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Additional key words: oak decline, tanoak decline, sudden oak death (SOD), stem water potential, tree failure
This report discusses findings after five 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 2004, we collected data on P. ramorum symptoms, tree condition, midday stem water potential (SWP), and various other factors in 150 circular plots (8 m radius). Each plot was centered around a case (symptomatic) or control (asymptomatic) plot center tree. Plots were located at 12 locations in the California counties of Marin, Sonoma, and Napa in areas where P. ramorum canker was prevalent in 2000.
Between September 2000 and September 2004, the percentage of symptomatic coast live oak trees in the plots increased slightly, from 23% to 24%. Over the same period, the percentage of symptomatic tanoaks increased from 31% to 43%. Between 2000 and 2004, mortality due to P. ramorum increased from 4% to 9% in coast live oak and from 12% to 23% in tanoak. About 58% of coast live oak and 47% of tanoak study trees with disease symptoms in 2000 progressed to a more advanced disease severity class by 2004. Diffuse canopy dieback developed in many coast live oaks with advanced P. ramorum canker symptoms that survived for at least several years. This pattern of slow decline in infected trees is distinct from the "sudden oak death" pattern that was originally described in trees with P. ramorum canker.
Among trees for which we have the most detailed disease ratings, 3 of 17 (18%) symptomatic tanoaks and 16 of 60 (27%) coast live oaks showed no apparent disease progress between 2000 and 2004 based on either canker girdling rating or symptom class. In both species, some symptomatic trees developed callus tissue along at least part of the canker margin where canker expansion was apparently inhibited. Also, in some infected trees, cankers have not changed in size for several years and appear inactive. These resistant reactions represent another symptom development pattern distinct from the rapid decline and slow progressive decline patterns seen in other trees affected by P. ramorum canker.
Between September 2000 and September 2004, coast live oaks with P. ramorum canker symptoms failed at a significantly higher rate than trees without P. ramorum canker. With only one exception, failures in P. ramorum-affected trees occurred in dead trees or in live trees with evidence of Hypoxylon thouarsianum and/or bark or ambrosia beetle colonization. The failed part was dead at the time of failure in 77% of all scored failures. However, in living trees with P. ramorum canker symptoms, more than half of the failures occurred in live branches or stems.
Most coast live oaks and tanoaks with P. ramorum canker symptoms maintained relatively high stem water potential (SWP) levels and did not show progressive increases in water stress as disease progressed. For coast live oak, trees with low water stress (high SWP) were more likely to develop P. ramorum canker than were more water-stressed (low SWP) trees. Subsequent disease progress in symptomatic trees was not related to SWP.
Only two of numerous bark characteristics assessed were associated with P. ramorum canker in coast live oak. The presence of unweathered brown bark in bark furrows was the only bark surface characteristic that was positively correlated with disease. Unweathered brown bark in bark furrows appears to be associated with faster rates of bole radial growth. The correlation between unweathered bark and disease is consistent with other analyses indicating that faster-growing coast live oaks may have a greater risk of developing P. ramorum canker than slow-growing trees. Bark thickness was positively correlated with both the risk of developing P. ramorum canker and the likelihood of disease progress among infected trees. Because bark thickness also increases with stem diameter, it is possible that the lack of cankers on small coast live oak stems and branches could be related to their relatively thin bark.