Phytosphere Research

Factors related to Phytophthora canker (sudden oak death) disease risk and disease progress in coast live oak and tanoak

July 2002 (final revised version)

T. J. Swiecki and E. A. Bernhardt
Phytosphere Research, Vacaville CA

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Additional key words: oak decline, tanoak decline, SOD


This report presents data from the second year of observations in a case-control study to examine the role of water stress and various other factors on the development of Phytophthora stem canker disease (commonly called sudden oak death) in coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorus). The study compares subject trees that exhibited symptoms of Phytophthora infection (case trees) with symptomless (control) trees. In September 2000 and September 2001, we collected data in 150 circular plots (8 m radius) in areas where disease caused by Phytophthora ramorum was prevalent. Each plot was centered around a case or control subject tree. Plots were established at 10 locations in Marin County, and 1 location each in Sonoma and Napa Counties.

Various plot and tree factors were associated with disease in the subject tree in logistic regression models for coast live oak. Vegetation-related plot variables that were positively correlated with disease in coast live oak included the count of California bay (Umbellularia californica) trees in the plot, the number of plot trees with Phytophthora canker symptoms, and the presence of poison oak (Toxicodendron diversilobum) in the plot. Tree-related factors that were associated with disease included multiple stems, large stem cross-sectional area, high levels of canopy exposure, and high stem water potential (SWP). In addition to these factors, logistic models based on plot trees other than the subject trees showed a negative association between Phytophthora canker symptoms and decline symptoms associated with agents other than Phytophthora.

The direction of the effects of a number of variables in several different analyses suggests the possibility that Phytophthora canker in coast live oak is more likely to occur in trees that are vigorous and/or fast-growing (larger, more dominant, less water-stressed, not in decline due to other agents) than in trees that are suppressed and/or slow-growing. Significant positive correlations between canopy dieback and Phytophthora canker may indicate that diffuse dieback in the canopy is an early indicator of Phytophthora canker for both coast live oak and tanoak.

Disease progress in trees with symptoms of Phytophthora canker was more rapid for tanoak than for coast live oak. Phytophthora-related mortality between 2000 and 2001 was greater for tanoak (10% of cases, 19% of symptomatic plot trees) than for coast live oak (3.8% of cases, 6% of symptomatic plot trees). New Phytophthora canker symptoms were also more common in tanoak than in coast live oak (8% and 1.6% of previously asymptomatic plot trees, respectively). Somewhat less than half of the coast live oak and tanoak cases showed no obvious advancement of disease symptoms between 2000 and 2001. Preliminary disease progress models for coast live oak indicate that most of the factors associated with disease occurrence are not associated with disease progress in trees that are already symptomatic. This pattern would be consistent with a disease model in which infection events occur infrequently and disease progress is due primarily to canker expansion rather than the initiation of additional cankers.

Subject tree SWP readings for 2001 were 0.54 MPa lower on average than 2000 SWP readings across both species. SWP readings for individual trees in both years were highly correlated. SWP readings made on multiple trees within plots were correlated, suggesting that plot soil moisture levels account for much of the variation in SWP between plots. Trees with Phytophthora canker symptoms did not show a significant overall reduction in SWP between 2000 and 2001 relative to asymptomatic trees, and trees that died between 2000 and 2001 had higher than average SWP readings in 2000. Disease progress was not correlated with changes in SWP. These results and other observations indicate that canopy water stress associated with the final drying of the top occurs over a period of less than a year.

This study was conducted with funding provided by the USDA Forest Service, Pacific Southwest Research Station and Phytosphere Research under cost share agreement 01-JV-11272162-163.