Why is my rosemary turning grey?

Grey rosemary stems usually indicate root rot caused by overwatering — India's humidity makes this the most common killer of rosemary. Check roots: if brown and mushy, the plant is in decline. Salvage by taking healthy green tip cuttings and re-rooting. Grey powdery coating on leaves is powdery mildew — improve airflow and spray with diluted milk (1:10 ratio with water). During monsoon, move rosemary under a covered area and reduce watering to once a week.