Is the entire plant showing these symptoms or only some of the older leaves? If it’s the entire plant, or a great deal of the plant, it’s likely that it’s getting too much water. Sage leaves yellow if the plant is being kept too wet and plants that are getting splashed with water on a frequent basis are likely to develop leaf-spot fungus.
Sage does best when grown in full sun and watered once a week, fairly deeply. Most plants don’t do well if hand-watered or under irrigation where the foliage is splashed with water frequently.
If it’s only a few leaves that look this way clip them off, and the plant is growing in the ground and not a pot, water the plant deeply only once a week, watering in the morning so that the foliage will dry. If the plant is in a container let it dry out a bit in between waterings – if the nights are cool, for example, you’ll only need to water this plant every two or three days. Make sure the pot has an open drainage hole – most plants don’t like to grow in pots without a drainage hole.