Nail fungus is also known as Onychomycosis . When fungus infects the areas between your toes and the skin of your feet, it's called athlete's foot. Nail fungus Canada is a common condition that begins as a white or yellow spot under the tip of your fingernail or toenail. As the fungal infection goes deeper, nail fungus may cause your nail to discolor, thicken and crumble at the edge. It is increasingly common with increased age. It rarely affects children.