what food should psoriasis patients avoid?

For sure, there are certain foods that aggravate some psoriasis sufferers skin but others can get away with eating them OK. General advice though is avoid foods :
Eliminate wheat for a month to see if this helps. If it does, stay off it for 6 months & then gradually reintroduce it.
Avoid citrus fruits and tomatoes in the same way.
Cut down on saturated and hydrogenated fats, especially red meat, mass produced veg oils, take-aways, mass-produced burgers, cakes, pastries, pies & full fat dairy. As much as possible, eliminate alcohol & sugary foods & drinks. Find which is the culprit, cut it out for 6 months & then gradually reintroduce.
Eat fresh produce wherever possible. Fish with omega 3 is good. Brown rice too. Pectin rich foods such as apples & carrots will help. Figs, prunes, kiwi & papaya are good for the skin.
Hope this helps.

Psoriasis and Raw Food