It happened overnight last Wednesday to Thursday at the premises of Woodfuel Co-operative on the Heathhall Industrial Estate.
The 24m x 2m tunnel which was used to store briquettes was ‘seriously damaged’, say police.
Officers are investigating and PC Scott Riddick said: “This appears to have been a fairly determined attack on this polytunnel and it has been slashed along the length of it.
“We want to hear from anyone who may have seen or heard anything suspicious in the area over Wednesday night/Thursday morning.”
Anyone with information should call police at Dumfries on 101.