Consequences of Phytophthora ramorum Infection in Coast Live Oaks

Sudden oak death, caused by Phytophthora ramorum, has infected and killed large numbers of oaks (Quercus spp.) and tanoaks (Lithocarpus densiflorus) in California since the mid 1990s. Since March 2000 we have been investigating the interactions between patterns of disease progression and broader landscape-scale patterns of disease incidence and expansion in study plots in Marin County. The incidence of new infections has continued to increase in both coast live oaks (Q. agrifolia) and California black oaks (Q. kelloggii), and very rapidly in tanoaks. The net result of more than10 years of P. ramorum presence in these forests has led to dramatically altered overstory structure. Larger diameter trees are more likely to be infected and once infected, are more likely to be attacked by beetles. Beetle attacks dramatically lowered the estimated median survival of infected coast live oaks, from 7 years to less than 3 years, with comparable results for tanoaks. In 2001 we established landscapescale monitoring in Marin and Santa Cruz Cos. to assess spatial and temporal dynamics of disease spread within vegetatively heterogeneous forests. In Marin Co., the proportion of coast live oaks that was infected decreased by 2004, while the proportions increased in tanoaks and Shreve oaks (Q. parvula var. Shrevei) in Santa Cruz Co. by 2005. To evaluate the role of bark and ambrosia beetles associated with this disease in coast live oaks, we inoculated asymptomatic trees with P. ramorum, used an insecticide to experimentally prevent beetle attacks, and placed traps on inoculated and wounded (uninoculated) trees. Traps on infected trees caught 97 percent of all beetles trapped. The size of the bleeding cankers was positively correlated with the response of beetles to infected trees. Both the number of beetles trapped on infected trees prior to any attacks and the intensity of beetle attacks predicted progression to advanced disease stage later in the year. We conclude that beetles naturally adapted to infest recently killed oaks are exploiting a new resource and in the process are accelerating the rate at which P. ramorum kills these trees.