Pustular psoriasis is rather difficult to treat and till date no medicine is available to cure the conditions,,,but there are treatment which may improve and give you long time remission...like topical steroids, vitamin-D analogs, PUVA, ultraviolet light B (UVB), acitretin, methotrexate or cyclosporine depending on the severity...maximum benefit with minimum of medicine should be the goal...talking about your lesion over foot this could be psoriasis but psoriatic patch is more greyish white scaly with underlying erythema rather than dark as you mentioned...You should better show it to a dermatologist to rule out any other fungal infection...thanks