I have just had a look, and the best thing to do is type in specific carbohydrate diet and psoriasis. This brings up web sites that talk about how the diet can help psoriasis. If you think it sounds credible then you can go and look at the diet.
I personally never had Psoriasis until I began to have digestion problems. I have only had it three times and only new what it was because my Dad has always had it, and my Aunty has it. They have both also had inflamatory bowel disease. The connection is to do with having a leaky gut because of inflamation. This allows food particles and bacteria to get into the blood stream and cause an allergic reaction. I will point out that my Dad did not have the bowel problem until his early forties, but he has had Psoriasis since being a child.