The introduced pathogen Phytophthora ramorum causes a lethal trunk canker disease known as sudden oak death (SOD) (Rizzo and others 2002). Tanoak (Notholithocarpus densiflorus), coast live oak (Quercus agrifolia), Shreve oak (Q. parvula var. shreveii), California black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis) are native California trees that are killed by SOD (Sidebar 1-1—Taxonomic Relationships between Susceptible Trunk Canker Hosts). In areas invaded by P. ramorum, SOD has quickly become the most common cause of mortality of these trees (Swiecki and Bernhardt 2010). Mortality rates are greater than 50% in some areas and continue to increase (Maloney and others 2005, Swiecki and Bernhardt 2008a).
Phytophthora ramorum also causes lesions on leaves, twigs, and small stems of many other native species, including tanoak, California bay (Umbellularia californica), madrone(Arbutus menziesii), California huckleberry (Vaccinium ovatum), manzanita (Arctostaphylos spp.), toyon (Heteromeles arbutifolia), poison oak (Toxicodendron diversilobum), Douglas-fir (Pseudotsuga menziesii var. menziesii), and coast redwood (Sequoia sempervirens) (Sidebar 1-2). In addition, P. ramorum causes foliar and stem diseases in a wide variety of non-native plants, including many ornamentals (Sidebar 1-3). This wide host range contributes to the ability of P. ramorum to invade and become established in new areas. Phytophthora ramorum causes only minor damage on most of these foliar hosts. However, pathogen survival and spread is strongly influenced by some foliar hosts, especially California bay and tanoak.
Disease can result when P. ramorum comes in contact with appropriate tissues of a susceptible host. However, environmental conditions must be favorable for a sufficient length of time for infection to occur. The rest of Part 1 describes the processes that contribute to disease development. By understanding the SOD disease process, you will be able to determine what steps can be taken to manage this disease at a particular site.