Disease Risk Factors and Disease Progress in Coast Live Oak and Tanoak Affected by Phytophthora ramorum Canker

This paper reports on 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 percent to 24 percent. Over the same period, the percentage of symptomatic tanoaks increased from 31 percent to 43 percent. Between 2000 and 2004, mortality due to P. ramorum increased from 4 percent to 9 percent in coast live oak and from 12 percent to 23 percent in tanoak. About 58 percent of coast live oak and 47 percent of tanoak study trees with disease symptoms in 2000 progressed to a more advanced disease severity class by 2004. In both species, some symptomatic trees developed callus tissue along at least part of the canker margin where canker expansion was apparently inhibited. In some infected trees, cankers have not changed in size for several years and appear inactive. Diffuse canopy dieback commonly develops in coast live oaks with advanced P. ramorum canker symptoms that survive for at least several years. Most coast live oaks and tanoaks with P. ramorum canker symptoms have maintained relatively high stem water potential (SWP) levels and do not show progressive increases in water stress as disease develops. For coast live oak, trees with low water stress (high SWP) are more likely to develop P. ramorum canker, but subsequent disease progress in symptomatic trees is not related to SWP. Only two of numerous bark characteristics were associated with P. ramorum 1 A version of this paper was presented at the Sudden Oak Death Second Science Symposium: The State of Our Knowledge, January 18-21, 2005, Monterey, California 2 Phytosphere Research, 1027 Davis Street, Vacaville CA 95687; [email protected] GENERAL TECHNICAL REPORT PSW-GTR-196 384 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. This characteristic may be associated with faster rates of bole radial growth, and 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