The Potential of Ambrosia Beetles (Monarthrum scutellare and Monarthrum dentiger) to Vector the Phytophthora that causes Sudden Oak Death Tamara Rich Abstract Potential vectoring of invasive fungus, genus Phytophthora, in infected California oak trees by ambrosia beetles, Monarthrum scutellare, was observed. A newly discovered invasive fungus in the genus Phytophthora is responsible for the sudden death of the Live Oak, Tanoak, and Black Oak trees in Coastal California. Fungal infection and beetle tunnels of ambrosia beetles, Monarthrum scutellare and M. dentiger have been associated with the death of the trees. I studied whether beetles may serve as vectors of the invasive Phytophthora fungus. Beetles were tested in partially artificial conditions. Twelve infected logs were cut from Live Oak, Tanoak, and Black Oak trees in Marin water district and incubated in an 80 degree isolation chamber. Beetles that emerged were collected and plated onto Phytophthora selected media. Fungal growth was observed and compared to known growths of Phytophthora. Approximately 4,000 beetles were tested carrying approximately fifteen different fungi growing on selected media. No definite matches to the known species of pathogen were found. Overall, results of this study did not support the hypothesis that ambrosia beetles vector the invading fungus.Introduction For the past five years, Oak trees along the northern California coast line have been under attack of an invasive, non-native, fungus-like oomycete of the genus Phytophthora. This fungus and three species of bark and ambrosia beetles have been associated with the oak death, though their roles are still not defined (McPherson et al, 2000). This study focuses on Oak-beetle-Phytophthora interactions in the Marin County Municipal Water District. The trees are distributed along 1,500 coast miles in California and Oregon. The spread of the fungal infection is confirmed as far north as Kings Ridge in Sonoma County and as far south as Pfeiffer Big Sur State Park (http://camfer.cnr.berkeley.edu/oaks/.). Phytophthora (“plant killer”) is a genus of plant pathogens with swimming zoospores and thick walled resting clamydospores (Hansen et al, 2000). This pathogenic species was identified in June of 2000 as a Phytophthora genus, which is responsible for the Irish potato famine of 1779-1841. In more recent years it has been found to cause of the rapid death of evergreen Oaks in the Mediterranean region and deciduous oaks in Central and Western Europe (Raven et al, 1999). On the west coast of North America the Phytophthora lateralis has infected and destroyed thousands of acres of Port-Orford-Cedar in Oregon and Northern California (Hansen et al, 2000). P. lateralis is a suspect parent of this new Phytophthora strain, potentially caused by hybridization with another Phytophthora strain (McPherson, per comm.). Several Phytophthora species have been observed to hybridize in alder trees, expressing its habitat flexibility (Brasier, 1999). The differences between behavior of P. lateralis and this new species is that the latter exists only as a root rot and is vectored via soil and water splash, not by wind. It has been shown to survive for seven years in potted soil, but dies within days of being exposed to the sun on the soil surface (Hansen, 1996). The Phytophthora infesting the California coast Oaks does not attack subterranean roots but instead infests the soft wood under the bark near the base of the trunk (McPherson et al, 2000). It is not known how the spores are vectored, via soil, water splash, wind, or by beetles. The spawning of the fungus into the sapwood of the trees, bypassing the protective bark layers with remarkable success suggests the fungus is carried into the sapwood by a specialized mobilizing agent which also proliferates in the lower portion of the trunk.In the case of Sudden Oak Death (SOD) three stages of progressive symptoms have been observed. The first stage is the infection by the Phytophthora in the lower trunk, forming a growth in the cambium layer where it spreads along the surface of the soft wood creating a canker of dark, decomposing wood. This area is identified from the outside by the seeping of the tree, which is caused by the trees defense mechanisms attempting to push out the invading organism. In the second stage, the ambrosia beetle, Monarthrum scutellare, the minor Oak ambrosia beetle, Monarthrum Dentiger, and the Western Oak bark beetle, Pseudopityphthorus pubipennis, colonize the lower trunk in large numbers. They produce reddish-brown piles of dust called frass resulting from their tunneling activity (Palkovsky and Svihra). These beetles are natural parasites to this habitat, usually attacking dead or dying trees. In the case of SOD they are noted to attack green leafed, seemingly healthy but infected trees. The third stage is the growth of hypoxylon fungus growing from the sites of infection. Hypoxylon is a wood decay fungus that commonly feeds from fallen logs or dead standing trees, however it is now found on infected, green to brown leafed trees. Oak trees are necessary for the existence of ambrosia and bark beetles. The ambrosia beetles tunnel 1-2 ½” into the soft wood, where they lay their eggs and feed from non-aggressive, nutritious fungi. The bark beetles tunnel through the bark, into the cambium where they lay their eggs and feed from the phloem. Here the larvae develop over a period of months. In coastal California, the adult beetles emerge in March-April, with a second brood produced in August-September. The beetles emerge as adults from one host and attack a new, suitable host carrying fungi and microorganisms picked up in the tunnels. When they find a host, they tunnel into the bark and wood, lay eggs inside the cavity, and the cycle continues. The bark and ambrosia beetles are known to be vectors of several pathogenic fungi. Bark beetles of the Ips. spp. have been shown to vector the pitch canker pathogen, Fusarium subglutinans, to Monterey Pine in California (Storer et al, unpub). This disease causes mortality, stem deformation, reduced growth, and seedling mortality (Storer et al, 1995). The southern pine beetle, Dendroctonus frontalis, infects pine trees with the blue stain fungus, Ceratocystis minor, into the phloem where it spreads to the soft and hard wood causing wilting of the tree (Barras, 1970). There is no information in the
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