The Michigan Department of Agriculture and Rural Development has revised the quarantine in an attempt to protect walnut trees from a fatal disease. Thousand cankers disease has killed numerous walnut trees in the western US.
The quarantine generally prohibits shipment of walnut nursery stock, certain walnut timber products, and hardwood firewood into Michigan from infested states. Walnut furniture, veneer, kiln-dried walnut lumber without bark and walnut nuts and nutmeats are exempt.
The quarantine stated in 2010.
For more information; http://www.thousandcankers.com/state-info.php?state=Michigan