I have harvested yeast several times from various fruits, my favorite being raspberries. I will drop a few raspberries into some starter wort and it usually takes a week or so at 70ish temps to show any signs of fermentation. Let me just warn you that the results from this step usually smell pretty bad. Sometimes vomit or sweaty socks, or even strong cheese. Once fermentation is done, usually around the 2 week mark, cold crash, decant, and pitch into another starter. Do this 3 or 4 times to build up the cell count and you should be good to go. Taste each step and make sure the flavor profile is going the way you want. I've had about 50% success this way and most wild yeasts do have Belgian Saison characteristics.