What Causes Dandruff In Facial Hair?

Hello i'm a 29 year old male and i have a problem with my beard. I have oily skin thats my skin type. My beard has a bad dandruff problem everywhere even my mustache has the problem. It gets worse when i grow my beard out longer, but i try not to do that so i keep it cut every two days so the dandruff will be less visible but every once in a while i wanna grow my beard out. I wanted to know is this a fungus problem or is it because i have oily skin.