This is the problem. If a dog is suspected to have developed a food allergy, then you need to feed only a novel food for quite a length of time, in order to rule out all the foods the dog has ever eaten. We had exactly this problem when Penny developed skin problems last year. The skin specialist asked us to think of a food - an available one! - that she hadn't ever eaten. Kangaroo? She has that quite often. Duck? She's had it once or twice. Camel? She's even had that! And so on...
Fortunately, the specialist decided the problem most likely wasn't a food allergy. Since then we've been very careful about bothersome plants, like the ubiquitous and horrible Wandering Trad, and we wipe her 'private areas' after we come inside, with baby wipes. Worth the trouble, to keep her from licking herself raw!
The article, written by Dr Jennifer Coates, says that this problem of people giving their dogs all sorts of foods means that vets have to prescribe special dry foods. She says:
Veterinarians do still have the ability to prescribe limited antigen diets made from protein sources that have been hydrolyzed (i.e., broken down into such tiny pieces that they don’t stimulate the immune system). To avoid confusion, I’m starting to turn to these more than the novel ingredient foods that I have used in the past.Now, at last, I understand why our vet had Penny on the seemingly boring special diet last year, before he sent us to see the skin specialist.