Pityrosporum is a yeast infection. Currently, it is usually referred to as Malassezia, but that is only because mycologists have to change the name of everything every few years in order to have something to do.
It is possible to culture yeast from normal ears, so this diagnosis has to be considered carefully when it is made. As many as 30 to 40% of normal dogs ears have Malassezia organisms in them. I have not seen figures for the percentage of normal cat ears from which yeast can be cultured but it is likely to be similar.
In most cases Malassezia is a secondary problem. It can occur in conjunction with allergies, immune mediated disease, chronic bacterial infections, long-term antibiotic use and seborrhea. It may also occur as a primary infection in some dogs and cats. It seems to be pretty common after ears are treated for Pseudomonas infection (a bacteria), probably because treatment for that condition requires long term antibiotic use with broad spectrum antibiotics.
Treating the primary problem is the first step in treating Malassezia. If that can be done, then keeping the ear canal dry and acidic is often enough to keep the Malassezia growth down. When that doesn't work there are medications that work pretty reliably. The most commonly used one currently is probably clotrimazole (used in Otomax Rx). Thiabendazole (in Tresaderm Rx) has been reported to be effective.
It can take extensive diagnostic work to correctly identify the underlying cause of yeast infections when they are recurrent. Tests for immune system disorders like feline leukemia or feline immundeficiency viruses, hormonal disease, allergies, bacterial infections, parasites (ear mites, demodectic mange, etc.) and careful review of medication histories may be necessary to establish the diagnosis of the primary problem.