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Additional key words: oak decline, tanoak decline, sudden oak death (SOD), stem water potential, tree failure, disease progress, regeneration
This report discusses findings after six years of observations in a case-control study examining the role of tree and site factors on the development of Phytophthora ramorum stem canker (sudden oak death) in coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorus). In September of each year from 2000 through 2005, we collected data on P. ramorum symptoms, tree condition, midday stem water potential (SWP), and various other factors in 150 circular plots (8 m radius). Each plot was centered around a case (symptomatic) or control (asymptomatic) plot center tree. Plots were located at 12 locations in the California counties of Marin, Sonoma, and Napa in areas where P. ramorum canker was prevalent in 2000.
Between September 2000 and September 2005, the percentage of coast live oaks with P. ramorum canker symptoms increased slightly, from 23% to 26%. Most of this increase occurred between September 2004 and September 2005 and was due to a large increase in newly-symptomatic coast live oaks at one location. Between 2000 and 2005, tanoaks showed a significantly larger increase in disease incidence, from 31% to 45%.
Most of the P. ramorum-related mortality that has occurred since 2000 was seen in trees that had P. ramorum canker symptoms in 2000; 58% of the tanoaks and 90% of the coast live oaks killed by P. ramorum between 2001 and 2005 were symptomatic in 2000. Overall, symptomatic tanoaks tend to survive for a shorter time after the onset of symptoms than do coast live oaks. Among live trees that had P. ramorum canker symptoms in 2000, 73% of coast live oaks but only 50% of tanoaks were still alive in 2005.
Mortality of tanoak and coast live oak has increased in a linear manner between September 2000 and September 2005. Assuming that this linear trend existed prior to 2000, initial mortality was estimated to occur in tanoak plots around 1995 and in coast live oak plots around 1996.
Most recent coast live oak failures in oak woodlands affected by P. ramorum have occurred in trees infected by P. ramorum. The number of initial failures has been declining over several years following a peak that occurred in the 2001-2002 observation interval. Since about 2003, the cumulative failure rate in P. ramorum-infected coast live oaks has been approximately equal to the cumulative mortality rate due to P. ramorum. In 2005, the cumulative rates of both mortality and failure were about 10%.
SWP of coast live oak trees continued to show a pattern of change from year to year which is closely tied to annual rainfall. Average SWP at all locations in September 2005 was at or near minimum levels (indicating relatively low water stress) observed over the study; the 2004-2005 rainfall total was also the highest recorded over this period.
Data for the period from September 2000 to September 2005 suggest P. ramorum has not impacted seedling populations of either coast live oak or tanoak. Seedling populations in coast live oak plots have varied widely between and within locations over time. Despite fluctuations during the intervening years, seedling densities in 2005 do not differ significantly from those observed in 2000. Tanoak seedling densities have not fluctuated as widely as coast live oak seedling numbers. Counts of tanoak seedlings in plots also showed no significant change between 2000 and 2005. In 2005, all tanoak plots that had P. ramorum-related mortality had tanoak seedlings present. In contrast, 8% of all coast live oak plots had P. ramorum-related mortality but lacked coast live oak seedlings.
This study was conducted with funding provided by the USDA Forest Service, Pacific Southwest Research Station and Phytosphere Research under cost share agreement 02-JV-11272138-063.