I didn't know that, can you post the source for that?
I don't think I have a written source, but it was a point that was widely discussed at a recent DEFRA/ Forest Research conference I attended on P. Ramorum. Generally, the (suspected) incidents of P.r. are/were up and down the west coast and trail off eastwards along a gradually declining rainfall gradient. However, since the conference, a number of suspected sites have since been give the all clear, so the outbreak map
doesn't look as bad as it did.
We've not had long to study this disease yet, but it is thought that warm wet summers have allowed the disease to establish in the SW, and the last three (four?) summers have seen above average rainfall and above average humidity which has resulted in a large increase in the number of infections to the extent that whole stands became infected. The sporalation rate of P.r. in larch is also much more than of Rhododendron (another host for the disease), and it being in the tree canopy it is in a more exposed position to spread on a damp wind.
Bit more here, but most of it refers to the American version, Sudden Oak Death: http://en.wikipedia.org/wiki/Phytophthora_ramorum