Hello, 

Thanks for reaching out! Here is a helpful UMN Extension article on managing apple scab: https://extension.umn.edu/plant-diseases/apple-scab

Scroll down to the "Using Fungicides" section for more information regarding your question. It is not recommended to use a fungicide if your tree is ornamental or the fruit quality is not a concern (such as being used for cider). Make sure to accompany any fungicide use with good sanitation and cultural control practices. 

If you do use a fungicide, spraying should start when the first green leaf tips appear in the spring. Always follow the fungicide label directions attached to the container you are using. Be sure that the plant you wish to treat is listed on the label of the fungicide you intend to use. Remember, the label is the law.

Good luck!