I have a small potted sage plant whose leaves have cm-wide blotches of what I think is fungus. What do I do?

You may not have fungus. Sage has a heavy duty root system and heavy nutrient requirements. Your plant may be potbound. When a plant is potbound, it will begin to sacrifice its leaves before it will give up its roots. Blotches are a symptom of nutrient deficiency as well as fungus. Check to see if the plant is potbound. Try and lift it out of the pot and if it is held in the pot by an abudanza of roots, it needs repotting.

I have a small potted sage plant whose leaves have cm-wide blotches of what I think is fungus. What do I do?
best thing is to plant it out