Black or brown spots on leaves and flowers and watery patches in leaf tissue are a sure sign that a pathogen has invaded that plant.

Over-watering, careless repotting, direct sunlight on a wet leaf, low temperatures, poor ventilation and stagnant air can all precipitate fungal or bacterial invasion. A number of fungi have been identified as orchid pathogens, particularly in tropical countries. Usually the rot or infection has gone too far for treatment by the time it becomes obvious.

The best treatment is to remove the affected tissue and about 1cm (0.5 in.) of adjacent healthy tissue, and disinfect the cut surface with flowers of sulphur, Captan or Physan. Damaged parts should be removed and burned to avoid infecting other plants. Badly affected plants should be removed and discarded to avoid infecting the rest of the collection.