A laboratory study using detached leaves of some common broadleaf hosts found in the PNW was undertaken in summer of 2009 and 2010. Leaves were inoculated with a zoospore suspension of an NA1 isolate of P. ramorum and lesion area and infection frequency was evaluated. If P. ramorum was recovered from a leaf that did not show a visible lesion, it was considered to be infected asymptomatically. Sporulation potential of foliage of each plant species was also determined.
In general, the results of this study indicate that western Washington forests are not at high risk for damage caused by P. ramorum, based on the host plants tested. However, this is a subset of the many plant species that occur, and there may be a host species that is either extremely susceptible to infection or a prolific sporulator, that was not tested in this study. Plants posing the smallest risk of P. ramorum establishment were generally invasives and/or riparian species. The highest risk plants were commonly found in forested environments. These were fairly susceptible to infection and produced more chlamydospores than sporangia in their foliage.
Chlamydospore production was higher than sporangia production on many hosts in western Washington forests that were examined in this study. In other systems, such as bay laurel (Umbellularia californica) in California, and Rhododendron ponticum in the UK, P. ramorum outbreaks are driven by high concentrations of sporangia produced on foliage of these hosts. None of the Washington hosts tested produced as many sporangia as U. californica. Chlamydospores are a means by which P. ramorum can persist on a site in soil and decaying foliage, but will probably not produce large amounts of inoculum unless they germinate directly into sporangia, which can occur in flooded soils.
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