What causes such a fungus, I am assuming that is what it is, to grow on tomatoes? Too much water, too much fertilizer, diseased plants, etc? I have several tomato plants and a few of them are producing tomatoes that look great at fist glance, but when picked have a flat spot on the bottom that is mostly black with some white and brown spots.
Tomatoes Have What Looks Like Fungus on Bottom
Certified GKH Gardening Expert
Your tomatoes probably have what's called Blossom End Rot: a water-soaked spot on the bottom of the fruit where the blossom was. It's typically caused by poorly draining soil that doesn't contain enough calcium.
This article will tell you more about Blossom End Rot: