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Additional key words: oak decline, tanoak decline, sudden oak death (SOD)
We have completed four 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 2003, we collected data on P. ramorum symptoms, tree condition, stem water potential, and various other factors in 150 circular plots (8 m radius) in areas where P. ramorum canker was prevalent in 2000. Each plot is centered around a case (symptomatic) or control (asymptomatic) subject tree. Plots were established at 10 locations in Marin County, and 1 location each in Sonoma and Napa Counties.
Since September 2000, the percentage of symptomatic coast live oak trees in the plots has increased from 22.9% to 24.4%. Two-thirds of this increase in disease incidence occurred between September 2002 and September 2003, following the first relatively wet spring of the study. Between 2000 and 2003, the incidence of P. ramorum canker in tanoak has increased from 33% to 39%. For tanoak, increases in disease incidence were approximately equal in each observation interval, about a 2% increase in disease incidence per year.
Mortality due to P. ramorum among all monitored coast live oak increased from 3% to 8% between 2000 and 2003. Over the same period, P. ramorum-related mortality in all monitored tanoak has increased from 12% to 22.5% in 2003.
Between September 2000 and September 2003, substantial failures occurred in 32% of coast live oaks with P. ramorum symptoms but only 2% of coast live oaks lacking P. ramorum symptoms. Over this time interval, 73% of trees that were dead as a result P. ramorum canker in 2000 have failed, and 65% of the trees with late P. ramorum symptoms in 2000 (cankers with beetle boring and/or Hypoxylon thouarsianum) had failed. In contrast, only 10.5% of the coast live oaks with early P. ramorum symptoms (bleeding cankers only) in 2000 had failed by 2003 and less than 1% of healthy asymptomatic trees failed over this period. Bole failures were most common overall (58% of all failures) and root failures were the least common failure type (1.7% of the failures).
Because P. ramorum primarily causes a bark canker in coast live oak, we initiated a study to determine whether observable bark characteristics are related to P. ramorum canker occurrence or progress. In 2003, we evaluated both bark thickness at 1 m height and a variety of morphological bark characteristics in a subset of study trees. In this sample, we found that P. ramorum canker was more likely to occur in trees with greater bark thickness. We also observed that coast live oak bark thickness increases in a nonlinear manner as tree diameter increases, which suggests that relationships between tree diameter and disease could be related to bark thickness. Only one of the bark morphological characteristics we assessed, the presence of unweathered bark in bark furrows, was positively correlated with disease. This characteristic seems to be associated with faster rates of tree radial growth, and is consistent with other analyses indicating that faster-growing coast live oaks have a greater risk of developing P. ramorum canker than slow-growing trees.
Based on four successive years of stem water potential measurements on the same set of trees, it appears that SWP values measured in September are mainly influenced by rainfall over the three preceding rainy seasons. SWP readings for individual trees in all three years are also highly correlated. SWP readings indicate that water stress is not a significant predisposing factor for the development of P. ramorum canker in coast live oak. Most coast live oaks with P. ramorum canker symptoms have maintained relatively high SWP levels and do not show progressive increases in water stress as disease develops. We hypothesize that SWP levels in many of these trees with advanced symptoms of disease remain high because of the progressive diffuse canopy dieback that develops in trees with advanced P. ramorum canker symptoms. Leaf area loss resulting from branch dieback reduces evapotranspiration while roots continue to function, which allows trees to maintain high SWP levels.