ALDEVAUX

I think nursery stock can be infected at the time of propagation (infected cuttings, soil contamination and contaminated cutting tools) or any time after by the usual methods of dissemination of the pathogen.
As for other methods of infection into the plants by P. ramorum, I think we have to wait until the research has been fully accomplished to know exactly how they can be achieved. Maybe the fungus can inter the sap of trees by contaminated insects that wound the bark (like dutch elm disease) or by other mechanical wounds that can remain wet long enough for the fungus to enter.
What I was referring to in my last posts was only in reference for what is well known from the studies carried out with Phytophtora infestans which causes potato blight. P. ramorum may have other methods of infection which will explain why it can infect plants in areas, like in California where it is relatively drier.
If this is the case, I think susceptible bonsai plants should never have open wounds on the bark of trunks or on branches that remain wet for long periods.
Regards, Alain