Question: Can I add insecticidal soap to a fungicide spray such as sulfur, so I can "kill two birds with one stone," so to speak? Or must they be used separately?

Answer: It's not a good idea to mix pesticides. There might be a chemical reaction between the compounds, or the combination could be phytotoxic (react with the sun to burn leaf surfaces). Be sure to identify the cause of any problems you are having, then choose the proper pesticide for the job, following label directions carefully. There are some commercial mixes available; these have been tested for safety.