I've always thought of you as the expert on vandas so I wouldn't be able to tell you anything you don't already know. I use Diathane, which is a topical fungicide, and Thymol, which is a systemic fungicide. I think that, just like people, some individual plants seem to be more susceptible to diseases than others. I hope you can get it under control before the summer rains hit. I have a heck of a time controlling fungus once the Florida rainy season starts. Good luck.