Lars, I would hit it hard with a combo of systemics. I use Dithane M-45 and Thiomyl about a tsp per gallon. The next time I spray I use BanRot and Thiomyl same ratio. I spray every 10 days and bump that up to every week during the winter month (January) and when we are in the height of our rainy season.
I have great success with Vandas this way.