The Ecological Association between Ambrosia Beetles (Monarthrum scutellare and M. dentiger) and Phytophthora ramorum, the Pathogen that causes Sudden Oak Death Hanna Mershman Abstract A recently discovered fungus-like oomycete, Phytophthora ramorum, has been identified as the pathogen responsible for the epidemic levels of oak mortality throughout California and southern portions of Oregon. The means of pathogen dispersal is currently unknown. This study explored the possibility of bark beetles (Pseudopityophthorus pubipennis) and ambrosia beetles (Monarthrum scutellare and M. dentiger) as vectors of P. ramorum, which results in the disease complex termed Sudden Oak Death (SOD). To confirm beetles as vectors of the pathogen, logs of coast live oak (Quercus agrifolia) exhibiting SOD infection and evidence of beetle colonization were placed in isolation chambers and emergent beetles were collected and plated on a selective media designed to promote P. ramorum growth. Additionally, logs were cross-sectioned to determine if the pathogen could be isolated from tunnel walls. Of the 1,344 beetles sampled, none were shown to associate with the pathogen. Since 98% of the beetles sampled belonged to M. scutellare, no conclusions could be made concerning M. dentiger and P. pubipennis due to inadequate sample size. P. ramorum was not isolated from tunnel wall shavings. These results suggest that M. scutellare is not a vector of P. ramorum and therefore neither capable of spreading the pathogen within the vascular tissue of the host nor capable of transporting the pathogen to a new host.Introduction Since 1995, oak mortality has risen to epidemic levels from as far south as Monterey County, California, to as far north as southern portions of Oregon (Garbelotto et al. 2001). Evidence suggests that a previously unknown pathogen, Phytophthora ramorum (McPherson, 2001), is responsible for this rapid decline in oak health. Infection by the pathogen results in a disease complex termed Sudden Oak Death (SOD). Susceptible oak species include black oak (Quercus kellogii), coast live oak (Quercus agrifolia), Shreve oak (Quercus parvula var. shrevei), and tan oak (Lithocarpus densiflorus). However, the pathogen is not restricted to these four species in the family Fagaceae. The pathogen has also been confirmed to attack huckleberry (Vaccinium ovatum), Rhododendron spp., madrone (Arbutus menziesii), bay laurel (Umbellularia californica), buckeye (Aesculus californica), bigleaf maple (Acer macrophyllum), and manzanita (Arctostaphylos manzanita). The host list continues to lengthen as researchers discover new susceptible species. Recently, coastal redwoods (Sequoia sempervirens) were identified as potential hosts; research has yet to confirm these statements (California Oak Mortality Task Force, 2002). The number of species the pathogen is capable of infecting remains unknown. Hardest hit counties include Marin, Santa Cruz, Monterey, Napa, San Mateo, Santa Clara, and Sonoma (Figure 1). Although the disease distribution is patchy, infected areas can experience 40-80% oak stand mortality (Garbelotto et al. 2001). As a result, within these hard-hit counties, thousands of dead and dying oaks have generated a wealth of social, economic, and environmental problems (Kan-Rice, 2001). At present, there is no cure for this affliction. The consequences of changes in forest structure as a result of thousands of dying and dead oaks are uncertain. Changes in fire ecology, wildlife habitat, and aesthetics are issues raising awareness of the disease among the scientific community and public (Kan-Rice, 2001). With so many lingering questions, research to combat this epidemic is urgent. Sudden Oak Death has devastated many oak woodland areas. However, ecological devastation as a consequence of exotic pathogens is nothing extraordinary. Unlike native pathogens which have co-evolved with their hosts, a new or exotic pathogen finds its host plants unable to adequately defend themselves. For example, the exotic Asian fungus CryphonectriaFigure 1. Areas of Confirmed SOD Infection as of April 18, 2002 (Monitoring SOD in California, 2002) parasitica, from the time of its introduction to United States in 1900 to 1940, killed 3.5 billion American chestnut trees (Castanea dentata). The blight successfully downgraded the most important tree species of the eastern forest to insignificance (American Chestnut Cooperator’s Foundation, 2002). Similar notable cases of ecological devastation as a result of exotic pathogen introduction include Dutch elm disease, White Pine Blister Rust, and Pine Pitch Canker. Today, researchers struggle to prevent similar catastrophic losses and species extinction from Sudden Oak Death. The pathogen causing Sudden Oak Death belongs to the large genus of plant-pathogenic fungi called Phytophthora (“plant destroyer”). Although Phytophthora ramorum is a recently discovered species, members of the genus Phytophthora have caused economic and social problems for more than a century. Many species of the genus are known to cause diseases on an increasing number of host plants, including major food crops, tropical fruit trees, nut trees,berries, and ornamental plants (Zentmeyer, 1983). In 1845 P. infestans caused a potato blight devastating potato crops throughout Western Europe. Ireland was hardest hit, suffering widespread famine and massive emigration (Gregory, 1983). The remarkable success of Phytophthora species can be attributed to their plasticity and tolerance of environmental change. The lifecycle of Phytophthora species is complex, involving a variety of structures, which can transition back and forth depending on external conditions (Gregory, 1983). At present, knowledge of P. ramorum’s lifecycle remains incomplete. Furthermore, the dispersal mechanism of Phytophthora spores vary among species, including wind (P. infestans and P. phaseoli), water (P. coloasiae), ants (P. magekarya), and even snails (Gregory, 1983; El-Hamalawi et. al., 1996). This research project addresses the mechanism of Phytophthora ramorum dispersal. Two Ambrosia beetles (Monarthrum scutellare and M. dentiger) and the Western Oak bark beetle (Pseudopityophthorus pubipennis) are commonly found in infected oak species and are hypothesized to serve as P. ramorum vectors. Here, I will explore the possibility of an oak/pathogen/beetle association. I will investigate the beetle
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