Pityriasis. Light micrograph of a section through a skin sample containing the fungus Pityrosporum orbiculare. This yeast fungus, which is also known as Malassezia furfur, causes the skin disease pityriasis or tinea versicolor. It manifests as spots or discoloured patches on the skin's surface. P. orbiculare may also cause folliculitis (inflammation of the hair follicles). The condition is aggravated by humid environments, excessive sweating or oily skin. Diagnosis is made by observing skin scrapings under a microscope, and treatment is with topical or oral anti-fungal drugs.