Understanding and Managing Sudden Oak Death in California

SIDEBAR 1-1—Taxonomic relationships between susceptible trunk canker hosts

In California, Phytophthora ramorum is capable of killing 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). These species are all members of the plant family Fagaceae.

Tanoak, native to California and southern Oregon, is found only in North America. Until recently it was considered a member of the genus Lithocarpus. tanoak was assigned to the newly created genus Notholithocarpus (Manos and other 2008) as its only member. Under current taxonomy, all Lithocarpus species are native to East and Southeast Asia. Tanoak is unique among the trunk canker hosts in that it also develops P. ramorum infections on twigs and leaves.

The true oaks, genus Quercus, include about 600 species of trees and shrubs. The genus has been divided into two subgenera: Quercus and Cyclobalanopsis. All North American and European oaks are in subgenus Quercus, as are oaks of Africa, South and Central America, and many oaks of Asia. Oaks in subgenus Cyclobalanopsis, which is treated as a separate genus by some taxonomists, are native to eastern and southeastern Asia (Nixon 1993).

The subgenus Quercus has been divided into varying numbers of sections. Representatives of three of these sections (table 1-1) occur in California and include at least 21 oak species plus a number of natural hybrids and varieties. Oaks in the red oak section, including species that are native to the Eastern United States, are susceptible to P. ramorum trunk cankers. Canyon live oak, the only widespread tree species from the intermediate oaks in California, is also susceptible to lethal trunk cankers. Oaks in the white oak section have not been found infected by this pathogen in the field. Table 1-1 summarizes information on the susceptibility of California's tree species in the Fagaceae to P. ramorum trunk cankers. Most of California's shrub oak species occur outside of the known range of SOD and have not been tested for susceptibility.

Table 1-1—Susceptibility of California native tree species in the Fagaceae (oak family) to lethal trunk cankers caused by Phytophthora ramorum under field conditions

Group Species P. ramorum susceptibility
Non oaks Notholithocarpus densiflorus (=Lithocarpus densiflorus) - tanoak lethal trunk cankers and twig/foliar infections
Quercus section Lobatae Q. agrifolia - coast live oak lethal trunk cankers1
red (or black) oaks Q. kelloggii - California black oak lethal trunk cankers1
  Q. parvula var. shreveii - Shreve oak lethal trunk cankers1
  Q. wislizeni - Interior live oak susceptible in greenhouse tests, disease not yet observed in field
Quercus section Quercus Q. douglasii - blue oak not susceptible
white oaks Q. engelmanii - Engelmann oak not susceptible (?)2
  Q. garryana - Oregon white oak not susceptible3
  Q. lobata - valley oak not susceptible
Quercus section Protobalanus Q. cedrosensis - Cedros Island oak unknown
intermediate oaks Q. chrysolepis - canyon live oak lethal trunk cankers1
  Q. tomentella - island oak unknown

Note: Natural oak hybrids and shrub oak species are not listed.

1Twig and/or foliar infections are uncommonly observed under conditions where shoots are exposed to high spore loads from other hosts such as California bay (David Rizzo, personal communication, Vettraino and others 2008).

2Q. engelmannii does not occur in areas currently infested by P. ramorum. Based on the lack of susceptibility of other related California white oaks (Q. douglasii, Q. garryana, Q. lobata) under field conditions, Q. engelmannii is likely not susceptible.

3Q. garryana develops cankers in laboratory inoculations, but disease has not been observed in the field (Hansen and others 2005).