Upvote:6
To bring in live plants (or roots for growing) you do need a USDA permit. They are pretty easy to obtain, a bit of online paperwork and a visit to the nearest USDA office to verify your identity. You will need to know the scientific name for wasabi, as your permit is specific to certain plant families or genera that you list while applying.
You will need to make sure your root is completely clean of any dirt or wood debris. Wrap it in some newspaper or paper towels, wet them a bit and drop it in a plastic bag in your suitcase or carry on. Then declare it upon arrival in Atlanta. You will go through the AG Inspection line where they will make sure you aren't carrying any pests or disease on your root(s).
I imagine the wasabi you will be getting will be a commercially hybridized variety for farming, so likely will not fall under the CITES treaty, but wouldn't hurt to research that before you start the whole process.