Horticultural oil is used, which is sometimes called dormant oil when used at a slightly-higher concentration during the dormant season (late fall through winter). You can learn more on the linked page. When and how often to apply dormant oil may depend on what pest or disease is being targeted.

Dormant oil may not effectively suppress all problems, such as Peach Leaf Curl (a significant infection if you're growing peaches, for example). In that situation, to prevent Peach Leaf Curl, you would instead apply liquid copper fungicide or Bordeaux mix (copper sulfate and lime) before bud swell (which may happen by mid-week this week based on rapidly increasing day temps during this warm spell). Check the label information thoroughly for any product you spray, as some ingredients may interact with each other in a way that can damage the plant if the applications are not separated by enough time.

Miri