March 28, 2001
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Additional key words: sudden oak death, SOD
We conducted a case-control study to examine the role of water stress and various other factors on the development of Phytophthora ramorum cankers in coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorus). The study compares subject trees that exhibited symptoms of P. ramorum infection (case trees) with symptomless (control) trees. In September 2000, we collected data in 150 circular plots (8 m radius) in areas where disease caused by Phytophthora ramorum is prevalent. Each plot was centered around a subject case or control tree. We collected data at 10 locations in Marin County, and 1 location each in Sonoma and Napa Counties. Subject tree variables, data on host and nonhost trees in the plot, and other plot characteristics were fitted into logistic regression models.
For Q. agrifolia, the incidence of decline and death related to P. ramorum was almost equal to rates of decline and recent mortality due to other agents. For L. densiflorus, decline and mortality related to P. ramorum was far more common than decline and mortality due to other agents. Phytophthora infection in plot trees other than the subject (case or control) tree was more common in case plots than in control plots. This result indicates that P. ramorum infected trees are spatially aggregated, at least on the scale of the plot size used in this study (0.02 ha). However, counts of plot trees with advanced symptoms of decline due to P. ramorum canker were not associated with disease in the subject tree. Furthermore, within plots, counts of trees with early and advanced symptoms of P. ramorum canker were not correlated. Thus, our data do not indicate that inoculum produced within the plot plays a clear role in the spatial aggregation of diseased trees.
Midday stem water potential in Q. agrifolia subject trees ranged from -0.25 to -3.1 MPa. Stem water potential was higher (indicating lower water stress) in cases than controls. Furthermore, the amount of trunk girdling caused by P. ramorum cankers also increased as stem water potential increased. These results indicate that Q. agrifolia trees in moister sites are at a higher risk for disease than are those in drier sites. Other variables significantly associated with disease in Q. agrifolia include tree canopy dieback and the amount of the canopy shaded by other trees.
Stem water potential in L. densiflorus subject trees ranged from -0.425 to -1.65 MPa. No significant relationships between stem water potential and disease were evident for L. densiflorus, but relatively few L. densiflorus subject trees were included in the study.