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Resistance of Umbellularia californica (bay laurel) to Phytophthora ramorum

Hüberli, D., Davidson, J.M., Garbelotto, M., Swain, S. and Van Sant, W. (2002) Resistance of Umbellularia californica (bay laurel) to Phytophthora ramorum. In: Sudden Oak Death Science Symposium, 15 - 18 December, Monterey, California.


Recent research suggests California bay laurel trees, Umbellularia californica, act as a critical vector of Phytophthora ramorum, the causal agent of Sudden Oak Death. Severe stem cankers on Quercus agrifolia (coast live oak) are strongly associated with heavily infested adjacent bay trees. This discovery has inspired us to conduct research on both genetic and environmental resistance factors to P. ramorum among bays. By understanding the conditions most conducive for P. ramorum infecting bays, we can determine which oak woodlands contain the highest risk of potential infection by the pathogen.

The methodology used includes collection of healthy bay samples throughout the known geographic range of the pathogen and then inoculating the leaves with zoospores of P. ramorum in a controlled environment. Leaf lesions are then measured to determine susceptibility of trees. Preliminary results suggest there is a range of resistance among infested geographic regions. Comparisons between trees with high infection levels and those which show little or no symptoms were also completed. It is hypothesized that a number of factors contribute to these epidemics including host resistance, genetic structure of host species in the forests, pathogen variation, and environmental conditions. These issues are currently being assessed.

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