Establishment of an emerging generalist pathogen in redwood forest communities


Patricia E. Maloney (tel. +1 530 754 894; fax +1 530 752 5674; e-mail


  • 1Phytophthora ramorum (causal agent of sudden oak death) is an emerging generalist pathogen in coastal forests of California and Oregon, USA, that causes lethal stem infections on oaks (Quercus spp.) and tanoak (Lithocarpus densiflorus) as well as non-lethal foliar infections on a broad range of trees and shrubs.
  • 2We studied P. ramorum over its known range in coastal redwood forests to determine forest compositional variables that are important to its epidemiology within the geographical area that it has already invaded. Redwood forests are dominated by coast redwood (Sequoia sempervirens), tanoak and California bay laurel (Umbellularia californica).
  • 3A total of 120 permanent plots (500 m2 each) were established in redwood forests at 12 sites within the main epidemic area in California. Over 5000 trees were mapped and examined for the presence of P. ramorum during spring 2002 and resampled in spring 2003.
  • 4Mean incidence of P. ramorum across all plots was 0.17 ± 0.01 in 2002 and 0.24 ± 0.02 in 2003. The highest infection levels by P. ramorum were found on California bay laurel (range 0.42–0.69) and tanoak (0.32–0.45). The highest levels of mortality were associated with tanoak and ranged from 0 to 66%, with 62.5% of that mortality associated with P. ramorum infection.
  • 5Disease incidence above 30% was most often associated with bay laurel importance value. In plots with few bay laurel stems, high disease levels were associated with the presence of understorey tanoaks. Bay laurel and small tanoaks are thought to represent the main source of inoculum for further spread of P. ramorum.
  • 6Differential host mortality due to this emerging generalist pathogen will exert considerable influence on redwood forest dynamics, with potentially dramatic shifts in forest composition and structure and subsequent cascading ecological and evolutionary effects.