Business.With a minor in Sales.
Opposing argument: Depending on how big a business you are planning on growing or how much work you intend to leave for yourself you may actually want to have a professional sales and office manager and look at a degree in chemistry with a minor in biology. While I do agree that the mechanics of brewing are fairly easy to learn, being a head brewer in a growing business is a giant chunk of work alone, let alone having to be the GM and the Sales staff. In my case I am the head brewer of a brewery striving to put out 5000 bbls this year while one of the co-founders is the GM/business and sales manager. I can assure you there is no way in hell I could do his job and retain the mantle of head brewer.
As it too often happens, a brewer follows his passion to open a brewery and then suddenly realizes he has opened a business and the business side isn't always very much fun and doesn't have anything to do with brewing. So if you plan on actually running the business prepare yourself to hire a brewer. Or, if you want to be the brewer pair yourself with someone who knows how to run the business and sales side.