I think, unfortunately, you are right, it's just that it's playing out slowly enough that it's hard for us to see. Over time, natural selection will work its magic and the trees within the species that are more resistant to the disease will reproduce and the genes for resistance will spread throughout the population. The unfortunate thing is that this is not something that happens in a human lifetime, or even many human lifetimes, and it's not going to save individual trees living right now.

Plants don't have an immune system, at least the way we normal think of one. As far as we know, there isn't a way to "vaccinate" them against diseases. Maybe that will change with molecular techniques, but not today.