I purchased my first Better Boy Tomato plant recently in a 5 gal container. I kept it in the container for 6 weeks but it got to big I transplanted it into the ground. It is doing well, still growing and lots of tomatoes (still green) but some of them have brown rotting spots on the bottom of them. I planted it in the ground mixing flower/vegatable mulch and I use tomato plant food once a month. What is causing the rotting?