Evaluation of stem water potential and other tree and stand variables as risk factors for Phytophthora ramorum canker development in coast live oak

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 symptomatic (case) and symptomless (control) coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorus). Midday stem water potential (SWP) in Q. agrifolia subject trees ranged from –0.25 to –3.1 megapascals (MPa). SWP was higher (indicating lower water stress) in cases than controls. Other variables significantly associated with disease in Q. agrifolia included tree canopy dieback, the amount of the canopy shaded by other trees, and the density of California bay in the plot. Cases were also significantly associated with counts of other plot trees with early P. ramorum canker symptoms, indicating that spatial aggregation of diseased trees occurs on the scale of the plot area (0.02 hectare). For Q. agrifolia, the incidence of decline and death related to P. ramorum in case plots 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.