Here are some of the pests/diseases that might harm your grown white sage and some ways on how to manage them:
Aphids: Remove aphids by a direct stream of water sprayed in the plant. You can also use other predators to keep the aphids away. To do this, put a sticky band around the base of the stem to attract the ants (this is available in local nurseries). Whiteflies: You can either vacuum the insects or spray them with water.  Flat and spider mites: Shake the bugs loose from the plant. Use a piece of white paper under the leaves to avoid letting them fall into the other parts of the plant. Powdery mildew: Avoid overwatering the plant to avoid attracting fungus in your plants. You can also use neem oil as a preventive measure. Mint rust: Remove the affected area to prevent spread. Avoid watering the plant too much and provide enough space for the air to flow on the leaves.