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 hardhit 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 Cryphonectria Figure 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 tunneling and gallery establishment within infected trees to determine whether the pathogen can be isolated from larvae and gallery walls Such an inquiry may help determine whether beetles and larvae are capable of transporting the pathogen further into the sapwood of the host perhaps
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