It took me a year to learn whether the petunias were coming or going. ... Usually the brown blooms need pinching, not at the flower, but at the stem where it came from. If it truly is the "bud", then the plants is probably not getting enough water. Petunies in warm, especially breezy, climates need water every day. They have so much foilage that can dry out from the sun and/or the wind. I love petunias, but they do need daily attention to provide beautiful and continuous blooms for you. It's worth it! Just remember not to pinch just the dead bloom, but pinch or cut where it came out from the mother stem. Good luck!