Umbellularia californica) in controlled trials. Survey and soil sampling was conducted to determine the presence of other damaging Phytophthora species.  

Results/Conclusions

By 2013, Phytophthora ramorum was present in 15 of 16 randomly distributed plots in the Peninsula Watershed; of the 952 coast live oak monitored, 17% were dead from P. ramorum and annual infection rate averaged 16.2%.  Phytophthora nemorosa was also detected on the plots.  For both Phytophthora species, infection is associated with high rainfall levels.  Survival patterns are being investigated to hone P. ramorum management strategies. 

The phosphite study was the largest test of its efficacy against P. ramorum in a native tanoak forest stand. Phosphite failed to prevent infection or reduce disease severity; 25% of the tanoaks were killed by 2013.  Treatment has been discontinued.  Additional experiments are underway in other areas to determine the overall effectiveness of phosphite to manage P. ramorum in tanoak.

The soil-borne pathogen Phytophthora cinnamomi and 6 other Phytophthora species were recovered from soil near dead and declining native plants on the watershed, most commonly Pacific madrone (Arbutus menziesii).  The greatest diversity of Phytophthora species was found near a nursery and horticultural garden, which is suspected to be the source of the pathogens.  Efforts to better understand the threats posed by each species and use that knowledge to protect the forest, continue.