You may wish to bring a

You may wish to bring a sample to your cooperative extension service office. Radishes rarely have pests or problems. The only time the root can have a black color (unless it's meant to be a black radish) is black rot, which starts at the root. This is caused by a soil-borne fungus. The solutions are crop rotations every 3 to 4 years (so do not plant radishes in this spot for a while!) and good soil drainage so they don't get wet feet. The long-rooted radish are more prone to this disease; the rounded types are usually safer.