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Additional key words: SOD, oak and tanoak decline, regeneration
The recently described plant pathogen Phytophthora ramorum causes extensive bark cankers in coast live oak, black oak, and tanoak that are associated with premature mortality of these species. This bark canker disease, which is commonly referred to as "sudden oak death" (SOD) has the potential to severely impact many of the ecologically important woodlands and forests of Sonoma County. In this project, we established permanent research/monitoring plots in woodland and forest types at risk from P. ramorum canker to monitor disease progression and assess impacts of disease over time. We also assessed tree decline and mortality due to other agents.
We established a total of 250 fixed-area plots (0.02 ha each) at 11 study locations in various portions of Sonoma County. SOD host trees (i.e., coast live oak, black oak, and tanoak) lacked any symptoms of P. ramorum canker at four locations. Four locations had trees with SOD symptoms, but the presence of P. ramorum at these locations has not yet been confirmed by laboratory analysis. Three locations with SOD symptoms had preexisting laboratory confirmations for P. ramorum's presence.
Among the seven locations with likely P. ramorum symptoms, apparent SOD infection rates ranged from 3% to 45%. Symptoms of P. ramorum canker were most common on tanoak and least common on black oak. For coast live oak but not tanoak, SOD symptoms were significantly more common among overstory trees than understory trees.
Among the 11 study locations, the incidence of decline and mortality of SOD hosts from agents other than P. ramorum ranged from 17% to 33%. In most locations, decline and mortality from these agents, mostly native wood decay fungi, is currently more important than SOD. However, we observed relatively little overlap between trees affected by P. ramorum and trees declining as a result of other pathogens. If P. ramorum is less likely to affect trees that are already in decline due to other factors, the overall impact of SOD will be increased in stands with high background levels of decline and mortality.
Seedlings of tanoak and coast live oak are typically present in the understory beneath these species. This increases the chances that tanoak and coast live oak mortality may be replaced by the same species. However, disease and competition with seedlings of other tree species may inhibit regeneration of tanoak and coast live oak. Black oak seedlings are relatively uncommon beneath black oak canopy. Black oak regeneration appears to be insufficient to maintain current densities at the study locations, including those in which P. ramorum is not currently a problem.