Black color mold on drying rack from wet towel. do i need to toss it?

The towel?: Or the drying rack? Neither. Just clean them. A bleach solution or Oxiclean is fine for the towel. The drying rack,you might need to get a little more aggressive with ammonia based cleaners. There are commercial black mold cleaning products that you can purchase at home improvement stores. You can decide if the rack is worth keeping. The towel....no problem. Just disinfect in the wash.