T. J. Swiecki1, and E. A. Bernhardt2
In September of 2000, 2001, and 2002, we recorded detailed symptom information and measured late summer midday stem water potentials (SWP) of coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorus) with and without symptoms of Phytophthora ramorum stem canker (sudden oak death).
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 (19% of symptomatic plot trees) than for coast live oak (6% of symptomatic plot trees). Apparently 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.
Tree SWP readings for 2001 were 0.54 MPa lower on average than 2000 SWP readings across both species. Overall average SWP readings in 2002 were intermediate between 2000 and 2001 readings. SWP readings for individual trees in all three years were highly correlated. SWP readings made on multiple trees within plots were also 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 from 2000 to 2001 was not correlated with changes in SWP. From these results and other observations, we conclude that trees with Phytophthora canker develop substantial water stress in the canopy within the final year before the top is killed. Hence, canopy water stress appears to be a relatively late symptom of disease.
