I read about this in a New Zealand magazine as a treatment for crown rot in bromeliads
13 March 2019 06:17 PM
there are no home-garden products registered in Australia containing the active ingredient chlorothalonil. In your case it's best to fix growing conditions rather than try to control with chemicals. Cool over-wet conditions are the cause of the problem. Suggest thin out older plants and replant pups in clean bark-based orchid potting material in an improved position with less water applied to the pots.