I think there is some specificity such as P. lateralis is known to cause root rot in Lawson Cypress and P. ramorum causes bleeding cankers and in various oaks and P. cinnamomi causes collar rot and cankers, etc. in many plants.
It just can get crazy because the Phyto. can hybridize so easily (which is what's happening out West to the oaks)or there can be several Phytos. at a site until one beats out the others.
As always, though, it is never just one "event" that brings something to full manifestation. With P. ramorum many people feel that the years of strict fire retention have contributed to the outbreak. Fires change the soil Ph (with the ashes)to a more alkaline state and that makes it less conducive to P.ramorum establishment. The soils out here have become too acidic without Nature's natural cycle of fires.
Of course, now we've had millions of acres burn so we'll see what happens, but it seems a little late...
-Diane-
