Guinness World Records have told Colin and Donna Craig-Brown that what they believed to be a potato is in fact a gourd tuber.
Guinness wrote to the pair to confirm scientific testing found their find was not a potato.
The discovery of the tuber happened last year when Mr Craig-Brown hit it with a hoe while gardening on their small farm near Hamilton.
He said it looks and tasted like a potato while admitting he did not know what gourd tuber tasted like.
"What can you say?" Mr Craig-Brown told the Associated Press.
"We can't say we don't believe you, because we gave them the DNA stuff."
After months of submitting photos and paperwork, the couple got the bad news from Guinness in an email last week.
"Dear Colin, sadly the specimen is not a potato and is in fact the tuber of a type of gourd," they wrote.
"For this reason we do unfortunately have to disqualify the application."
The couple had named their find Doug, which they took to spelling Dug, after the way it was unearthed.
The tuber became something of a local celebrity after the couple began posting photos of it on Facebook with a hat on and even built a cart to tow it around.
An official weigh-in at a local farming store put Dug at 7.8 kilograms.
The existing Guinness record for the world's largest potato is a 2011 monster from Britain that weighed in at just under 5kg.