WELLINGTON, New Zealand—When is a potato not a potato? When it’s a tuber of a gourd, according to Guinness World Records. A New Zealand couple who believed they had dug up the world’s largest potato in the garden of their small farm near Hamilton have had their dreams turned to mash after Guinness wrote to say that scientific testing had found it wasn’t, in fact, a potato after all. Colin Craig-Brown, who first hit the tuber with a hoe last August when gardening with his wife Donna, said it sure looked and tasted like a potato. Mind you, he added, he’s never tasted a gourd tuber. “What can you say?” said Craig-Brown. “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,” the email …