I think your plants are suffering from sunburn followed by attack of fungus.The phal leaves being more succulent are more susceptible to the fungus or bacteria attack. The dendrochilum,having tougher leaves, I am sure will recover. Sometimes a sudden exposure to strong sunlight will result in burning of the leaves; ditto with strong concentration of insecticide. The resultant injury may give rise to infection from bacteria and fungus. In such cases cut down on watering and treat infected part with fungicide or cinnamon powder.Physan, if available., is the best treatment.
To prevent mite infestation give plenty of ventilation and wet the underside of the leaves when watering, particularly of phals, paphs and dendrobiums.